https://pc5214.org/api.php?action=feedcontributions&feedformat=atom&user=MayInnPC5214 wiki - User contributions [en]2026-04-23T18:45:56ZUser contributionsMediaWiki 1.42.1https://pc5214.org/index.php?title=Kerr_Microscope&diff=3014Kerr Microscope2022-04-30T01:05:49Z<p>MayInn: /* Lab Session Logs */</p>
<hr />
<div><blockquote> "I was led some time ago to think it very likely, that if a beam of plane-polarized light were reflected under proper conditions from the surface of intensely magnetized iron, it would have its plane of polarization turned through a sensible angle in the process of reflection." - John Kerr<ref>J.Kerr, Philosophical Magazine 3 (1877) p.312.</ref></blockquote><br />
<br />
Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| 2x Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
<br />
<b>Magnetic domains</b><br />
[[File:Mokemodes_imaging.png|thumb|MOKE domain imaging of (100) silicon-iron sheet at different imaging modes. <ref>Rudolf Schafer. [http://magnetism.eu/esm/2007-cluj/abs/Schaeffer2-abs.pdf| Magneto-optical microscopy and its application.].</ref>]]<br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains. When multiple magnetic domains are formed, magnetostatic energy in the system decreases as the net magnetization of the system is reduced. Common instances where we can find magnetic domains would be in the random arrangement of magnetic domains on refrigerator magnets, and in specific forms within magnetic recording devices such as magnetic tapes and Video Home System (VHS) tapes. Several domain imaging techniques can be used to observe and study these magnetic domains, and the most inexpensive, time saving, and least intrusive method would be through Magneto-optical Kerr Effect imaging technique. <br />
<br />
<b>Magneto-Optical Kerr Effect (MOKE)</b><br />
<br />
Conceived by John Kerr in 1877<ref>P.Weinberger writes about Kerr's famous communications to the Philosophical Magazine - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>, the magneto-optic Kerr effect (MOKE) describes the rotation of light polarization when reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
<b>MOKE microscopy</b><br />
<br />
Interaction between an incident light and the magnetization of a magnetic sample causes a change in polarization state of the incident light. MOKE microscopy makes use of the fact that varying magnetizations corresponding to different magnetic domains on the sample gives rise to different degrees of incident light polarization. By detecting and imaging these reflected or transmitted interacted light, an image of magnetic domains with varying intensities can be observed, thereby allowing for domain imaging to be conducted. Basic components of a MOKE microscope includes, firstly the light source lamp, which is fed through a microscope array with polarizers and analysers onto a sample with an accompanying magnet to change the applied magnetic field, and the resultant light is collected in a camera. Subsequent image processing is required to elucidate weak changes in contrasts. <br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner, in accordance to data acquisition conditions and the sampled materials' suitability.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED connected in conjunction with a current limiting resistor. The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole after Mirror 2.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
* Added micrometer screw translation stage for sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the <i>tube length</i>, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>. A second parameter that must be kept in mind is the <i>working distance</i>, which is the distance that the sample must be placed in front of the objective. For the 10x and 60x objective, these are 1.5mm and 0.15mm respectively<ref>10x Objective - [https://www.edmundoptics.com/p/10x-din-plan-commercial-grade-objective/5386/| Edmund Optics]</ref><ref>60x Objective - [https://www.edmundoptics.com.sg/p/60x-din-achromactic-finite-intl-standard-objective/3137/| Edmund Optics]</ref>. Hence, when using the 60x objective, the sample is practically kissing the objective.<br />
<br />
The use of a micrometer screw translation stage allowed for finer control over the position of our sample to a precision of <math>\pm 10</math> microns (5 micron contribution from both ends of the measurement).<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample whose external field will be controlled by a magnet stack. Its expected overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<b>Series of permanent magnets</b> <br />
[[File:Magnetseperationfield2.png|thumb|Magnetic field variation away from the magnet stack.]]<br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. </b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. After obtaining these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
(@Joel)<br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
Since Kerr imaging is based on the non-linear response of the sample with respect to the incident light intensity, we considered two alternative lighting configurations -- one in which the laser beam was collimated, and another in which the laser beam was focused onto the sample plane to maximize the incident intensity.<br />
<br />
<gallery widths=420px heights=250px><br />
File: Nonmagnetic images.png | Measured images from non-magnetic sample illuminated by a collimated beam.<br />
File: Nonmagnetic focused images.png | Measured images from non-magnetic sample illuminated by a focused beam.<br />
File: Magnetic images.png | Measured images from magnetic sample illuminated by a collimated beam.<br />
File: Magnetic focused images.png | Measured images from magnetic sample illuminated by a focused beam.<br />
</gallery><br />
<br />
The four figures above depict the raw measurements captured by our Kerr microscopy setup for the four different cases corresponding to the type of sample we were imaging as well as the lighting configuration. The labeled distance of <math>x\,\mu </math>m above each individual image denotes the estimated separation distance between the bar magnet and the back of the sample. In all cases, the magnification was kept the same. The vastly different features seen between the 4 cases are likely due to different areas of each sample being imaged upon changing the lighting and sample configurations.<br />
<br />
<gallery widths=500px heights=340px><br />
File: Intensity vs distance.png | Summed image intensities against sample-magnet distance.<br />
</gallery><br />
<br />
In order to obtain a more quantitative evaluation of whether the Kerr effect is present in our measurements, we plotted the total summed intensity in each captured image and plotted the result with respect to the sample-magnet distance (i.e. external magnetic field strength). For simpler comparison, we also normalized the graphs such that the maximum intensity is 1. Interestingly, we observed a rather consistent intensity across all sample-magnet distances for the focused laser beam, as opposed to the increasing trend for the collimated laser beam case. The sudden jump of intensity from 1 to ~0.7 in the top right quadrant is likely due to an accidental change in our setup apparatus whilst we were moving the sample stage from 250 <math>\mu</math>m to 300 <math>\mu</math>m.<br />
<br />
The non-increasing trend observed in the top two quadrants are consistent with what we expect from theory. For a non-magnetic sample, the MOKE should not be present as silicon on its own is not magnetic, and therefore should not result in a change in light polarization. Based on the magnetic field strength as a function of distance previously measured, we believe that the field strength experienced by the sample even at the furthest distance of 500 <math>\mu</math>m is strong enough to saturate the magnetization of the sample. Therefore, we are unable to observe any changes in the intensity of light for this range of distances. Due to time limitation, we were unable to go back to the lab to collect more data for distances further away where the field strength should be small enough for us to theoretically recover the hysteresis curve we expect from MOKE.<br />
<br />
For the increasing trends seen in the bottom two quadrants, we suspect that these are evidence of systematic errors during our measurement process because they are inconsistent with what we expect to observe from our simple setup. For the non-magnetic material illuminated with collimated light, it should also result in an unchanged intensity regardless of the external magnetic field. On the other hand for the magnetic sample,<br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of ten microns. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
===29 Mar 2022===<br />
<br />
*Alignment attempts and measuring of imaged frame size.<br />
*Attempt to image small features such as those on a $2 bank note.<br />
<br />
===5 Apr 2022===<br />
<br />
*The polarizer sheet has too many scratches and marks to work with, to switch it out to another better quality polarizer.<br />
*To change light source to a fibre coupled laser source instead and check out how it fairs in comparison.<br />
<br />
===8 Apr 2022===<br />
<br />
# Try to misalign beam to prevent interference or whatever is happening at NPBS interface <br />
# Switch to LED light source. Might be challenging to couple beam into fiber, but should get rid of interference fringes<br />
# Illuminate at angle rather than on-axis. Might be hard to aim beam since sample is so close to objective. This might help reduce any interference between incoming and reflected beam<br />
<br />
===13 Apr 2022===<br />
<br />
Final alignment and field dependent imaging session.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=3013Kerr Microscope2022-04-30T00:57:53Z<p>MayInn: /* Theory */</p>
<hr />
<div><blockquote> "I was led some time ago to think it very likely, that if a beam of plane-polarized light were reflected under proper conditions from the surface of intensely magnetized iron, it would have its plane of polarization turned through a sensible angle in the process of reflection." - John Kerr<ref>J.Kerr, Philosophical Magazine 3 (1877) p.312.</ref></blockquote><br />
<br />
Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| 2x Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
<br />
<b>Magnetic domains</b><br />
[[File:Mokemodes_imaging.png|thumb|MOKE domain imaging of (100) silicon-iron sheet at different imaging modes. <ref>Rudolf Schafer. [http://magnetism.eu/esm/2007-cluj/abs/Schaeffer2-abs.pdf| Magneto-optical microscopy and its application.].</ref>]]<br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains. When multiple magnetic domains are formed, magnetostatic energy in the system decreases as the net magnetization of the system is reduced. Common instances where we can find magnetic domains would be in the random arrangement of magnetic domains on refrigerator magnets, and in specific forms within magnetic recording devices such as magnetic tapes and Video Home System (VHS) tapes. Several domain imaging techniques can be used to observe and study these magnetic domains, and the most inexpensive, time saving, and least intrusive method would be through Magneto-optical Kerr Effect imaging technique. <br />
<br />
<b>Magneto-Optical Kerr Effect (MOKE)</b><br />
<br />
Conceived by John Kerr in 1877<ref>P.Weinberger writes about Kerr's famous communications to the Philosophical Magazine - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>, the magneto-optic Kerr effect (MOKE) describes the rotation of light polarization when reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
<b>MOKE microscopy</b><br />
<br />
Interaction between an incident light and the magnetization of a magnetic sample causes a change in polarization state of the incident light. MOKE microscopy makes use of the fact that varying magnetizations corresponding to different magnetic domains on the sample gives rise to different degrees of incident light polarization. By detecting and imaging these reflected or transmitted interacted light, an image of magnetic domains with varying intensities can be observed, thereby allowing for domain imaging to be conducted. Basic components of a MOKE microscope includes, firstly the light source lamp, which is fed through a microscope array with polarizers and analysers onto a sample with an accompanying magnet to change the applied magnetic field, and the resultant light is collected in a camera. Subsequent image processing is required to elucidate weak changes in contrasts. <br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner, in accordance to data acquisition conditions and the sampled materials' suitability.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED connected in conjunction with a current limiting resistor. The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole after Mirror 2.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
* Added micrometer screw translation stage for sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the <i>tube length</i>, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>. A second parameter that must be kept in mind is the <i>working distance</i>, which is the distance that the sample must be placed in front of the objective. For the 10x and 60x objective, these are 1.5mm and 0.15mm respectively<ref>10x Objective - [https://www.edmundoptics.com/p/10x-din-plan-commercial-grade-objective/5386/| Edmund Optics]</ref><ref>60x Objective - [https://www.edmundoptics.com.sg/p/60x-din-achromactic-finite-intl-standard-objective/3137/| Edmund Optics]</ref>. Hence, when using the 60x objective, the sample is practically kissing the objective.<br />
<br />
The use of a micrometer screw translation stage allowed for finer control over the position of our sample to a precision of <math>\pm 10</math> microns (5 micron contribution from both ends of the measurement).<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample whose external field will be controlled by a magnet stack. Its expected overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<b>Series of permanent magnets</b> <br />
[[File:Magnetseperationfield2.png|thumb|Magnetic field variation away from the magnet stack.]]<br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. </b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. After obtaining these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
(@Joel)<br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
Since Kerr imaging is based on the non-linear response of the sample with respect to the incident light intensity, we considered two alternative lighting configurations -- one in which the laser beam was collimated, and another in which the laser beam was focused onto the sample plane to maximize the incident intensity.<br />
<br />
<gallery widths=420px heights=250px><br />
File: Nonmagnetic images.png | Measured images from non-magnetic sample illuminated by a collimated beam.<br />
File: Nonmagnetic focused images.png | Measured images from non-magnetic sample illuminated by a focused beam.<br />
File: Magnetic images.png | Measured images from magnetic sample illuminated by a collimated beam.<br />
File: Magnetic focused images.png | Measured images from magnetic sample illuminated by a focused beam.<br />
</gallery><br />
<br />
The four figures above depict the raw measurements captured by our Kerr microscopy setup for the four different cases corresponding to the type of sample we were imaging as well as the lighting configuration. The labeled distance of <math>x\,\mu </math>m above each individual image denotes the estimated separation distance between the bar magnet and the back of the sample. In all cases, the magnification was kept the same. The vastly different features seen between the 4 cases are likely due to different areas of each sample being imaged upon changing the lighting and sample configurations.<br />
<br />
<gallery widths=500px heights=340px><br />
File: Intensity vs distance.png | Summed image intensities against sample-magnet distance.<br />
</gallery><br />
<br />
In order to obtain a more quantitative evaluation of whether the Kerr effect is present in our measurements, we plotted the total summed intensity in each captured image and plotted the result with respect to the sample-magnet distance (i.e. external magnetic field strength). For simpler comparison, we also normalized the graphs such that the maximum intensity is 1. Interestingly, we observed a rather consistent intensity across all sample-magnet distances for the focused laser beam, as opposed to the increasing trend for the collimated laser beam case. The sudden jump of intensity from 1 to ~0.7 in the top right quadrant is likely due to an accidental change in our setup apparatus whilst we were moving the sample stage from 250 <math>\mu</math>m to 300 <math>\mu</math>m.<br />
<br />
The non-increasing trend observed in the top two quadrants are consistent with what we expect from theory. For a non-magnetic sample, the MOKE should not be present as silicon on its own is not magnetic, and therefore should not result in a change in light polarization. Based on the magnetic field strength as a function of distance previously measured, we believe that the field strength experienced by the sample even at the furthest distance of 500 <math>\mu</math>m is strong enough to saturate the magnetization of the sample. Therefore, we are unable to observe any changes in the intensity of light for this range of distances. Due to time limitation, we were unable to go back to the lab to collect more data for distances further away where the field strength should be small enough for us to theoretically recover the hysteresis curve we expect from MOKE.<br />
<br />
For the increasing trends seen in the bottom two quadrants, we suspect that these are evidence of systematic errors during our measurement process because they are inconsistent with what we expect to observe from our simple setup. For the non-magnetic material illuminated with collimated light, it should also result in an unchanged intensity regardless of the external magnetic field. On the other hand for the magnetic sample,<br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of ten microns. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
===29 Mar 2022===<br />
<br />
Alignment attempts and measuring of imaged frame size.<br />
<br />
===8 Apr 2022===<br />
<br />
# Try to misalign beam to prevent interference or whatever is happening at NPBS interface <br />
# Switch to LED light source. Might be challenging to couple beam into fiber, but should get rid of interference fringes<br />
# Illuminate at angle rather than on-axis. Might be hard to aim beam since sample is so close to objective. This might help reduce any interference between incoming and reflected beam<br />
<br />
===13 Apr 2022===<br />
<br />
Final alignment and field dependent imaging session.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=3012Kerr Microscope2022-04-30T00:52:06Z<p>MayInn: /* Theory */</p>
<hr />
<div><blockquote> "I was led some time ago to think it very likely, that if a beam of plane-polarized light were reflected under proper conditions from the surface of intensely magnetized iron, it would have its plane of polarization turned through a sensible angle in the process of reflection." - John Kerr<ref>J.Kerr, Philosophical Magazine 3 (1877) p.312.</ref></blockquote><br />
<br />
Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| 2x Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
<br />
<b>Magnetic domains</b><br />
[[File:Mokemodes_imaging.png|thumb|MOKE domain imaging of (100) silicon-iron sheet at different imaging modes. <ref>Rudolf Schafer. [http://magnetism.eu/esm/2007-cluj/abs/Schaeffer2-abs.pdf| Magneto-optical microscopy and its application.].</ref>]]<br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains. When multiple magnetic domains are formed, magnetostatic energy in the system decreases as the net magnetization of the system is reduced. Common instances where we can find magnetic domains would be in the random arrangement of magnetic domains on refrigerator magnets, and in specific forms within magnetic recording devices such as magnetic tapes and Video Home System (VHS) tapes. Several domain imaging techniques can be used to observe and study these magnetic domains, and the most inexpensive, time saving, and least intrusive method would be through Magneto-optical Kerr Effect imaging technique. <br />
<br />
<b>Magneto-Optical Kerr Effect (MOKE)</b><br />
<br />
Conceived by John Kerr in 1877<ref>P.Weinberger writes about Kerr's famous communications to the Philosophical Magazine - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>, the magneto-optic Kerr effect (MOKE) describes the rotation of light polarization when reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
<b>MOKE microscopy</b><br />
<br />
Interaction between an incident light and the magnetization of a magnetic sample causes a change in polarization state of the incident light. Varying magnetizations corresponding to different magnetic domains on the sample gives rise to different degrees of incident light polarization. By detecting and imaging these reflected or transmitted interacted light, an image of magnetic domains with varying intensities can be observed, thereby allowing for domain imaging to be conducted. <br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner, in accordance to data acquisition conditions and the sampled materials' suitability.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED connected in conjunction with a current limiting resistor. The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole after Mirror 2.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
* Added micrometer screw translation stage for sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the <i>tube length</i>, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>. A second parameter that must be kept in mind is the <i>working distance</i>, which is the distance that the sample must be placed in front of the objective. For the 10x and 60x objective, these are 1.5mm and 0.15mm respectively<ref>10x Objective - [https://www.edmundoptics.com/p/10x-din-plan-commercial-grade-objective/5386/| Edmund Optics]</ref><ref>60x Objective - [https://www.edmundoptics.com.sg/p/60x-din-achromactic-finite-intl-standard-objective/3137/| Edmund Optics]</ref>. Hence, when using the 60x objective, the sample is practically kissing the objective.<br />
<br />
The use of a micrometer screw translation stage allowed for finer control over the position of our sample to a precision of <math>\pm 10</math> microns (5 micron contribution from both ends of the measurement).<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample whose external field will be controlled by a magnet stack. Its expected overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<b>Series of permanent magnets</b> <br />
[[File:Magnetseperationfield2.png|thumb|Magnetic field variation away from the magnet stack.]]<br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. </b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. After obtaining these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
(@Joel)<br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
Since Kerr imaging is based on the non-linear response of the sample with respect to the incident light intensity, we considered two alternative lighting configurations -- one in which the laser beam was collimated, and another in which the laser beam was focused onto the sample plane to maximize the incident intensity.<br />
<br />
<gallery widths=420px heights=250px><br />
File: Nonmagnetic images.png | Measured images from non-magnetic sample illuminated by a collimated beam.<br />
File: Nonmagnetic focused images.png | Measured images from non-magnetic sample illuminated by a focused beam.<br />
File: Magnetic images.png | Measured images from magnetic sample illuminated by a collimated beam.<br />
File: Magnetic focused images.png | Measured images from magnetic sample illuminated by a focused beam.<br />
</gallery><br />
<br />
The four figures above depict the raw measurements captured by our Kerr microscopy setup for the four different cases corresponding to the type of sample we were imaging as well as the lighting configuration. The labeled distance of <math>x\,\mu </math>m above each individual image denotes the estimated separation distance between the bar magnet and the back of the sample. In all cases, the magnification was kept the same. The vastly different features seen between the 4 cases are likely due to different areas of each sample being imaged upon changing the lighting and sample configurations.<br />
<br />
<gallery widths=500px heights=340px><br />
File: Intensity vs distance.png | Summed image intensities against sample-magnet distance.<br />
</gallery><br />
<br />
In order to obtain a more quantitative evaluation of whether the Kerr effect is present in our measurements, we plotted the total summed intensity in each captured image and plotted the result with respect to the sample-magnet distance (i.e. external magnetic field strength). For simpler comparison, we also normalized the graphs such that the maximum intensity is 1. Interestingly, we observed a rather consistent intensity across all sample-magnet distances for the focused laser beam, as opposed to the increasing trend for the collimated laser beam case. The sudden jump of intensity from 1 to ~0.7 in the top right quadrant is likely due to an accidental change in our setup apparatus whilst we were moving the sample stage from 250 <math>\mu</math>m to 300 <math>\mu</math>m.<br />
<br />
The non-increasing trend observed in the top two quadrants are consistent with what we expect from theory. For a non-magnetic sample, the MOKE should not be present as silicon on its own is not magnetic, and therefore should not result in a change in light polarization. Based on the magnetic field strength as a function of distance previously measured, we believe that the field strength experienced by the sample even at the furthest distance of 500 <math>\mu</math>m is strong enough to saturate the magnetization of the sample. Therefore, we are unable to observe any changes in the intensity of light for this range of distances. Due to time limitation, we were unable to go back to the lab to collect more data for distances further away where the field strength should be small enough for us to theoretically recover the hysteresis curve we expect from MOKE.<br />
<br />
For the increasing trends seen in the bottom two quadrants, we suspect that these are evidence of systematic errors during our measurement process because they are inconsistent with what we expect to observe from our simple setup. For the non-magnetic material illuminated with collimated light, it should also result in an unchanged intensity regardless of the external magnetic field. On the other hand for the magnetic sample,<br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of ten microns. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
===29 Mar 2022===<br />
<br />
Alignment attempts and measuring of imaged frame size.<br />
<br />
===8 Apr 2022===<br />
<br />
# Try to misalign beam to prevent interference or whatever is happening at NPBS interface <br />
# Switch to LED light source. Might be challenging to couple beam into fiber, but should get rid of interference fringes<br />
# Illuminate at angle rather than on-axis. Might be hard to aim beam since sample is so close to objective. This might help reduce any interference between incoming and reflected beam<br />
<br />
===13 Apr 2022===<br />
<br />
Final alignment and field dependent imaging session.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=3011Kerr Microscope2022-04-30T00:51:37Z<p>MayInn: /* Theory */</p>
<hr />
<div><blockquote> "I was led some time ago to think it very likely, that if a beam of plane-polarized light were reflected under proper conditions from the surface of intensely magnetized iron, it would have its plane of polarization turned through a sensible angle in the process of reflection." - John Kerr<ref>J.Kerr, Philosophical Magazine 3 (1877) p.312.</ref></blockquote><br />
<br />
Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| 2x Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
<br />
<b>Magnetic domains</b><br />
[[File:Mokemodes_imaging.png|thumb|MOKE domain imaging of (100) silicon-iron sheet at different imaging modes. <ref>Rudolf Schafer. - [http://magnetism.eu/esm/2007-cluj/abs/Schaeffer2-abs.pdf| Magneto-optical microscopy and its application.].</ref>]]<br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains. When multiple magnetic domains are formed, magnetostatic energy in the system decreases as the net magnetization of the system is reduced. Common instances where we can find magnetic domains would be in the random arrangement of magnetic domains on refrigerator magnets, and in specific forms within magnetic recording devices such as magnetic tapes and Video Home System (VHS) tapes. Several domain imaging techniques can be used to observe and study these magnetic domains, and the most inexpensive, time saving, and least intrusive method would be through Magneto-optical Kerr Effect imaging technique. <br />
<br />
<b>Magneto-Optical Kerr Effect (MOKE)</b><br />
<br />
Conceived by John Kerr in 1877<ref>P.Weinberger writes about Kerr's famous communications to the Philosophical Magazine - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>, the magneto-optic Kerr effect (MOKE) describes the rotation of light polarization when reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
<b>MOKE microscopy</b><br />
<br />
Interaction between an incident light and the magnetization of a magnetic sample causes a change in polarization state of the incident light. Varying magnetizations corresponding to different magnetic domains on the sample gives rise to different degrees of incident light polarization. By detecting and imaging these reflected or transmitted interacted light, an image of magnetic domains with varying intensities can be observed, thereby allowing for domain imaging to be conducted. <br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner, in accordance to data acquisition conditions and the sampled materials' suitability.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED connected in conjunction with a current limiting resistor. The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole after Mirror 2.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
* Added micrometer screw translation stage for sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the <i>tube length</i>, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>. A second parameter that must be kept in mind is the <i>working distance</i>, which is the distance that the sample must be placed in front of the objective. For the 10x and 60x objective, these are 1.5mm and 0.15mm respectively<ref>10x Objective - [https://www.edmundoptics.com/p/10x-din-plan-commercial-grade-objective/5386/| Edmund Optics]</ref><ref>60x Objective - [https://www.edmundoptics.com.sg/p/60x-din-achromactic-finite-intl-standard-objective/3137/| Edmund Optics]</ref>. Hence, when using the 60x objective, the sample is practically kissing the objective.<br />
<br />
The use of a micrometer screw translation stage allowed for finer control over the position of our sample to a precision of <math>\pm 10</math> microns (5 micron contribution from both ends of the measurement).<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample whose external field will be controlled by a magnet stack. Its expected overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<b>Series of permanent magnets</b> <br />
[[File:Magnetseperationfield2.png|thumb|Magnetic field variation away from the magnet stack.]]<br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. </b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. After obtaining these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
(@Joel)<br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
Since Kerr imaging is based on the non-linear response of the sample with respect to the incident light intensity, we considered two alternative lighting configurations -- one in which the laser beam was collimated, and another in which the laser beam was focused onto the sample plane to maximize the incident intensity.<br />
<br />
<gallery widths=420px heights=250px><br />
File: Nonmagnetic images.png | Measured images from non-magnetic sample illuminated by a collimated beam.<br />
File: Nonmagnetic focused images.png | Measured images from non-magnetic sample illuminated by a focused beam.<br />
File: Magnetic images.png | Measured images from magnetic sample illuminated by a collimated beam.<br />
File: Magnetic focused images.png | Measured images from magnetic sample illuminated by a focused beam.<br />
</gallery><br />
<br />
The four figures above depict the raw measurements captured by our Kerr microscopy setup for the four different cases corresponding to the type of sample we were imaging as well as the lighting configuration. The labeled distance of <math>x\,\mu </math>m above each individual image denotes the estimated separation distance between the bar magnet and the back of the sample. In all cases, the magnification was kept the same. The vastly different features seen between the 4 cases are likely due to different areas of each sample being imaged upon changing the lighting and sample configurations.<br />
<br />
<gallery widths=500px heights=340px><br />
File: Intensity vs distance.png | Summed image intensities against sample-magnet distance.<br />
</gallery><br />
<br />
In order to obtain a more quantitative evaluation of whether the Kerr effect is present in our measurements, we plotted the total summed intensity in each captured image and plotted the result with respect to the sample-magnet distance (i.e. external magnetic field strength). For simpler comparison, we also normalized the graphs such that the maximum intensity is 1. Interestingly, we observed a rather consistent intensity across all sample-magnet distances for the focused laser beam, as opposed to the increasing trend for the collimated laser beam case. The sudden jump of intensity from 1 to ~0.7 in the top right quadrant is likely due to an accidental change in our setup apparatus whilst we were moving the sample stage from 250 <math>\mu</math>m to 300 <math>\mu</math>m.<br />
<br />
The non-increasing trend observed in the top two quadrants are consistent with what we expect from theory. For a non-magnetic sample, the MOKE should not be present as silicon on its own is not magnetic, and therefore should not result in a change in light polarization. Based on the magnetic field strength as a function of distance previously measured, we believe that the field strength experienced by the sample even at the furthest distance of 500 <math>\mu</math>m is strong enough to saturate the magnetization of the sample. Therefore, we are unable to observe any changes in the intensity of light for this range of distances. Due to time limitation, we were unable to go back to the lab to collect more data for distances further away where the field strength should be small enough for us to theoretically recover the hysteresis curve we expect from MOKE.<br />
<br />
For the increasing trends seen in the bottom two quadrants, we suspect that these are evidence of systematic errors during our measurement process because they are inconsistent with what we expect to observe from our simple setup. For the non-magnetic material illuminated with collimated light, it should also result in an unchanged intensity regardless of the external magnetic field. On the other hand for the magnetic sample,<br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of ten microns. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
===29 Mar 2022===<br />
<br />
Alignment attempts and measuring of imaged frame size.<br />
<br />
===8 Apr 2022===<br />
<br />
# Try to misalign beam to prevent interference or whatever is happening at NPBS interface <br />
# Switch to LED light source. Might be challenging to couple beam into fiber, but should get rid of interference fringes<br />
# Illuminate at angle rather than on-axis. Might be hard to aim beam since sample is so close to objective. This might help reduce any interference between incoming and reflected beam<br />
<br />
===13 Apr 2022===<br />
<br />
Final alignment and field dependent imaging session.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=3010Kerr Microscope2022-04-30T00:50:20Z<p>MayInn: /* Theory */</p>
<hr />
<div><blockquote> "I was led some time ago to think it very likely, that if a beam of plane-polarized light were reflected under proper conditions from the surface of intensely magnetized iron, it would have its plane of polarization turned through a sensible angle in the process of reflection." - John Kerr<ref>J.Kerr, Philosophical Magazine 3 (1877) p.312.</ref></blockquote><br />
<br />
Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| 2x Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
<br />
<b>Magnetic domains</b><br />
[[File:Mokemodes_imaging.png|thumb|MOKE domain imaging of (100) silicon-iron sheet at different imaging modes. <ref>Rudolf Schafer. Magneto-optical microscopy and its application. - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>]]<br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains. When multiple magnetic domains are formed, magnetostatic energy in the system decreases as the net magnetization of the system is reduced. Common instances where we can find magnetic domains would be in the random arrangement of magnetic domains on refrigerator magnets, and in specific forms within magnetic recording devices such as magnetic tapes and Video Home System (VHS) tapes. Several domain imaging techniques can be used to observe and study these magnetic domains, and the most inexpensive, time saving, and least intrusive method would be through Magneto-optical Kerr Effect imaging technique. <br />
<br />
<b>Magneto-Optical Kerr Effect (MOKE)</b><br />
<br />
Conceived by John Kerr in 1877<ref>P.Weinberger writes about Kerr's famous communications to the Philosophical Magazine - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>, the magneto-optic Kerr effect (MOKE) describes the rotation of light polarization when reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
<b>MOKE microscopy</b><br />
<br />
Interaction between an incident light and the magnetization of a magnetic sample causes a change in polarization state of the incident light. Varying magnetizations corresponding to different magnetic domains on the sample gives rise to different degrees of incident light polarization. By detecting and imaging these reflected or transmitted interacted light, an image of magnetic domains with varying intensities can be observed, thereby allowing for domain imaging to be conducted. <br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner, in accordance to data acquisition conditions and the sampled materials' suitability.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED connected in conjunction with a current limiting resistor. The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole after Mirror 2.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
* Added micrometer screw translation stage for sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the <i>tube length</i>, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>. A second parameter that must be kept in mind is the <i>working distance</i>, which is the distance that the sample must be placed in front of the objective. For the 10x and 60x objective, these are 1.5mm and 0.15mm respectively<ref>10x Objective - [https://www.edmundoptics.com/p/10x-din-plan-commercial-grade-objective/5386/| Edmund Optics]</ref><ref>60x Objective - [https://www.edmundoptics.com.sg/p/60x-din-achromactic-finite-intl-standard-objective/3137/| Edmund Optics]</ref>. Hence, when using the 60x objective, the sample is practically kissing the objective.<br />
<br />
The use of a micrometer screw translation stage allowed for finer control over the position of our sample to a precision of <math>\pm 10</math> microns (5 micron contribution from both ends of the measurement).<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample whose external field will be controlled by a magnet stack. Its expected overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<b>Series of permanent magnets</b> <br />
[[File:Magnetseperationfield2.png|thumb|Magnetic field variation away from the magnet stack.]]<br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. </b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. After obtaining these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
(@Joel)<br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
Since Kerr imaging is based on the non-linear response of the sample with respect to the incident light intensity, we considered two alternative lighting configurations -- one in which the laser beam was collimated, and another in which the laser beam was focused onto the sample plane to maximize the incident intensity.<br />
<br />
<gallery widths=420px heights=250px><br />
File: Nonmagnetic images.png | Measured images from non-magnetic sample illuminated by a collimated beam.<br />
File: Nonmagnetic focused images.png | Measured images from non-magnetic sample illuminated by a focused beam.<br />
File: Magnetic images.png | Measured images from magnetic sample illuminated by a collimated beam.<br />
File: Magnetic focused images.png | Measured images from magnetic sample illuminated by a focused beam.<br />
</gallery><br />
<br />
The four figures above depict the raw measurements captured by our Kerr microscopy setup for the four different cases corresponding to the type of sample we were imaging as well as the lighting configuration. The labeled distance of <math>x\,\mu </math>m above each individual image denotes the estimated separation distance between the bar magnet and the back of the sample. In all cases, the magnification was kept the same. The vastly different features seen between the 4 cases are likely due to different areas of each sample being imaged upon changing the lighting and sample configurations.<br />
<br />
<gallery widths=500px heights=340px><br />
File: Intensity vs distance.png | Summed image intensities against sample-magnet distance.<br />
</gallery><br />
<br />
In order to obtain a more quantitative evaluation of whether the Kerr effect is present in our measurements, we plotted the total summed intensity in each captured image and plotted the result with respect to the sample-magnet distance (i.e. external magnetic field strength). For simpler comparison, we also normalized the graphs such that the maximum intensity is 1. Interestingly, we observed a rather consistent intensity across all sample-magnet distances for the focused laser beam, as opposed to the increasing trend for the collimated laser beam case. The sudden jump of intensity from 1 to ~0.7 in the top right quadrant is likely due to an accidental change in our setup apparatus whilst we were moving the sample stage from 250 <math>\mu</math>m to 300 <math>\mu</math>m.<br />
<br />
The non-increasing trend observed in the top two quadrants are consistent with what we expect from theory. For a non-magnetic sample, the MOKE should not be present as silicon on its own is not magnetic, and therefore should not result in a change in light polarization. Based on the magnetic field strength as a function of distance previously measured, we believe that the field strength experienced by the sample even at the furthest distance of 500 <math>\mu</math>m is strong enough to saturate the magnetization of the sample. Therefore, we are unable to observe any changes in the intensity of light for this range of distances. Due to time limitation, we were unable to go back to the lab to collect more data for distances further away where the field strength should be small enough for us to theoretically recover the hysteresis curve we expect from MOKE.<br />
<br />
For the increasing trends seen in the bottom two quadrants, we suspect that these are evidence of systematic errors during our measurement process because they are inconsistent with what we expect to observe from our simple setup. For the non-magnetic material illuminated with collimated light, it should also result in an unchanged intensity regardless of the external magnetic field. On the other hand for the magnetic sample,<br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of ten microns. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
===29 Mar 2022===<br />
<br />
Alignment attempts and measuring of imaged frame size.<br />
<br />
===8 Apr 2022===<br />
<br />
# Try to misalign beam to prevent interference or whatever is happening at NPBS interface <br />
# Switch to LED light source. Might be challenging to couple beam into fiber, but should get rid of interference fringes<br />
# Illuminate at angle rather than on-axis. Might be hard to aim beam since sample is so close to objective. This might help reduce any interference between incoming and reflected beam<br />
<br />
===13 Apr 2022===<br />
<br />
Final alignment and field dependent imaging session.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=3009Kerr Microscope2022-04-30T00:48:47Z<p>MayInn: /* Theory */</p>
<hr />
<div><blockquote> "I was led some time ago to think it very likely, that if a beam of plane-polarized light were reflected under proper conditions from the surface of intensely magnetized iron, it would have its plane of polarization turned through a sensible angle in the process of reflection." - John Kerr<ref>J.Kerr, Philosophical Magazine 3 (1877) p.312.</ref></blockquote><br />
<br />
Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| 2x Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
<br />
<b>Magnetic domains</b><br />
[[File:Mokemodes_imaging.png|thumb|MOKE domain imaging of (100) silicon-iron sheet at different imaging modes.]]<br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains. When multiple magnetic domains are formed, magnetostatic energy in the system decreases as the net magnetization of the system is reduced. Common instances where we can find magnetic domains would be in the random arrangement of magnetic domains on refrigerator magnets, and in specific forms within magnetic recording devices such as magnetic tapes and Video Home System (VHS) tapes. Several domain imaging techniques can be used to observe and study these magnetic domains, and the most inexpensive, time saving, and least intrusive method would be through Magneto-optical Kerr Effect imaging technique. <br />
<br />
<b>Magneto-Optical Kerr Effect (MOKE)</b><br />
<br />
Conceived by John Kerr in 1877<ref>P.Weinberger writes about Kerr's famous communications to the Philosophical Magazine - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>, the magneto-optic Kerr effect (MOKE) describes the rotation of light polarization when reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
<b>MOKE microscopy</b><br />
<br />
Interaction between an incident light and the magnetization of a magnetic sample causes a change in polarization state of the incident light. Varying magnetizations corresponding to different magnetic domains on the sample gives rise to different degrees of incident light polarization. By detecting and imaging these reflected or transmitted interacted light, an image of magnetic domains with varying intensities can be observed, thereby allowing for domain imaging to be conducted. <br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner, in accordance to data acquisition conditions and the sampled materials' suitability.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED connected in conjunction with a current limiting resistor. The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole after Mirror 2.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
* Added micrometer screw translation stage for sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the <i>tube length</i>, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>. A second parameter that must be kept in mind is the <i>working distance</i>, which is the distance that the sample must be placed in front of the objective. For the 10x and 60x objective, these are 1.5mm and 0.15mm respectively<ref>10x Objective - [https://www.edmundoptics.com/p/10x-din-plan-commercial-grade-objective/5386/| Edmund Optics]</ref><ref>60x Objective - [https://www.edmundoptics.com.sg/p/60x-din-achromactic-finite-intl-standard-objective/3137/| Edmund Optics]</ref>. Hence, when using the 60x objective, the sample is practically kissing the objective.<br />
<br />
The use of a micrometer screw translation stage allowed for finer control over the position of our sample to a precision of <math>\pm 10</math> microns (5 micron contribution from both ends of the measurement).<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample whose external field will be controlled by a magnet stack. Its expected overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<b>Series of permanent magnets</b> <br />
[[File:Magnetseperationfield2.png|thumb|Magnetic field variation away from the magnet stack.]]<br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. </b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. After obtaining these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
(@Joel)<br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
Since Kerr imaging is based on the non-linear response of the sample with respect to the incident light intensity, we considered two alternative lighting configurations -- one in which the laser beam was collimated, and another in which the laser beam was focused onto the sample plane to maximize the incident intensity.<br />
<br />
<gallery widths=420px heights=250px><br />
File: Nonmagnetic images.png | Measured images from non-magnetic sample illuminated by a collimated beam.<br />
File: Nonmagnetic focused images.png | Measured images from non-magnetic sample illuminated by a focused beam.<br />
File: Magnetic images.png | Measured images from magnetic sample illuminated by a collimated beam.<br />
File: Magnetic focused images.png | Measured images from magnetic sample illuminated by a focused beam.<br />
</gallery><br />
<br />
The four figures above depict the raw measurements captured by our Kerr microscopy setup for the four different cases corresponding to the type of sample we were imaging as well as the lighting configuration. The labeled distance of <math>x\,\mu </math>m above each individual image denotes the estimated separation distance between the bar magnet and the back of the sample. In all cases, the magnification was kept the same. The vastly different features seen between the 4 cases are likely due to different areas of each sample being imaged upon changing the lighting and sample configurations.<br />
<br />
<gallery widths=500px heights=340px><br />
File: Intensity vs distance.png | Summed image intensities against sample-magnet distance.<br />
</gallery><br />
<br />
In order to obtain a more quantitative evaluation of whether the Kerr effect is present in our measurements, we plotted the total summed intensity in each captured image and plotted the result with respect to the sample-magnet distance (i.e. external magnetic field strength). For simpler comparison, we also normalized the graphs such that the maximum intensity is 1. Interestingly, we observed a rather consistent intensity across all sample-magnet distances for the focused laser beam, as opposed to the increasing trend for the collimated laser beam case. The sudden jump of intensity from 1 to ~0.7 in the top right quadrant is likely due to an accidental change in our setup apparatus whilst we were moving the sample stage from 250 <math>\mu</math>m to 300 <math>\mu</math>m.<br />
<br />
The non-increasing trend observed in the top two quadrants are consistent with what we expect from theory. For a non-magnetic sample, the MOKE should not be present as silicon on its own is not magnetic, and therefore should not result in a change in light polarization. Based on the magnetic field strength as a function of distance previously measured, we believe that the field strength experienced by the sample even at the furthest distance of 500 <math>\mu</math>m is strong enough to saturate the magnetization of the sample. Therefore, we are unable to observe any changes in the intensity of light for this range of distances. Due to time limitation, we were unable to go back to the lab to collect more data for distances further away where the field strength should be small enough for us to theoretically recover the hysteresis curve we expect from MOKE.<br />
<br />
For the increasing trends seen in the bottom two quadrants, we suspect that these are evidence of systematic errors during our measurement process because they are inconsistent with what we expect to observe from our simple setup. For the non-magnetic material illuminated with collimated light, it should also result in an unchanged intensity regardless of the external magnetic field. On the other hand for the magnetic sample,<br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of ten microns. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
===29 Mar 2022===<br />
<br />
Alignment attempts and measuring of imaged frame size.<br />
<br />
===8 Apr 2022===<br />
<br />
# Try to misalign beam to prevent interference or whatever is happening at NPBS interface <br />
# Switch to LED light source. Might be challenging to couple beam into fiber, but should get rid of interference fringes<br />
# Illuminate at angle rather than on-axis. Might be hard to aim beam since sample is so close to objective. This might help reduce any interference between incoming and reflected beam<br />
<br />
===13 Apr 2022===<br />
<br />
Final alignment and field dependent imaging session.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=3008Kerr Microscope2022-04-30T00:48:20Z<p>MayInn: /* Theory */</p>
<hr />
<div><blockquote> "I was led some time ago to think it very likely, that if a beam of plane-polarized light were reflected under proper conditions from the surface of intensely magnetized iron, it would have its plane of polarization turned through a sensible angle in the process of reflection." - John Kerr<ref>J.Kerr, Philosophical Magazine 3 (1877) p.312.</ref></blockquote><br />
<br />
Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| 2x Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
<br />
<b>Magnetic domains</b><br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains. When multiple magnetic domains are formed, magnetostatic energy in the system decreases as the net magnetization of the system is reduced. Common instances where we can find magnetic domains would be in the random arrangement of magnetic domains on refrigerator magnets, and in specific forms within magnetic recording devices such as magnetic tapes and Video Home System (VHS) tapes. Several domain imaging techniques can be used to observe and study these magnetic domains, and the most inexpensive, time saving, and least intrusive method would be through Magneto-optical Kerr Effect imaging technique. <br />
<br />
<b>Magneto-Optical Kerr Effect (MOKE)</b><br />
<br />
Conceived by John Kerr in 1877<ref>P.Weinberger writes about Kerr's famous communications to the Philosophical Magazine - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>, the magneto-optic Kerr effect (MOKE) describes the rotation of light polarization when reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
<b>MOKE microscopy</b><br />
<br />
[[File:Mokemodes_imaging.png|thumb|MOKE domain imaging of (100) silicon-iron sheet at different imaging modes.]]<br />
<br />
Interaction between an incident light and the magnetization of a magnetic sample causes a change in polarization state of the incident light. Varying magnetizations corresponding to different magnetic domains on the sample gives rise to different degrees of incident light polarization. By detecting and imaging these reflected or transmitted interacted light, an image of magnetic domains with varying intensities can be observed, thereby allowing for domain imaging to be conducted. <br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner, in accordance to data acquisition conditions and the sampled materials' suitability.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED connected in conjunction with a current limiting resistor. The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole after Mirror 2.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
* Added micrometer screw translation stage for sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the <i>tube length</i>, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>. A second parameter that must be kept in mind is the <i>working distance</i>, which is the distance that the sample must be placed in front of the objective. For the 10x and 60x objective, these are 1.5mm and 0.15mm respectively<ref>10x Objective - [https://www.edmundoptics.com/p/10x-din-plan-commercial-grade-objective/5386/| Edmund Optics]</ref><ref>60x Objective - [https://www.edmundoptics.com.sg/p/60x-din-achromactic-finite-intl-standard-objective/3137/| Edmund Optics]</ref>. Hence, when using the 60x objective, the sample is practically kissing the objective.<br />
<br />
The use of a micrometer screw translation stage allowed for finer control over the position of our sample to a precision of <math>\pm 10</math> microns (5 micron contribution from both ends of the measurement).<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample whose external field will be controlled by a magnet stack. Its expected overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<b>Series of permanent magnets</b> <br />
[[File:Magnetseperationfield2.png|thumb|Magnetic field variation away from the magnet stack.]]<br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. </b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. After obtaining these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
(@Joel)<br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
Since Kerr imaging is based on the non-linear response of the sample with respect to the incident light intensity, we considered two alternative lighting configurations -- one in which the laser beam was collimated, and another in which the laser beam was focused onto the sample plane to maximize the incident intensity.<br />
<br />
<gallery widths=420px heights=250px><br />
File: Nonmagnetic images.png | Measured images from non-magnetic sample illuminated by a collimated beam.<br />
File: Nonmagnetic focused images.png | Measured images from non-magnetic sample illuminated by a focused beam.<br />
File: Magnetic images.png | Measured images from magnetic sample illuminated by a collimated beam.<br />
File: Magnetic focused images.png | Measured images from magnetic sample illuminated by a focused beam.<br />
</gallery><br />
<br />
The four figures above depict the raw measurements captured by our Kerr microscopy setup for the four different cases corresponding to the type of sample we were imaging as well as the lighting configuration. The labeled distance of <math>x\,\mu </math>m above each individual image denotes the estimated separation distance between the bar magnet and the back of the sample. In all cases, the magnification was kept the same. The vastly different features seen between the 4 cases are likely due to different areas of each sample being imaged upon changing the lighting and sample configurations.<br />
<br />
<gallery widths=500px heights=340px><br />
File: Intensity vs distance.png | Summed image intensities against sample-magnet distance.<br />
</gallery><br />
<br />
In order to obtain a more quantitative evaluation of whether the Kerr effect is present in our measurements, we plotted the total summed intensity in each captured image and plotted the result with respect to the sample-magnet distance (i.e. external magnetic field strength). For simpler comparison, we also normalized the graphs such that the maximum intensity is 1. Interestingly, we observed a rather consistent intensity across all sample-magnet distances for the focused laser beam, as opposed to the increasing trend for the collimated laser beam case. The sudden jump of intensity from 1 to ~0.7 in the top right quadrant is likely due to an accidental change in our setup apparatus whilst we were moving the sample stage from 250 <math>\mu</math>m to 300 <math>\mu</math>m.<br />
<br />
The non-increasing trend observed in the top two quadrants are consistent with what we expect from theory. For a non-magnetic sample, the MOKE should not be present as silicon on its own is not magnetic, and therefore should not result in a change in light polarization. Based on the magnetic field strength as a function of distance previously measured, we believe that the field strength experienced by the sample even at the furthest distance of 500 <math>\mu</math>m is strong enough to saturate the magnetization of the sample. Therefore, we are unable to observe any changes in the intensity of light for this range of distances. Due to time limitation, we were unable to go back to the lab to collect more data for distances further away where the field strength should be small enough for us to theoretically recover the hysteresis curve we expect from MOKE.<br />
<br />
For the increasing trends seen in the bottom two quadrants, we suspect that these are evidence of systematic errors during our measurement process because they are inconsistent with what we expect to observe from our simple setup. For the non-magnetic material illuminated with collimated light, it should also result in an unchanged intensity regardless of the external magnetic field. On the other hand for the magnetic sample,<br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of ten microns. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
===29 Mar 2022===<br />
<br />
Alignment attempts and measuring of imaged frame size.<br />
<br />
===8 Apr 2022===<br />
<br />
# Try to misalign beam to prevent interference or whatever is happening at NPBS interface <br />
# Switch to LED light source. Might be challenging to couple beam into fiber, but should get rid of interference fringes<br />
# Illuminate at angle rather than on-axis. Might be hard to aim beam since sample is so close to objective. This might help reduce any interference between incoming and reflected beam<br />
<br />
===13 Apr 2022===<br />
<br />
Final alignment and field dependent imaging session.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=File:Mokemodes_imaging.png&diff=3007File:Mokemodes imaging.png2022-04-30T00:46:52Z<p>MayInn: Obtained from http://magnetism.eu/esm/2007-cluj/abs/Schaeffer2-abs.pdf</p>
<hr />
<div>== Summary ==<br />
Obtained from http://magnetism.eu/esm/2007-cluj/abs/Schaeffer2-abs.pdf</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=3006Kerr Microscope2022-04-30T00:40:58Z<p>MayInn: /* Theory */</p>
<hr />
<div><blockquote> "I was led some time ago to think it very likely, that if a beam of plane-polarized light were reflected under proper conditions from the surface of intensely magnetized iron, it would have its plane of polarization turned through a sensible angle in the process of reflection." - John Kerr<ref>J.Kerr, Philosophical Magazine 3 (1877) p.312.</ref></blockquote><br />
<br />
Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| 2x Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
<br />
<b>Magnetic domains</b><br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains. When multiple magnetic domains are formed, magnetostatic energy in the system decreases as the net magnetization of the system is reduced. Common instances where we can find magnetic domains would be in the random arrangement of magnetic domains on refrigerator magnets, and in specific forms within magnetic recording devices such as magnetic tapes and Video Home System (VHS) tapes. Several domain imaging techniques can be used to observe and study these magnetic domains, and the most inexpensive, time saving, and least intrusive method would be through Magneto-optical Kerr Effect imaging technique. <br />
<br />
<b>Magneto-Optical Kerr Effect (MOKE)</b><br />
<br />
Conceived by John Kerr in 1877<ref>P.Weinberger writes about Kerr's famous communications to the Philosophical Magazine - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>, the magneto-optic Kerr effect (MOKE) describes the rotation of light polarization when reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
<b>MOKE microscopy</b><br />
<br />
Interaction between an incident light and the magnetization of a magnetic sample causes a change in polarization state of the incident light. Varying magnetizations corresponding to different magnetic domains on the sample gives rise to different degrees of incident light polarization. By detecting and imaging these reflected or transmitted interacted light, an image of magnetic domains with varying intensities can be observed, thereby allowing for domain imaging to be conducted. <br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner, in accordance to data acquisition conditions and the sampled materials' suitability.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED connected in conjunction with a current limiting resistor. The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole after Mirror 2.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
* Added micrometer screw translation stage for sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the <i>tube length</i>, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>. A second parameter that must be kept in mind is the <i>working distance</i>, which is the distance that the sample must be placed in front of the objective. For the 10x and 60x objective, these are 1.5mm and 0.15mm respectively<ref>10x Objective - [https://www.edmundoptics.com/p/10x-din-plan-commercial-grade-objective/5386/| Edmund Optics]</ref><ref>60x Objective - [https://www.edmundoptics.com.sg/p/60x-din-achromactic-finite-intl-standard-objective/3137/| Edmund Optics]</ref>. Hence, when using the 60x objective, the sample is practically kissing the objective.<br />
<br />
The use of a micrometer screw translation stage allowed for finer control over the position of our sample to a precision of <math>\pm 10</math> microns (5 micron contribution from both ends of the measurement).<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample whose external field will be controlled by a magnet stack. Its expected overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<b>Series of permanent magnets</b> <br />
[[File:Magnetseperationfield2.png|thumb|Magnetic field variation away from the magnet stack.]]<br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. </b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. After obtaining these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
(@Joel)<br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
Since Kerr imaging is based on the non-linear response of the sample with respect to the incident light intensity, we considered two alternative lighting configurations -- one in which the laser beam was collimated, and another in which the laser beam was focused onto the sample plane to maximize the incident intensity.<br />
<br />
<gallery widths=420px heights=250px><br />
File: Nonmagnetic images.png | Measured images from non-magnetic sample illuminated by a collimated beam.<br />
File: Nonmagnetic focused images.png | Measured images from non-magnetic sample illuminated by a focused beam.<br />
File: Magnetic images.png | Measured images from magnetic sample illuminated by a collimated beam.<br />
File: Magnetic focused images.png | Measured images from magnetic sample illuminated by a focused beam.<br />
</gallery><br />
<br />
The four figures above depict the raw measurements captured by our Kerr microscopy setup for the four different cases corresponding to the type of sample we were imaging as well as the lighting configuration. The labeled distance of <math>x\,\mu </math>m above each individual image denotes the estimated separation distance between the bar magnet and the back of the sample. In all cases, the magnification was kept the same. The vastly different features seen between the 4 cases are likely due to different areas of each sample being imaged upon changing the lighting and sample configurations.<br />
<br />
<gallery widths=500px heights=340px><br />
File: Intensity vs distance.png | Summed image intensities against sample-magnet distance.<br />
</gallery><br />
<br />
In order to obtain a more quantitative evaluation of whether the Kerr effect is present in our measurements, we plotted the total summed intensity in each captured image and plotted the result with respect to the sample-magnet distance (i.e. external magnetic field strength). For simpler comparison, we also normalized the graphs such that the maximum intensity is 1. Interestingly, we observed a rather consistent intensity across all sample-magnet distances for the focused laser beam, as opposed to the increasing trend for the collimated laser beam case. The sudden jump of intensity from 1 to ~0.7 in the top right quadrant is likely due to an accidental change in our setup apparatus whilst we were moving the sample stage from 250 <math>\mu</math>m to 300 <math>\mu</math>m.<br />
<br />
The non-increasing trend observed in the top two quadrants are consistent with what we expect from theory. For a non-magnetic sample, the MOKE should not be present as silicon on its own is not magnetic, and therefore should not result in a change in light polarization. Based on the magnetic field strength as a function of distance previously measured, we believe that the field strength experienced by the sample even at the furthest distance of 500 <math>\mu</math>m is strong enough to saturate the magnetization of the sample. Therefore, we are unable to observe any changes in the intensity of light for this range of distances. Due to time limitation, we were unable to go back to the lab to collect more data for distances further away where the field strength should be small enough for us to theoretically recover the hysteresis curve we expect from MOKE.<br />
<br />
For the increasing trends seen in the bottom two quadrants, we suspect that these are evidence of systematic errors during our measurement process because they are inconsistent with what we expect to observe from our simple setup. For the non-magnetic material illuminated with collimated light, it should also result in an unchanged intensity regardless of the external magnetic field. On the other hand for the magnetic sample,<br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of ten microns. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
===29 Mar 2022===<br />
<br />
Alignment attempts and measuring of imaged frame size.<br />
<br />
===8 Apr 2022===<br />
<br />
# Try to misalign beam to prevent interference or whatever is happening at NPBS interface <br />
# Switch to LED light source. Might be challenging to couple beam into fiber, but should get rid of interference fringes<br />
# Illuminate at angle rather than on-axis. Might be hard to aim beam since sample is so close to objective. This might help reduce any interference between incoming and reflected beam<br />
<br />
===13 Apr 2022===<br />
<br />
Final alignment and field dependent imaging session.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=3005Kerr Microscope2022-04-30T00:40:18Z<p>MayInn: /* Theory */</p>
<hr />
<div><blockquote> "I was led some time ago to think it very likely, that if a beam of plane-polarized light were reflected under proper conditions from the surface of intensely magnetized iron, it would have its plane of polarization turned through a sensible angle in the process of reflection." - John Kerr<ref>J.Kerr, Philosophical Magazine 3 (1877) p.312.</ref></blockquote><br />
<br />
Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| 2x Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
<br />
<b>Magnetic domains</b><br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains. When multiple magnetic domains are formed, magnetostatic energy in the system decreases as the net magnetization of the system is reduced. Common instances where we can find magnetic domains would be in the random arrangement of magnetic domains on [[refrigerator magnet]]s, and in specific forms within magnetic recording devices such as [[magnetic tape]]s and Video Home System ([[VHS]]) tapes. Several domain imaging techniques can be used to observe and study these magnetic domains, and the most inexpensive, time saving, and least intrusive method would be through Magneto-optical Kerr Effect imaging technique. <br />
<br />
<b>Magneto-Optical Kerr Effect (MOKE)</b><br />
<br />
Conceived by John Kerr in 1877<ref>P.Weinberger writes about Kerr's famous communications to the Philosophical Magazine - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>, the magneto-optic Kerr effect (MOKE) describes the rotation of light polarization when reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
<b>MOKE microscopy</b><br />
<br />
Interaction between an incident light and the magnetization of a magnetic sample causes a change in polarization state of the incident light. Varying magnetizations corresponding to different magnetic domains on the sample gives rise to different degrees of incident light polarization. By detecting and imaging these reflected or transmitted interacted light, an image of magnetic domains with varying intensities can be observed, thereby allowing for domain imaging to be conducted. <br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner, in accordance to data acquisition conditions and the sampled materials' suitability.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED connected in conjunction with a current limiting resistor. The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole after Mirror 2.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
* Added micrometer screw translation stage for sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the <i>tube length</i>, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>. A second parameter that must be kept in mind is the <i>working distance</i>, which is the distance that the sample must be placed in front of the objective. For the 10x and 60x objective, these are 1.5mm and 0.15mm respectively<ref>10x Objective - [https://www.edmundoptics.com/p/10x-din-plan-commercial-grade-objective/5386/| Edmund Optics]</ref><ref>60x Objective - [https://www.edmundoptics.com.sg/p/60x-din-achromactic-finite-intl-standard-objective/3137/| Edmund Optics]</ref>. Hence, when using the 60x objective, the sample is practically kissing the objective.<br />
<br />
The use of a micrometer screw translation stage allowed for finer control over the position of our sample to a precision of <math>\pm 10</math> microns (5 micron contribution from both ends of the measurement).<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample whose external field will be controlled by a magnet stack. Its expected overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<b>Series of permanent magnets</b> <br />
[[File:Magnetseperationfield2.png|thumb|Magnetic field variation away from the magnet stack.]]<br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. </b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. After obtaining these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
(@Joel)<br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
Since Kerr imaging is based on the non-linear response of the sample with respect to the incident light intensity, we considered two alternative lighting configurations -- one in which the laser beam was collimated, and another in which the laser beam was focused onto the sample plane to maximize the incident intensity.<br />
<br />
<gallery widths=420px heights=250px><br />
File: Nonmagnetic images.png | Measured images from non-magnetic sample illuminated by a collimated beam.<br />
File: Nonmagnetic focused images.png | Measured images from non-magnetic sample illuminated by a focused beam.<br />
File: Magnetic images.png | Measured images from magnetic sample illuminated by a collimated beam.<br />
File: Magnetic focused images.png | Measured images from magnetic sample illuminated by a focused beam.<br />
</gallery><br />
<br />
The four figures above depict the raw measurements captured by our Kerr microscopy setup for the four different cases corresponding to the type of sample we were imaging as well as the lighting configuration. The labeled distance of <math>x\,\mu </math>m above each individual image denotes the estimated separation distance between the bar magnet and the back of the sample. In all cases, the magnification was kept the same. The vastly different features seen between the 4 cases are likely due to different areas of each sample being imaged upon changing the lighting and sample configurations.<br />
<br />
<gallery widths=500px heights=340px><br />
File: Intensity vs distance.png | Summed image intensities against sample-magnet distance.<br />
</gallery><br />
<br />
In order to obtain a more quantitative evaluation of whether the Kerr effect is present in our measurements, we plotted the total summed intensity in each captured image and plotted the result with respect to the sample-magnet distance (i.e. external magnetic field strength). For simpler comparison, we also normalized the graphs such that the maximum intensity is 1. Interestingly, we observed a rather consistent intensity across all sample-magnet distances for the focused laser beam, as opposed to the increasing trend for the collimated laser beam case. The sudden jump of intensity from 1 to ~0.7 in the top right quadrant is likely due to an accidental change in our setup apparatus whilst we were moving the sample stage from 250 <math>\mu</math>m to 300 <math>\mu</math>m.<br />
<br />
The non-increasing trend observed in the top two quadrants are consistent with what we expect from theory. For a non-magnetic sample, the MOKE should not be present as silicon on its own is not magnetic, and therefore should not result in a change in light polarization. Based on the magnetic field strength as a function of distance previously measured, we believe that the field strength experienced by the sample even at the furthest distance of 500 <math>\mu</math>m is strong enough to saturate the magnetization of the sample. Therefore, we are unable to observe any changes in the intensity of light for this range of distances. Due to time limitation, we were unable to go back to the lab to collect more data for distances further away where the field strength should be small enough for us to theoretically recover the hysteresis curve we expect from MOKE.<br />
<br />
For the increasing trends seen in the bottom two quadrants, we suspect that these are evidence of systematic errors during our measurement process because they are inconsistent with what we expect to observe from our simple setup. For the non-magnetic material illuminated with collimated light, it should also result in an unchanged intensity regardless of the external magnetic field. On the other hand for the magnetic sample,<br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of ten microns. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
===29 Mar 2022===<br />
<br />
Alignment attempts and measuring of imaged frame size.<br />
<br />
===8 Apr 2022===<br />
<br />
# Try to misalign beam to prevent interference or whatever is happening at NPBS interface <br />
# Switch to LED light source. Might be challenging to couple beam into fiber, but should get rid of interference fringes<br />
# Illuminate at angle rather than on-axis. Might be hard to aim beam since sample is so close to objective. This might help reduce any interference between incoming and reflected beam<br />
<br />
===13 Apr 2022===<br />
<br />
Final alignment and field dependent imaging session.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2887Kerr Microscope2022-04-29T17:32:45Z<p>MayInn: /* Results and Analysis */</p>
<hr />
<div><blockquote> "I was led some time ago to think it very likely, that if a beam of plane-polarized light were reflected under proper conditions from the surface of intensely magnetized iron, it would have its plane of polarization turned through a sensible angle in the process of reflection." - John Kerr<ref>J.Kerr, Philosophical Magazine 3 (1877) p.312.</ref></blockquote><br />
<br />
Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| 2x Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
<br />
<b>Magnetic domains</b><br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains. When multiple magnetic domains are formed, magnetostatic energy in the system decreases as the net magnetization of the system is reduced. Common instances where we can find magnetic domains would be in the random arrangement of magnetic domains on fridge magnets, and in specific forms within magnetic recording devices such as [[Magnetic tape]]s and Video Home System ([[VHS]]) tapes. Several domain imaging techniques can be used to observe and study these magnetic domains, and the most inexpensive, time saving, and least intrusive method would be through Magneto-optical Kerr Effect imaging technique. <br />
<br />
<b>Magneto-Optical Kerr Effect (MOKE)</b><br />
<br />
Conceived by John Kerr in 1877<ref>P.Weinberger writes about Kerr's famous communications to the Philosophical Magazine - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>, the magneto-optic Kerr effect (MOKE) describes the rotation of light polarization when reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
<b>MOKE microscopy</b><br />
<br />
Interaction between an incident light and the magnetization of a magnetic sample causes a change in polarization state of the incident light. Varying magnetizations corresponding to different magnetic domains on the sample gives rise to different degrees of incident light polarization. By detecting and imaging these reflected or transmitted interacted light, an image of magnetic domains with varying intensities can be observed, thereby allowing for domain imaging to be conducted. <br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner, in accordance to data acquisition conditions and the sampled materials' suitability.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED connected in conjunction with a current limiting resistor. The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole after Mirror 2.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
* Added micrometer screw translation stage for sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the <i>tube length</i>, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>. A second parameter that must be kept in mind is the <i>working distance</i>, which is the distance that the sample must be placed in front of the objective. For the 10x and 60x objective, these are 1.5mm and 0.15mm respectively<ref>10x Objective - [https://www.edmundoptics.com/p/10x-din-plan-commercial-grade-objective/5386/| Edmund Optics]</ref><ref>60x Objective - [https://www.edmundoptics.com.sg/p/60x-din-achromactic-finite-intl-standard-objective/3137/| Edmund Optics]</ref>. Hence, when using the 60x objective, the sample is practically kissing the objective.<br />
<br />
The use of a micrometer screw translation stage allowed for finer control over the position of our sample to a precision of <math>\pm 10</math> microns (5 micron contribution from both ends of the measurement).<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample whose external field will be controlled by a magnet stack. Its expected overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<b>Series of permanent magnets</b> <br />
[[File:Magnetseperationfield2.png|thumb|Magnetic field variation away from the magnet stack.]]<br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. </b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. After obtaining these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
(@Joel)<br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
Since Kerr imaging is based on the non-linear response of the sample with respect to the incident light intensity, we considered two alternative lighting configurations -- one in which the laser beam was collimated, and another in which the laser beam was focused onto the sample plane to maximize the incident intensity.<br />
<br />
<gallery widths=420px heights=250px><br />
File: Nonmagnetic images.png | Measured images from non-magnetic sample illuminated by a collimated beam.<br />
File: Nonmagnetic focused images.png | Measured images from non-magnetic sample illuminated by a focused beam.<br />
File: Magnetic images.png | Measured images from magnetic sample illuminated by a collimated beam.<br />
File: Magnetic focused images.png | Measured images from magnetic sample illuminated by a focused beam.<br />
</gallery><br />
<br />
The four figures above depict the raw measurements captured by our Kerr microscopy setup for the four different cases corresponding to the type of sample we were imaging as well as the lighting configuration. The labeled distance of <math>x\,\mu </math>m above each individual image denotes the estimated separation distance between the bar magnet and the back of the sample. In all cases, the magnification was kept the same. The vastly different features seen between the 4 cases are likely due to different areas of each sample being imaged upon changing the lighting and sample configurations.<br />
<br />
<gallery widths=500px heights=340px><br />
File: Intensity vs distance.png | Summed image intensities against sample-magnet distance.<br />
</gallery><br />
<br />
In order to obtain a more quantitative evaluation of whether the Kerr effect is present in our measurements, we plotted the total summed intensity in each captured image and plotted the result with respect to the sample-magnet distance (i.e. external magnetic field strength). For simpler comparison, we also normalized the graphs such that the maximum intensity is 1. Interestingly, we observed a rather consistent intensity across all sample-magnet distances for the focused laser beam, as opposed to the increasing trend for the collimated laser beam case. The sudden jump of intensity from 1 to ~0.7 in the top right quadrant is likely due to an accidental change in our setup apparatus whilst we were moving the sample stage from 250 <math>\mu</math>m to 300 <math>\mu</math>m.<br />
<br />
The non-increasing trend observed in the top two quadrants are consistent with what we expect from theory. For a non-magnetic sample, the MOKE should not be present as silicon on its own is not magnetic, and therefore should not result in a change in light polarization. Based on the magnetic field strength as a function of distance previously measured, we believe that the field strength experienced by the sample even at the furthest distance of 500 <math>\mu</math>m is strong enough to saturate the magnetization of the sample. Therefore, we are unable to observe any changes in the intensity of light for this range of distances. Due to time limitation, we were unable to go back to the lab to collect more data for distances further away where the field strength should be small enough for us to theoretically recover the hysteresis curve we expect from MOKE.<br />
<br />
For the increasing trends seen in the bottom two quadrants, we suspect that these are evidence of systematic errors during our measurement process because they are inconsistent with what we expect to observe from our simple setup. For the non-magnetic material illuminated with collimated light, it should also result in an unchanged intensity regardless of the external magnetic field. On the other hand for the magnetic sample,<br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of ten microns. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
===29 Mar 2022===<br />
<br />
Alignment attempts and measuring of imaged frame size.<br />
<br />
===8 Apr 2022===<br />
<br />
# Try to misalign beam to prevent interference or whatever is happening at NPBS interface <br />
# Switch to LED light source. Might be challenging to couple beam into fiber, but should get rid of interference fringes<br />
# Illuminate at angle rather than on-axis. Might be hard to aim beam since sample is so close to objective. This might help reduce any interference between incoming and reflected beam<br />
<br />
===13 Apr 2022===<br />
<br />
Final alignment and field dependent imaging session.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=File:Magnetseperationfield2.png&diff=2886File:Magnetseperationfield2.png2022-04-29T17:31:32Z<p>MayInn: Magnetic field wrt separation</p>
<hr />
<div>== Summary ==<br />
Magnetic field wrt separation</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2883Kerr Microscope2022-04-29T17:29:38Z<p>MayInn: /* Experimental Setup */</p>
<hr />
<div><blockquote> "I was led some time ago to think it very likely, that if a beam of plane-polarized light were reflected under proper conditions from the surface of intensely magnetized iron, it would have its plane of polarization turned through a sensible angle in the process of reflection." - John Kerr<ref>J.Kerr, Philosophical Magazine 3 (1877) p.312.</ref></blockquote><br />
<br />
Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| 2x Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
<br />
<b>Magnetic domains</b><br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains. When multiple magnetic domains are formed, magnetostatic energy in the system decreases as the net magnetization of the system is reduced. Common instances where we can find magnetic domains would be in the random arrangement of magnetic domains on fridge magnets, and in specific forms within magnetic recording devices such as [[Magnetic tape]]s and Video Home System ([[VHS]]) tapes. Several domain imaging techniques can be used to observe and study these magnetic domains, and the most inexpensive, time saving, and least intrusive method would be through Magneto-optical Kerr Effect imaging technique. <br />
<br />
<b>Magneto-Optical Kerr Effect (MOKE)</b><br />
<br />
Conceived by John Kerr in 1877<ref>P.Weinberger writes about Kerr's famous communications to the Philosophical Magazine - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>, the magneto-optic Kerr effect (MOKE) describes the rotation of light polarization when reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
<b>MOKE microscopy</b><br />
<br />
Interaction between an incident light and the magnetization of a magnetic sample causes a change in polarization state of the incident light. Varying magnetizations corresponding to different magnetic domains on the sample gives rise to different degrees of incident light polarization. By detecting and imaging these reflected or transmitted interacted light, an image of magnetic domains with varying intensities can be observed, thereby allowing for domain imaging to be conducted. <br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner, in accordance to data acquisition conditions and the sampled materials' suitability.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED connected in conjunction with a current limiting resistor. The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole after Mirror 2.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
* Added micrometer screw translation stage for sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the <i>tube length</i>, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>. A second parameter that must be kept in mind is the <i>working distance</i>, which is the distance that the sample must be placed in front of the objective. For the 10x and 60x objective, these are 1.5mm and 0.15mm respectively<ref>10x Objective - [https://www.edmundoptics.com/p/10x-din-plan-commercial-grade-objective/5386/| Edmund Optics]</ref><ref>60x Objective - [https://www.edmundoptics.com.sg/p/60x-din-achromactic-finite-intl-standard-objective/3137/| Edmund Optics]</ref>. Hence, when using the 60x objective, the sample is practically kissing the objective.<br />
<br />
The use of a micrometer screw translation stage allowed for finer control over the position of our sample to a precision of <math>\pm 10</math> microns (5 micron contribution from both ends of the measurement).<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample whose external field will be controlled by a magnet stack. Its expected overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<b>Series of permanent magnets</b> <br />
[[File:Magnetseperationfield.png|thumb|Magnetic field variation away from the magnet stack.]]<br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. </b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. After obtaining these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
(@Joel)<br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
Since Kerr imaging is based on the non-linear response of the sample with respect to the incident light intensity, we considered two alternative lighting configurations -- one in which the laser beam was collimated, and another in which the laser beam was focused onto the sample plane to maximize the incident intensity.<br />
<br />
<gallery widths=420px heights=250px><br />
File: Nonmagnetic images.png | Measured images from non-magnetic sample illuminated by a collimated beam.<br />
File: Nonmagnetic focused images.png | Measured images from non-magnetic sample illuminated by a focused beam.<br />
File: Magnetic images.png | Measured images from magnetic sample illuminated by a collimated beam.<br />
File: Magnetic focused images.png | Measured images from magnetic sample illuminated by a focused beam.<br />
</gallery><br />
<br />
The four figures above depict the raw measurements captured by our Kerr microscopy setup for the four different cases corresponding to the type of sample we were imaging as well as the lighting configuration. The labeled distance of <math>x\,\mu </math>m above each individual image denotes the estimated separation distance between the bar magnet and the back of the sample. In all cases, the magnification was kept the same. The vastly different features seen between the 4 cases are likely due to different areas of each sample being imaged upon changing the lighting and sample configurations.<br />
<br />
<gallery widths=500px heights=340px><br />
File: Intensity vs distance.png | Summed image intensities against sample-magnet distance.<br />
</gallery><br />
<br />
In order to obtain a more quantitative evaluation of whether the Kerr effect is present in our measurements, we plotted the total summed intensity in each captured image and plotted the result with respect to the sample-magnet distance (i.e. external magnetic field strength). For simpler comparison, we also normalized the graphs such that the maximum intensity is 1. Interestingly, we observed a rather consistent intensity across all sample-magnet distances for the focused laser beam, as opposed to the increasing trend for the collimated laser beam case. The sudden jump of intensity from 1 to ~0.7 in the top right quadrant is likely due to an accidental change in our setup apparatus whilst we were moving the sample stage from 250 <math>\mu</math>m to 300 <math>\mu</math>m.<br />
<br />
The non-increasing trend observed in the top two quadrants are consistent with what we expect from theory. For a non-magnetic sample, the MOKE should not be present as silicon on its own is not magnetic, and therefore should not result in a change in light polarization. Based on the magnetic field strength as a function of distance previously measured, we believe that the field strength experienced by the sample even at the furthest distance of 500 <math>\mu</math>m is strong enough to saturate the magnetization of the sample. Therefore, we are unable to observe any changes in the intensity of light for this range of distances. Due to time limitation, we were unable to go back to the lab to collect more data for distances further away where the field strength should be small enough for us to theoretically recover the hysteresis curve we expect from MOKE.<br />
<br />
For the increasing trends seen in the bottom two quadrants, we suspect that these are evidence of systematic errors during our measurement process because they are inconsistent with what we expect to observe from our simple setup. For the non-magnetic material illuminated with collimated light, it should also result in an unchanged intensity regardless of the external magnetic field. On the other hand for the magnetic sample,<br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of ten microns. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
===29 Mar 2022===<br />
<br />
Alignment attempts and measuring of imaged frame size.<br />
<br />
===8 Apr 2022===<br />
<br />
# Try to misalign beam to prevent interference or whatever is happening at NPBS interface <br />
# Switch to LED light source. Might be challenging to couple beam into fiber, but should get rid of interference fringes<br />
# Illuminate at angle rather than on-axis. Might be hard to aim beam since sample is so close to objective. This might help reduce any interference between incoming and reflected beam<br />
<br />
===13 Apr 2022===<br />
<br />
Final alignment and field dependent imaging session.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2864Kerr Microscope2022-04-29T17:18:48Z<p>MayInn: /* Lab Session Logs */</p>
<hr />
<div><blockquote> "I was led some time ago to think it very likely, that if a beam of plane-polarized light were reflected under proper conditions from the surface of intensely magnetized iron, it would have its plane of polarization turned through a sensible angle in the process of reflection." - John Kerr<ref>J.Kerr, Philosophical Magazine 3 (1877) p.312.</ref></blockquote><br />
<br />
Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| 2x Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
<br />
<b>Magnetic domains</b><br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains. When multiple magnetic domains are formed, magnetostatic energy in the system decreases as the net magnetization of the system is reduced. Common instances where we can find magnetic domains would be in the random arrangement of magnetic domains on fridge magnets, and in specific forms within magnetic recording devices such as [[Magnetic tape]]s and Video Home System ([[VHS]]) tapes. Several domain imaging techniques can be used to observe and study these magnetic domains, and the most inexpensive, time saving, and least intrusive method would be through Magneto-optical Kerr Effect imaging technique. <br />
<br />
<b>Magneto-Optical Kerr Effect (MOKE)</b><br />
<br />
Conceived by John Kerr in 1877<ref>P.Weinberger writes about Kerr's famous communications to the Philosophical Magazine - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>, the magneto-optic Kerr effect (MOKE) describes the rotation of light polarization when reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
<b>MOKE microscopy</b><br />
<br />
Interaction between an incident light and the magnetization of a magnetic sample causes a change in polarization state of the incident light. Varying magnetizations corresponding to different magnetic domains on the sample gives rise to different degrees of incident light polarization. By detecting and imaging these reflected or transmitted interacted light, an image of magnetic domains with varying intensities can be observed, thereby allowing for domain imaging to be conducted. <br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner, in accordance to data acquisition conditions and the sampled materials' suitability.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED connected in conjunction with a current limiting resistor. The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole after Mirror 2.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
* Added micrometer screw translation stage for sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the <i>tube length</i>, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>. A second parameter that must be kept in mind is the <i>working distance</i>, which is the distance that the sample must be placed in front of the objective. For the 10x and 60x objective, these are 1.5mm and 0.15mm respectively<ref>10x Objective - [https://www.edmundoptics.com/p/10x-din-plan-commercial-grade-objective/5386/| Edmund Optics]</ref><ref>60x Objective - [https://www.edmundoptics.com.sg/p/60x-din-achromactic-finite-intl-standard-objective/3137/| Edmund Optics]</ref>. Hence, when using the 60x objective, the sample is practically kissing the objective.<br />
<br />
The use of a micrometer screw translation stage allowed for finer control over the position of our sample to a precision of <math>\pm 10</math> microns (5 micron contribution from both ends of the measurement).<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample. Although its specific composition is unknown, it is expected that the overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<b>Series of permanent magnets</b> <br />
[[File:Magnetseperationfield.png|thumb|Magnetic field variation away from the magnet stack.]]<br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. </b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. After obtaining these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
(@Joel)<br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
Since Kerr imaging is based on the non-linear response of the sample with respect to the incident light intensity, we considered two alternative lighting configurations -- one in which the laser beam was collimated, and another in which the laser beam was focused onto the sample plane to maximize the incident intensity.<br />
<br />
<gallery widths=420px heights=250px><br />
File: Nonmagnetic images.png | Measured images from non-magnetic sample illuminated by a collimated beam.<br />
File: Nonmagnetic focused images.png | Measured images from non-magnetic sample illuminated by a focused beam.<br />
File: Magnetic images.png | Measured images from magnetic sample illuminated by a collimated beam.<br />
File: Magnetic focused images.png | Measured images from magnetic sample illuminated by a focused beam.<br />
</gallery><br />
<br />
The four figures above depict the raw measurements captured by our Kerr microscopy setup for the four different cases corresponding to the type of sample we were imaging as well as the lighting configuration. The labeled distance of <math>x\,\mu </math>m above each individual image denotes the estimated separation distance between the bar magnet and the back of the sample. In all cases, the magnification was kept the same. The vastly different features seen between the 4 cases are likely due to different areas of each sample being imaged upon changing the lighting and sample configurations.<br />
<br />
<gallery widths=500px heights=340px><br />
File: Intensity vs distance.png | Summed image intensities against sample-magnet distance.<br />
</gallery><br />
<br />
In order to obtain a more quantitative evaluation of whether the Kerr effect is present in our measurements, we plotted the total summed intensity in each captured image and plotted the result with respect to the sample-magnet distance (i.e. external magnetic field strength). For simpler comparison, we also normalized the graphs such that the maximum intensity is 1. Interestingly, we observed a rather consistent intensity across all sample-magnet distances for the focused laser beam, as opposed to the increasing trend for the collimated laser beam case. The sudden jump of intensity from 1 to ~0.7 in the top right quadrant is likely due to an accidental change in our setup apparatus whilst we were moving the sample stage from 250 <math>\mu</math>m to 300 <math>\mu</math>m.<br />
<br />
The non-increasing trend observed in the top two quadrants are consistent with what we expect from theory. For a non-magnetic sample, the MOKE should not be present as silicon on its own is not magnetic, and therefore should not result in a change in light polarization. Based on the magnetic field strength as a function of distance previously measured, we believe that the field strength experienced by the sample even at the furthest distance of 500 <math>\mu</math>m is strong enough to saturate the magnetization of the sample. Therefore, we are unable to observe any changes in the intensity of light for this range of distances. Due to time limitation, we were unable to go back to the lab to collect more data for distances further away where the field strength should be small enough for us to theoretically recover the hysteresis curve we expect from MOKE.<br />
<br />
For the increasing trends seen in the bottom two quadrants, we suspect that these are evidence of systematic errors during our measurement process because they are inconsistent with what we expect to observe from our simple setup. For the non-magnetic material illuminated with collimated light, it should also result in an unchanged intensity regardless of the external magnetic field. On the other hand for the magnetic sample,<br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of ten microns. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
===29 Mar 2022===<br />
<br />
Alignment attempts and measuring of imaged frame size.<br />
<br />
===8 Apr 2022===<br />
<br />
# Try to misalign beam to prevent interference or whatever is happening at NPBS interface <br />
# Switch to LED light source. Might be challenging to couple beam into fiber, but should get rid of interference fringes<br />
# Illuminate at angle rather than on-axis. Might be hard to aim beam since sample is so close to objective. This might help reduce any interference between incoming and reflected beam<br />
<br />
===13 Apr 2022===<br />
<br />
Final alignment and field dependent imaging session.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2859Kerr Microscope2022-04-29T17:16:22Z<p>MayInn: /* Lab Session Logs */</p>
<hr />
<div><blockquote> "I was led some time ago to think it very likely, that if a beam of plane-polarized light were reflected under proper conditions from the surface of intensely magnetized iron, it would have its plane of polarization turned through a sensible angle in the process of reflection." - John Kerr<ref>J.Kerr, Philosophical Magazine 3 (1877) p.312.</ref></blockquote><br />
<br />
Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| 2x Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
<br />
<b>Magnetic domains</b><br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains. When multiple magnetic domains are formed, magnetostatic energy in the system decreases as the net magnetization of the system is reduced. Common instances where we can find magnetic domains would be in the random arrangement of magnetic domains on fridge magnets, and in specific forms within magnetic recording devices such as [[Magnetic tape]]s and Video Home System ([[VHS]]) tapes. Several domain imaging techniques can be used to observe and study these magnetic domains, and the most inexpensive, time saving, and least intrusive method would be through Magneto-optical Kerr Effect imaging technique. <br />
<br />
<b>Magneto-Optical Kerr Effect (MOKE)</b><br />
<br />
Conceived by John Kerr in 1877<ref>P.Weinberger writes about Kerr's famous communications to the Philosophical Magazine - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>, the magneto-optic Kerr effect (MOKE) describes the rotation of light polarization when reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
<b>MOKE microscopy</b><br />
<br />
Interaction between an incident light and the magnetization of a magnetic sample causes a change in polarization state of the incident light. Varying magnetizations corresponding to different magnetic domains on the sample gives rise to different degrees of incident light polarization. By detecting and imaging these reflected or transmitted interacted light, an image of magnetic domains with varying intensities can be observed, thereby allowing for domain imaging to be conducted. <br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner, in accordance to data acquisition conditions and the sampled materials' suitability.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED connected in conjunction with a current limiting resistor. The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole after Mirror 2.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
* Added micrometer screw translation stage for sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the <i>tube length</i>, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>. A second parameter that must be kept in mind is the <i>working distance</i>, which is the distance that the sample must be placed in front of the objective. For the 10x and 60x objective, these are 1.5mm and 0.15mm respectively<ref>10x Objective - [https://www.edmundoptics.com/p/10x-din-plan-commercial-grade-objective/5386/| Edmund Optics]</ref><ref>60x Objective - [https://www.edmundoptics.com.sg/p/60x-din-achromactic-finite-intl-standard-objective/3137/| Edmund Optics]</ref>. Hence, when using the 60x objective, the sample is practically kissing the objective.<br />
<br />
The use of a micrometer screw translation stage allowed for finer control over the position of our sample to a precision of <math>\pm 10</math> microns (5 micron contribution from both ends of the measurement).<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample. Although its specific composition is unknown, it is expected that the overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<b>Series of permanent magnets</b> <br />
[[File:Magnetseperationfield.png|thumb|Magnetic field variation away from the magnet stack.]]<br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. </b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. After obtaining these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
(@Joel)<br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
Since Kerr imaging is based on the non-linear response of the sample with respect to the incident light intensity, we considered two alternative lighting configurations -- one in which the laser beam was collimated, and another in which the laser beam was focused onto the sample plane to maximize the incident intensity.<br />
<br />
<gallery widths=420px heights=250px><br />
File: Nonmagnetic images.png | Measured images from non-magnetic sample illuminated by a collimated beam.<br />
File: Nonmagnetic focused images.png | Measured images from non-magnetic sample illuminated by a focused beam.<br />
File: Magnetic images.png | Measured images from magnetic sample illuminated by a collimated beam.<br />
File: Magnetic focused images.png | Measured images from magnetic sample illuminated by a focused beam.<br />
</gallery><br />
<br />
The four figures above depict the raw measurements captured by our Kerr microscopy setup for the four different cases corresponding to the type of sample we were imaging as well as the lighting configuration. The labeled distance of <math>x\,\mu </math>m above each individual image denotes the estimated separation distance between the bar magnet and the back of the sample. In all cases, the magnification was kept the same. The vastly different features seen between the 4 cases are likely due to different areas of each sample being imaged upon changing the lighting and sample configurations.<br />
<br />
<gallery widths=500px heights=340px><br />
File: Intensity vs distance.png | Summed image intensities against sample-magnet distance.<br />
</gallery><br />
<br />
In order to obtain a more quantitative evaluation of whether the Kerr effect is present in our measurements, we plotted the total summed intensity in each captured image and plotted the result with respect to the sample-magnet distance (i.e. external magnetic field strength). For simpler comparison, we also normalized the graphs such that the maximum intensity is 1. Interestingly, we observed a rather consistent intensity across all sample-magnet distances for the focused laser beam, as opposed to the increasing trend for the collimated laser beam case. The sudden jump of intensity from 1 to ~0.7 in the top right quadrant is likely due to an accidental change in our setup apparatus whilst we were moving the sample stage from 250 <math>\mu</math>m to 300 <math>\mu</math>m.<br />
<br />
The non-increasing trend observed in the top two quadrants are consistent with what we expect from theory. For a non-magnetic sample, the MOKE should not be present as silicon on its own is not magnetic, and therefore should not result in a change in light polarization. Based on the magnetic field strength as a function of distance previously measured, we believe that the field strength experienced by the sample even at the furthest distance of 500 <math>\mu</math>m is strong enough to saturate the magnetization of the sample. Therefore, we are unable to observe any changes in the intensity of light for this range of distances. Due to time limitation, we were unable to go back to the lab to collect more data for distances further away where the field strength should be small enough for us to theoretically recover the hysteresis curve we expect from MOKE.<br />
<br />
For the increasing trends seen in the bottom two quadrants, we suspect that these are evidence of systematic errors during our measurement process because they are inconsistent with what we expect to observe from our simple setup. For the non-magnetic material illuminated with collimated light, it should also result in an unchanged intensity regardless of the external magnetic field. On the other hand for the magnetic sample,<br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of ten microns. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
===8 Apr 2022===<br />
<br />
1. Try to misalign beam to prevent interference or whatever is happening at NPBS interface <br />
2. Switch to LED light source. Might be challenging to couple beam into fiber, but should get rid of interference fringes<br />
3. Illuminate at angle rather than on-axis. Might be hard to aim beam since sample is so close to objective. This might help reduce any interference between incoming and reflected beam<br />
<br />
===13 Apr 2022===<br />
<br />
Final alignment and field dependent imaging session.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2857Kerr Microscope2022-04-29T17:12:22Z<p>MayInn: /* Theory */</p>
<hr />
<div><blockquote> "I was led some time ago to think it very likely, that if a beam of plane-polarized light were reflected under proper conditions from the surface of intensely magnetized iron, it would have its plane of polarization turned through a sensible angle in the process of reflection." - John Kerr<ref>J.Kerr, Philosophical Magazine 3 (1877) p.312.</ref></blockquote><br />
<br />
Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| 2x Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
<br />
<b>Magnetic domains</b><br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains. When multiple magnetic domains are formed, magnetostatic energy in the system decreases as the net magnetization of the system is reduced. Common instances where we can find magnetic domains would be in the random arrangement of magnetic domains on fridge magnets, and in specific forms within magnetic recording devices such as [[Magnetic tape]]s and Video Home System ([[VHS]]) tapes. Several domain imaging techniques can be used to observe and study these magnetic domains, and the most inexpensive, time saving, and least intrusive method would be through Magneto-optical Kerr Effect imaging technique. <br />
<br />
<b>Magneto-Optical Kerr Effect (MOKE)</b><br />
<br />
Conceived by John Kerr in 1877<ref>P.Weinberger writes about Kerr's famous communications to the Philosophical Magazine - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>, the magneto-optic Kerr effect (MOKE) describes the rotation of light polarization when reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
<b>MOKE microscopy</b><br />
<br />
Interaction between an incident light and the magnetization of a magnetic sample causes a change in polarization state of the incident light. Varying magnetizations corresponding to different magnetic domains on the sample gives rise to different degrees of incident light polarization. By detecting and imaging these reflected or transmitted interacted light, an image of magnetic domains with varying intensities can be observed, thereby allowing for domain imaging to be conducted. <br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner, in accordance to data acquisition conditions and the sampled materials' suitability.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED connected in conjunction with a current limiting resistor. The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole after Mirror 2.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
* Added micrometer screw translation stage for sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the <i>tube length</i>, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>. A second parameter that must be kept in mind is the <i>working distance</i>, which is the distance that the sample must be placed in front of the objective. For the 10x and 60x objective, these are 1.5mm and 0.15mm respectively<ref>10x Objective - [https://www.edmundoptics.com/p/10x-din-plan-commercial-grade-objective/5386/| Edmund Optics]</ref><ref>60x Objective - [https://www.edmundoptics.com.sg/p/60x-din-achromactic-finite-intl-standard-objective/3137/| Edmund Optics]</ref>. Hence, when using the 60x objective, the sample is practically kissing the objective.<br />
<br />
The use of a micrometer screw translation stage allowed for finer control over the position of our sample to a precision of <math>\pm 10</math> microns (5 micron contribution from both ends of the measurement).<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample. Although its specific composition is unknown, it is expected that the overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<b>Series of permanent magnets</b> <br />
[[File:Magnetseperationfield.png|thumb|Magnetic field variation away from the magnet stack.]]<br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. </b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. After obtaining these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
(@Joel)<br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
Since Kerr imaging is based on the non-linear response of the sample with respect to the incident light intensity, we considered two alternative lighting configurations -- one in which the laser beam was collimated, and another in which the laser beam was focused onto the sample plane to maximize the incident intensity.<br />
<br />
<gallery widths=420px heights=250px><br />
File: Nonmagnetic images.png | Measured images from non-magnetic sample illuminated by a collimated beam.<br />
File: Nonmagnetic focused images.png | Measured images from non-magnetic sample illuminated by a focused beam.<br />
File: Magnetic images.png | Measured images from magnetic sample illuminated by a collimated beam.<br />
File: Magnetic focused images.png | Measured images from magnetic sample illuminated by a focused beam.<br />
</gallery><br />
<br />
The four figures above depict the raw measurements captured by our Kerr microscopy setup for the four different cases corresponding to the type of sample we were imaging as well as the lighting configuration. The labeled distance of <math>x\,\mu </math>m above each individual image denotes the estimated separation distance between the bar magnet and the back of the sample. In all cases, the magnification was kept the same. The vastly different features seen between the 4 cases are likely due to different areas of each sample being imaged upon changing the lighting and sample configurations.<br />
<br />
<gallery widths=500px heights=340px><br />
File: Intensity vs distance.png | Summed image intensities against sample-magnet distance.<br />
</gallery><br />
<br />
In order to obtain a more quantitative evaluation of whether the Kerr effect is present in our measurements, we plotted the total summed intensity in each captured image and plotted the result with respect to the sample-magnet distance (i.e. external magnetic field strength). For simpler comparison, we also normalized the graphs such that the maximum intensity is 1. Interestingly, we observed a rather consistent intensity across all sample-magnet distances for the focused laser beam, as opposed to the increasing trend for the collimated laser beam case. The sudden jump of intensity from 1 to ~0.7 in the top right quadrant is likely due to an accidental change in our setup apparatus whilst we were moving the sample stage from 250 <math>\mu</math>m to 300 <math>\mu</math>m.<br />
<br />
The non-increasing trend observed in the top two quadrants are consistent with what we expect from theory. For a non-magnetic sample, the MOKE should not be present as silicon on its own is not magnetic, and therefore should not result in a change in light polarization. Based on the magnetic field strength as a function of distance previously measured, we believe that the field strength experienced by the sample even at the furthest distance of 500 <math>\mu</math>m is strong enough to saturate the magnetization of the sample. Therefore, we are unable to observe any changes in the intensity of light for this range of distances. Due to time limitation, we were unable to go back to the lab to collect more data for distances further away where the field strength should be small enough for us to theoretically recover the hysteresis curve we expect from MOKE.<br />
<br />
For the increasing trends seen in the bottom two quadrants, we suspect that these are evidence of systematic errors during our measurement process because they are inconsistent with what we expect to observe from our simple setup. For the non-magnetic material illuminated with collimated light, it should also result in an unchanged intensity regardless of the external magnetic field. On the other hand for the magnetic sample,<br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of ten microns. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2856Kerr Microscope2022-04-29T17:04:09Z<p>MayInn: /* Results and Analysis */</p>
<hr />
<div><blockquote> "I was led some time ago to think it very likely, that if a beam of plane-polarized light were reflected under proper conditions from the surface of intensely magnetized iron, it would have its plane of polarization turned through a sensible angle in the process of reflection." - John Kerr<ref>J.Kerr, Philosophical Magazine 3 (1877) p.312.</ref></blockquote><br />
<br />
Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| 2x Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
<br />
<b>Magnetic domains</b><br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains. When multiple magnetic domains are formed, magnetostatic energy in the system decreases as the net magnetization of the system is reduced. Common instances where we can find magnetic domains would be in the random arrangement of magnetic domains on fridge magnets, and in specific forms within magnetic recording devices such as Video Home System (VHS) tapes. Several domain imaging techniques can be used to observe and study these magnetic domains, and the most inexpensive, time saving, and least intrusive method would be through Magneto-optical Kerr Effect imaging technique. <br />
<br />
<b>Magneto-Optical Kerr Effect (MOKE)</b><br />
<br />
Conceived by John Kerr in 1877<ref>P.Weinberger writes about Kerr's famous communications to the Philosophical Magazine - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>, the magneto-optic Kerr effect (MOKE) describes the rotation of light polarization when reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
<b>MOKE microscopy</b><br />
<br />
Interaction between an incident light and the magnetization of a magnetic sample causes a change in polarization state of the incident light. Varying magnetizations corresponding to different magnetic domains on the sample gives rise to different degrees of incident light polarization. By detecting and imaging these reflected or transmitted interacted light, an image of magnetic domains with varying intensities can be observed, thereby allowing for domain imaging to be conducted. <br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner, in accordance to data acquisition conditions and the sampled materials' suitability.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED connected in conjunction with a current limiting resistor. The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole after Mirror 2.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
* Added micrometer screw translation stage for sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the <i>tube length</i>, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>. A second parameter that must be kept in mind is the <i>working distance</i>, which is the distance that the sample must be placed in front of the objective. For the 10x and 60x objective, these are 1.5mm and 0.15mm respectively<ref>10x Objective - [https://www.edmundoptics.com/p/10x-din-plan-commercial-grade-objective/5386/| Edmund Optics]</ref><ref>60x Objective - [https://www.edmundoptics.com.sg/p/60x-din-achromactic-finite-intl-standard-objective/3137/| Edmund Optics]</ref>. Hence, when using the 60x objective, the sample is practically kissing the objective.<br />
<br />
The use of a micrometer screw translation stage allowed for finer control over the position of our sample to a precision of <math>\pm 10</math> microns (5 micron contribution from both ends of the measurement).<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample. Although its specific composition is unknown, it is expected that the overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<b>Series of permanent magnets</b> <br />
[[File:Magnetseperationfield.png|thumb|Magnetic field variation away from the magnet stack.]]<br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. </b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. After obtaining these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
(@Joel)<br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
Since Kerr imaging is based on the non-linear response of the sample with respect to the incident light intensity, we considered two alternative lighting configurations -- one in which the laser beam was collimated, and another in which the laser beam was focused onto the sample plane to maximize the incident intensity.<br />
<br />
<gallery widths=420px heights=250px><br />
File: Nonmagnetic images.png | Measured images from non-magnetic sample illuminated by a collimated beam.<br />
File: Nonmagnetic focused images.png | Measured images from non-magnetic sample illuminated by a focused beam.<br />
File: Magnetic images.png | Measured images from magnetic sample illuminated by a collimated beam.<br />
File: Magnetic focused images.png | Measured images from magnetic sample illuminated by a focused beam.<br />
</gallery><br />
<br />
The four figures above depict the raw measurements captured by our Kerr microscopy setup for the four different cases corresponding to the type of sample we were imaging as well as the lighting configuration. The labeled distance of <math>x\,\mu </math>m above each individual image denotes the estimated separation distance between the bar magnet and the back of the sample. In all cases, the magnification was kept the same. The vastly different features seen between the 4 cases are likely due to different areas of each sample being imaged upon changing the lighting and sample configurations.<br />
<br />
<gallery widths=500px heights=340px><br />
File: Intensity vs distance.png | Summed image intensities against sample-magnet distance.<br />
</gallery><br />
<br />
In order to obtain a more quantitative evaluation of whether the Kerr effect is present in our measurements, we plotted the total summed intensity in each captured image and plotted the result with respect to the sample-magnet distance (i.e. external magnetic field strength). For simpler comparison, we also normalized the graphs such that the maximum intensity is 1. Interestingly, we observed a rather consistent intensity across all sample-magnet distances for the focused laser beam, as opposed to the increasing trend for the collimated laser beam case. The sudden jump of intensity from 1 to ~0.7 in the top right quadrant is likely due to an accidental change in our setup apparatus whilst we were moving the sample stage from 250 <math>\mu</math>m to 300 <math>\mu</math>m.<br />
<br />
The non-increasing trend observed in the top two quadrants are consistent with what we expect from theory. For a non-magnetic sample, the MOKE should not be present as silicon on its own is not magnetic, and therefore should not result in a change in light polarization. Based on the magnetic field strength as a function of distance previously measured, we believe that the field strength experienced by the sample even at the furthest distance of 500 <math>\mu</math>m is strong enough to saturate the magnetization of the sample. Therefore, we are unable to observe any changes in the intensity of light for this range of distances. Due to time limitation, we were unable to go back to the lab to collect more data for distances further away where the field strength should be small enough for us to theoretically recover the hysteresis curve we expect from MOKE.<br />
<br />
For the increasing trends seen in the bottom two quadrants, we suspect that these are evidence of systematic errors during our measurement process because they are inconsistent with what we expect to observe from our simple setup. For the non-magnetic material illuminated with collimated light, it should also result in an unchanged intensity regardless of the external magnetic field. On the other hand for the magnetic sample,<br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of ten microns. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=File:Magnetseperationfield.png&diff=2855File:Magnetseperationfield.png2022-04-29T17:02:13Z<p>MayInn: magnetic field variation wrt separation</p>
<hr />
<div>== Summary ==<br />
magnetic field variation wrt separation</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2854Kerr Microscope2022-04-29T16:56:51Z<p>MayInn: /* Results and Analysis */</p>
<hr />
<div><blockquote> "I was led some time ago to think it very likely, that if a beam of plane-polarized light were reflected under proper conditions from the surface of intensely magnetized iron, it would have its plane of polarization turned through a sensible angle in the process of reflection." - John Kerr<ref>J.Kerr, Philosophical Magazine 3 (1877) p.312.</ref></blockquote><br />
<br />
Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| 2x Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
<br />
<b>Magnetic domains</b><br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains. When multiple magnetic domains are formed, magnetostatic energy in the system decreases as the net magnetization of the system is reduced. Common instances where we can find magnetic domains would be in the random arrangement of magnetic domains on fridge magnets, and in specific forms within magnetic recording devices such as Video Home System (VHS) tapes. Several domain imaging techniques can be used to observe and study these magnetic domains, and the most inexpensive, time saving, and least intrusive method would be through Magneto-optical Kerr Effect imaging technique. <br />
<br />
<b>Magneto-Optical Kerr Effect (MOKE)</b><br />
<br />
Conceived by John Kerr in 1877<ref>P.Weinberger writes about Kerr's famous communications to the Philosophical Magazine - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>, the magneto-optic Kerr effect (MOKE) describes the rotation of light polarization when reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
<b>MOKE microscopy</b><br />
<br />
Interaction between an incident light and the magnetization of a magnetic sample causes a change in polarization state of the incident light. Varying magnetizations corresponding to different magnetic domains on the sample gives rise to different degrees of incident light polarization. By detecting and imaging these reflected or transmitted interacted light, an image of magnetic domains with varying intensities can be observed, thereby allowing for domain imaging to be conducted. <br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner, in accordance to data acquisition conditions and the sampled materials' suitability.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED connected in conjunction with a current limiting resistor. The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole after Mirror 2.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
* Added micrometer screw translation stage for sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the <i>tube length</i>, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>. A second parameter that must be kept in mind is the <i>working distance</i>, which is the distance that the sample must be placed in front of the objective. For the 10x and 60x objective, these are 1.5mm and 0.15mm respectively<ref>10x Objective - [https://www.edmundoptics.com/p/10x-din-plan-commercial-grade-objective/5386/| Edmund Optics]</ref><ref>60x Objective - [https://www.edmundoptics.com.sg/p/60x-din-achromactic-finite-intl-standard-objective/3137/| Edmund Optics]</ref>. Hence, when using the 60x objective, the sample is practically kissing the objective.<br />
<br />
The use of a micrometer screw translation stage allowed for finer control over the position of our sample to a precision of <math>\pm 10</math> microns (5 micron contribution from both ends of the measurement).<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample. Although its specific composition is unknown, it is expected that the overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<b>Series of permanent magnets</b> <br />
[[File:Magnetseperationfield.png|thumb|Magnetic field variation away from the magnet stack.]]<br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. This is as described in <b> insert figure ref</b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. In this set of data, we observed the following... and chose the ideal polarization angle at an arbitrary rotation degree of ... After gleaning these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
(@Joel)<br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
Since Kerr imaging is based on the non-linear response of the sample with respect to the incident light intensity, we considered two alternative lighting configurations -- one in which the laser beam was collimated, and another in which the laser beam was focused onto the sample plane to maximize the incident intensity.<br />
<br />
<gallery widths=420px heights=250px><br />
File: Nonmagnetic images.png | Measured images from non-magnetic sample illuminated by a collimated beam.<br />
File: Nonmagnetic focused images.png | Measured images from non-magnetic sample illuminated by a focused beam.<br />
File: Magnetic images.png | Measured images from magnetic sample illuminated by a collimated beam.<br />
File: Magnetic focused images.png | Measured images from magnetic sample illuminated by a focused beam.<br />
</gallery><br />
<br />
The four figures above depict the raw measurements captured by our Kerr microscopy setup for the four different cases corresponding to the type of sample we were imaging as well as the lighting configuration. The labeled distance of <math>x\,\mu </math>m above each individual image denotes the estimated separation distance between the bar magnet and the back of the sample. In all cases, the magnification was kept the same. The vastly different features seen between the 4 cases are likely due to different areas of each sample being imaged upon changing the lighting and sample configurations.<br />
<br />
<gallery widths=500px heights=340px><br />
File: Intensity vs distance.png | Summed image intensities against sample-magnet distance.<br />
</gallery><br />
<br />
In order to obtain a more quantitative evaluation of whether the Kerr effect is present in our measurements, we plotted the total summed intensity in each captured image and plotted the result with respect to the sample-magnet distance (i.e. external magnetic field strength). For simpler comparison, we also normalized the graphs such that the maximum intensity is 1. Interestingly, we observed a rather consistent intensity across all sample-magnet distances for the focused laser beam, as opposed to the increasing trend for the collimated laser beam case. The sudden jump of intensity from 1 to ~0.7 in the top right quadrant is likely due to an accidental change in our setup apparatus whilst we were moving the sample stage from 250 <math>\mu</math>m to 300 <math>\mu</math>m.<br />
<br />
The non-increasing trend observed in the top two quadrants are consistent with what we expect from theory. For a non-magnetic sample, the MOKE should not be present as silicon on its own is not magnetic, and therefore should not result in a change in light polarization. Based on the magnetic field strength as a function of distance previously measured, we believe that the field strength experienced by the sample even at the furthest distance of 500 <math>\mu</math>m is strong enough to saturate the magnetization of the sample. Therefore, we are unable to observe any changes in the intensity of light for this range of distances. Due to time limitation, we were unable to go back to the lab to collect more data for distances further away where the field strength should be small enough for us to theoretically recover the hysteresis curve we expect from MOKE.<br />
<br />
For the increasing trends seen in the bottom two quadrants, we suspect that these are evidence of systematic errors during our measurement process because they are inconsistent with what we expect to observe from our simple setup. For the non-magnetic material illuminated with collimated light, it should also result in an unchanged intensity regardless of the external magnetic field. On the other hand for the magnetic sample,<br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of ten microns. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2853Kerr Microscope2022-04-29T16:55:42Z<p>MayInn: /* Results and Analysis */</p>
<hr />
<div><blockquote> "I was led some time ago to think it very likely, that if a beam of plane-polarized light were reflected under proper conditions from the surface of intensely magnetized iron, it would have its plane of polarization turned through a sensible angle in the process of reflection." - John Kerr<ref>J.Kerr, Philosophical Magazine 3 (1877) p.312.</ref></blockquote><br />
<br />
Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| 2x Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
<br />
<b>Magnetic domains</b><br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains. When multiple magnetic domains are formed, magnetostatic energy in the system decreases as the net magnetization of the system is reduced. Common instances where we can find magnetic domains would be in the random arrangement of magnetic domains on fridge magnets, and in specific forms within magnetic recording devices such as Video Home System (VHS) tapes. Several domain imaging techniques can be used to observe and study these magnetic domains, and the most inexpensive, time saving, and least intrusive method would be through Magneto-optical Kerr Effect imaging technique. <br />
<br />
<b>Magneto-Optical Kerr Effect (MOKE)</b><br />
<br />
Conceived by John Kerr in 1877<ref>P.Weinberger writes about Kerr's famous communications to the Philosophical Magazine - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>, the magneto-optic Kerr effect (MOKE) describes the rotation of light polarization when reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
<b>MOKE microscopy</b><br />
<br />
Interaction between an incident light and the magnetization of a magnetic sample causes a change in polarization state of the incident light. Varying magnetizations corresponding to different magnetic domains on the sample gives rise to different degrees of incident light polarization. By detecting and imaging these reflected or transmitted interacted light, an image of magnetic domains with varying intensities can be observed, thereby allowing for domain imaging to be conducted. <br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner, in accordance to data acquisition conditions and the sampled materials' suitability.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED connected in conjunction with a current limiting resistor. The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole after Mirror 2.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
* Added micrometer screw translation stage for sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the <i>tube length</i>, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>. A second parameter that must be kept in mind is the <i>working distance</i>, which is the distance that the sample must be placed in front of the objective. For the 10x and 60x objective, these are 1.5mm and 0.15mm respectively<ref>10x Objective - [https://www.edmundoptics.com/p/10x-din-plan-commercial-grade-objective/5386/| Edmund Optics]</ref><ref>60x Objective - [https://www.edmundoptics.com.sg/p/60x-din-achromactic-finite-intl-standard-objective/3137/| Edmund Optics]</ref>. Hence, when using the 60x objective, the sample is practically kissing the objective.<br />
<br />
The use of a micrometer screw translation stage allowed for finer control over the position of our sample to a precision of <math>\pm 10</math> microns (5 micron contribution from both ends of the measurement).<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample. Although its specific composition is unknown, it is expected that the overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<b>Series of permanent magnets</b> <br />
Magnetseperationfield.png<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. This is as described in <b> insert figure ref</b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. In this set of data, we observed the following... and chose the ideal polarization angle at an arbitrary rotation degree of ... After gleaning these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
(@Joel)<br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
Since Kerr imaging is based on the non-linear response of the sample with respect to the incident light intensity, we considered two alternative lighting configurations -- one in which the laser beam was collimated, and another in which the laser beam was focused onto the sample plane to maximize the incident intensity.<br />
<br />
<gallery widths=420px heights=250px><br />
File: Nonmagnetic images.png | Measured images from non-magnetic sample illuminated by a collimated beam.<br />
File: Nonmagnetic focused images.png | Measured images from non-magnetic sample illuminated by a focused beam.<br />
File: Magnetic images.png | Measured images from magnetic sample illuminated by a collimated beam.<br />
File: Magnetic focused images.png | Measured images from magnetic sample illuminated by a focused beam.<br />
</gallery><br />
<br />
The four figures above depict the raw measurements captured by our Kerr microscopy setup for the four different cases corresponding to the type of sample we were imaging as well as the lighting configuration. The labeled distance of <math>x\,\mu </math>m above each individual image denotes the estimated separation distance between the bar magnet and the back of the sample. In all cases, the magnification was kept the same. The vastly different features seen between the 4 cases are likely due to different areas of each sample being imaged upon changing the lighting and sample configurations.<br />
<br />
<gallery widths=500px heights=340px><br />
File: Intensity vs distance.png | Summed image intensities against sample-magnet distance.<br />
</gallery><br />
<br />
In order to obtain a more quantitative evaluation of whether the Kerr effect is present in our measurements, we plotted the total summed intensity in each captured image and plotted the result with respect to the sample-magnet distance (i.e. external magnetic field strength). For simpler comparison, we also normalized the graphs such that the maximum intensity is 1. Interestingly, we observed a rather consistent intensity across all sample-magnet distances for the focused laser beam, as opposed to the increasing trend for the collimated laser beam case. The sudden jump of intensity from 1 to ~0.7 in the top right quadrant is likely due to an accidental change in our setup apparatus whilst we were moving the sample stage from 250 <math>\mu</math>m to 300 <math>\mu</math>m.<br />
<br />
The non-increasing trend observed in the top two quadrants are consistent with what we expect from theory. For a non-magnetic sample, the MOKE should not be present as silicon on its own is not magnetic, and therefore should not result in a change in light polarization. Based on the magnetic field strength as a function of distance previously measured, we believe that the field strength experienced by the sample even at the furthest distance of 500 <math>\mu</math>m is strong enough to saturate the magnetization of the sample. Therefore, we are unable to observe any changes in the intensity of light for this range of distances. Due to time limitation, we were unable to go back to the lab to collect more data for distances further away where the field strength should be small enough for us to theoretically recover the hysteresis curve we expect from MOKE.<br />
<br />
For the increasing trends seen in the bottom two quadrants, we suspect that these are evidence of systematic errors during our measurement process because they are inconsistent with what we expect to observe from our simple setup. For the non-magnetic material illuminated with collimated light, it should also result in an unchanged intensity regardless of the external magnetic field. On the other hand for the magnetic sample,<br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of ten microns. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2826Kerr Microscope2022-04-29T15:12:46Z<p>MayInn: /* Theory */</p>
<hr />
<div><blockquote> "I was led some time ago to think it very likely, that if a beam of plane-polarized light were reflected under proper conditions from the surface of intensely magnetized iron, it would have its plane of polarization turned through a sensible angle in the process of reflection." - John Kerr<ref>J.Kerr, Philosophical Magazine 3 (1877) p.312.</ref></blockquote><br />
<br />
Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| 2x Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
<br />
<b>Magnetic domains</b><br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains. When multiple magnetic domains are formed, magnetostatic energy in the system decreases as the net magnetization of the system is reduced. Common instances where we can find magnetic domains would be in the random arrangement of magnetic domains on fridge magnets, and in specific forms within magnetic recording devices such as Video Home System (VHS) tapes. Several domain imaging techniques can be used to observe and study these magnetic domains, and the most inexpensive, time saving, and least intrusive method would be through Magneto-optical Kerr Effect imaging technique. <br />
<br />
<b>Magneto-Optical Kerr Effect (MOKE)</b><br />
<br />
Conceived by John Kerr in 1877<ref>P.Weinberger writes about Kerr's famous communications to the Philosophical Magazine - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>, the magneto-optic Kerr effect (MOKE) describes the rotation of light polarization when reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
<b>MOKE microscopy</b><br />
<br />
Interaction between an incident light and the magnetization of a magnetic sample causes a change in polarization state of the incident light. Varying magnetizations corresponding to different magnetic domains on the sample gives rise to different degrees of incident light polarization. By detecting and imaging these reflected or transmitted interacted light, an image of magnetic domains with varying intensities can be observed, thereby allowing for domain imaging to be conducted. <br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner, in accordance to data acquisition conditions and the sampled materials' suitability.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED connected in conjunction with a current limiting resistor. The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole after Mirror 2.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
* Added micrometer screw translation stage for sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the <i>tube length</i>, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>. A second parameter that must be kept in mind is the <i>working distance</i>, which is the distance that the sample must be placed in front of the objective. For the 10x and 60x objective, these are 1.5mm and 0.15mm respectively<ref>10x Objective - [https://www.edmundoptics.com/p/10x-din-plan-commercial-grade-objective/5386/| Edmund Optics]</ref><ref>60x Objective - [https://www.edmundoptics.com.sg/p/60x-din-achromactic-finite-intl-standard-objective/3137/| Edmund Optics]</ref>. Hence, when using the 60x objective, the sample is practically kissing the objective.<br />
<br />
The use of a micrometer screw translation stage allowed for finer control over the position of our sample to a precision of <math>\pm 10</math> microns (5 micron contribution from both ends of the measurement).<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample. Although its specific composition is unknown, it is expected that the overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<b>Series of permanent magnets</b> <br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. This is as described in <b> insert figure ref</b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. In this set of data, we observed the following... and chose the ideal polarization angle at an arbitrary rotation degree of ... After gleaning these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
(@Joel)<br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
Since Kerr imaging is based on the non-linear response of the sample with respect to the incident light intensity, we considered two alternative lighting configurations -- one in which the laser beam was collimated, and another in which the laser beam was focused onto the sample plane to maximize the incident intensity.<br />
<br />
<gallery widths=420px heights=250px><br />
File: Nonmagnetic images.png | Measured images from non-magnetic sample illuminated by a collimated beam.<br />
File: Nonmagnetic focused images.png | Measured images from non-magnetic sample illuminated by a focused beam.<br />
File: Magnetic images.png | Measured images from magnetic sample illuminated by a collimated beam.<br />
File: Magnetic focused images.png | Measured images from magnetic sample illuminated by a focused beam.<br />
</gallery><br />
<br />
The four figures above depict the raw measurements captured by our Kerr microscopy setup for the four different cases corresponding to the type of sample we were imaging as well as the lighting configuration. The labeled distance of <math>x\,\mu </math>m above each individual image denotes the estimated separation distance between the bar magnet and the back of the sample. In all cases, the magnification was kept the same. The vastly different features seen between the 4 cases are likely due to different areas of each sample being imaged upon changing the lighting and sample configurations.<br />
<br />
<gallery widths=500px heights=340px><br />
File: Intensity vs distance.png | Summed image intensities against sample-magnet distance.<br />
</gallery><br />
<br />
In order to obtain a more quantitative evaluation of whether the Kerr effect is present in our measurements, we plotted the total summed intensity in each captured image and plotted the result with respect to the sample-magnet distance (i.e. external magnetic field strength). For simpler comparison, we also normalized the graphs such that the maximum intensity is 1. Interestingly, we observed a rather consistent intensity across all sample-magnet distances for the focused laser beam, as opposed to the increasing trend for the collimated laser beam case. The sudden jump of intensity from 1 to ~0.7 in the top right quadrant is likely due to an accidental change in our setup apparatus whilst we were moving the sample stage from 250 <math>\mu</math>m to 300 <math>\mu</math>m.<br />
<br />
The non-increasing trend observed in the top two quadrants are consistent with what we expect from theory. For a non-magnetic sample, the MOKE should not be present as silicon on its own is not magnetic, and therefore should not result in a change in light polarization. Based on the magnetic field strength as a function of distance previously measured, we believe that the field strength experienced by the sample even at the furthest distance of 500 <math>\mu</math>m is strong enough to saturate the magnetization of the sample. Therefore, we are unable to observe any changes in the intensity of light for this range of distances. Due to time limitation, we were unable to go back to the lab to collect more data for distances further away where the field strength should be small enough for us to theoretically recover the hysteresis curve we expect from MOKE.<br />
<br />
For the increasing trends seen in the bottom two quadrants, we suspect that these are evidence of systematic errors during our measurement process because they are inconsistent with what we expect to observe from our simple setup. For the non-magnetic material illuminated with collimated light, it should also result in an unchanged intensity regardless of the external magnetic field. On the other hand for the magnetic sample,<br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of ten microns. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2818Kerr Microscope2022-04-29T14:33:14Z<p>MayInn: /* Theory */</p>
<hr />
<div><blockquote> "I was led some time ago to think it very likely, that if a beam of plane-polarized light were reflected under proper conditions from the surface of intensely magnetized iron, it would have its plane of polarization turned through a sensible angle in the process of reflection." - John Kerr<ref>J.Kerr, Philosophical Magazine 3 (1877) p.312.</ref></blockquote><br />
<br />
Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| 2x Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
<br />
<b>Magnetic domains</b><br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains. When multiple magnetic domains are formed, magnetostatic energy in the system decreases as the net magnetization of the system is reduced. Common instances where we can find magnetic domains would be in the random arrangement of magnetic domains on fridge magnets, and in specific forms within magnetic recording devices such as Video Home System (VHS) tapes. Several domain imaging techniques can be used to observe and study these magnetic domains, and the most inexpensive, time saving, and least intrusive method would be through Magneto-optical Kerr Effect imaging technique. <br />
<br />
<b>Magneto-Optical Kerr Effect (MOKE)</b><br />
<br />
Conceived by John Kerr in 1877<ref>P.Weinberger writes about Kerr's famous communications to the Philosophical Magazine - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>, the magneto-optic Kerr effect (MOKE) describes the rotation of light polarization when reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
<b>MOKE microscopy</b><br />
<br />
Interaction between an incident light and the magnetization of a magnetic sample causes a change in polarization state of the incident light. Varying magnetizations corresponding to different magnetic domains on the sample gives rise to different degrees of incident light polarization. By detecting and imaging these reflected or transmitted interacted light, an image of magnetic domains with varying intensities can be observed, thereby allowing for domain imaging to be conducted. <br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED connected in conjunction with a current limiting resistor. The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole after Mirror 2.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
* Added micrometer screw translation stage for sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the <i>tube length</i>, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>. A second parameter that must be kept in mind is the <i>working distance</i>, which is the distance that the sample must be placed in front of the objective. For the 10x and 60x objective, these are 1.5mm and 0.15mm respectively<ref>10x Objective - [https://www.edmundoptics.com/p/10x-din-plan-commercial-grade-objective/5386/| Edmund Optics]</ref><ref>60x Objective - [https://www.edmundoptics.com.sg/p/60x-din-achromactic-finite-intl-standard-objective/3137/| Edmund Optics]</ref>. Hence, when using the 60x objective, the sample is practically kissing the objective.<br />
<br />
The use of a micrometer screw translation stage allowed for finer control over the position of our sample to a precision of <math>\pm 10</math> microns (5 micron contribution from both ends of the measurement).<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample. Although its specific composition is unknown, it is expected that the overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<b>Series of permanent magnets</b> <br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. This is as described in <b> insert figure ref</b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. In this set of data, we observed the following... and chose the ideal polarization angle at an arbitrary rotation degree of ... After gleaning these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
(@Joel)<br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
Since Kerr imaging is based on the non-linear response of the sample with respect to the incident light intensity, we considered two alternative lighting configurations -- one in which the laser beam was collimated, and another in which the laser beam was focused onto the sample plane to maximize the incident intensity.<br />
<br />
<gallery widths=420px heights=250px><br />
File: Nonmagnetic images.png | Measured images from non-magnetic sample illuminated by a collimated beam.<br />
File: Nonmagnetic focused images.png | Measured images from non-magnetic sample illuminated by a focused beam.<br />
File: Magnetic images.png | Measured images from magnetic sample illuminated by a collimated beam.<br />
File: Magnetic focused images.png | Measured images from magnetic sample illuminated by a focused beam.<br />
</gallery><br />
<br />
The four figures above depict the raw measurements captured by our Kerr microscopy setup for the four different cases corresponding to the type of sample we were imaging as well as the lighting configuration. The labeled distance of <math>x\,\mu </math>m above each individual image denotes the estimated separation distance between the bar magnet and the back of the sample. In all cases, the magnification was kept the same. The vastly different features seen between the 4 cases are likely due to different areas of each sample being imaged upon changing the lighting and sample configurations.<br />
<br />
<gallery widths=500px heights=340px><br />
File: Intensity vs distance.png | Summed image intensities against sample-magnet distance.<br />
</gallery><br />
<br />
In order to obtain a more quantitative evaluation of whether the Kerr effect is present in our measurements, we plotted the total summed intensity in each captured image and plotted the result with respect to the sample-magnet distance (i.e. external magnetic field strength). For simpler comparison, we also normalized the graphs such that the maximum intensity is 1. Interestingly, we observed a rather consistent intensity across all sample-magnet distances for the focused laser beam, as opposed to the increasing trend for the collimated laser beam case. The sudden jump of intensity from 1 to ~0.7 in the top right quadrant is likely due to an accidental change in our setup apparatus whilst we were moving the sample stage from 250 <math>\mu</math>m to 300 <math>\mu</math>m.<br />
<br />
The non-increasing trend observed in the top two quadrants are consistent with what we expect from theory. For a non-magnetic sample, the MOKE should not be present as silicon on its own is not magnetic, and therefore should not result in a change in light polarization. Based on the magnetic field strength as a function of distance previously measured, we believe that the field strength experienced by the sample even at the furthest distance of 500 <math>\mu</math>m is strong enough to saturate the magnetization of the sample. Therefore, we are unable to observe any changes in the intensity of light for this range of distances. Due to time limitation, we were unable to go back to the lab to collect more data for distances further away where the field strength should be small enough for us to theoretically recover the hysteresis curve we expect from MOKE.<br />
<br />
For the increasing trends seen in the bottom two quadrants, we suspect that these are evidence of systematic errors during our measurement process because they are inconsistent with what we expect to observe from our simple setup. For the non-magnetic material illuminated with collimated light, it should also result in an unchanged intensity regardless of the external magnetic field. On the other hand for the magnetic sample,<br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of ten microns. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2475Kerr Microscope2022-04-29T05:44:47Z<p>MayInn: /* Theory */</p>
<hr />
<div><blockquote> "I was led some time ago to think it very likely, that if a beam of plane-polarized light were reflected under proper conditions from the surface of intensely magnetized iron, it would have its plane of polarization turned through a sensible angle in the process of reflection." - John Kerr<ref>J.Kerr, Philosophical Magazine 3 (1877) p.312.</ref></blockquote><br />
<br />
Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| 2x Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
<br />
<b>Magnetic domains</b><br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains. When multiple magnetic domains are formed, magnetostatic energy in the system decreases as the net magnetization of the system is reduced. Common instances where we can find magnetic domains would be in the random arrangement of magnetic domains on fridge magnets, and in specific forms within magnetic recording devices such as Video Home System (VHS) tapes. Several domain imaging techniques can be used to observe and study these magnetic domains, and the most inexpensive, time saving, and least intrusive method would be through Magneto-optical Kerr Effect imaging technique. <br />
<br />
<b>Magneto-Optical Kerr Effect (MOKE)</b><br />
<br />
Conceived by John Kerr in 1877<ref>P.Weinberger writes about Kerr's famous communications to the Philosophical Magazine - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>, the magneto-optic Kerr effect (MOKE) describes the rotation of light polarization when reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
<b>MOKE microscopy</b><br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED connected in conjunction with a current limiting resistor. The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole after Mirror 2.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
* Added micrometer screw translation stage for sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the <i>tube length</i>, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>. A second parameter that must be kept in mind is the <i>working distance</i>, which is the distance that the sample must be placed in front of the objective. For the 10x and 60x objective, these are 1.5mm and 0.15mm respectively<ref>10x Objective - [https://www.edmundoptics.com/p/10x-din-plan-commercial-grade-objective/5386/| Edmund Optics]</ref><ref>60x Objective - [https://www.edmundoptics.com.sg/p/60x-din-achromactic-finite-intl-standard-objective/3137/| Edmund Optics]</ref>. Hence, when using the 60x objective, the sample is practically kissing the objective.<br />
<br />
The use of a micrometer screw translation stage allowed for finer control over the position of our sample to a precision of <math>\pm 10</math> microns (5 micron contribution from both ends of the measurement).<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample. Although its specific composition is unknown, it is expected that the overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<br />
(@Joel)<br />
<br />
<br />
<br />
<b>Series of permanent magnets</b> <br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. This is as described in <b> insert figure ref</b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. In this set of data, we observed the following... and chose the ideal polarization angle at an arbitrary rotation degree of ... After gleaning these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of ten microns. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2459Kerr Microscope2022-04-29T04:55:26Z<p>MayInn: /* Theory */</p>
<hr />
<div><blockquote> "I was led some time ago to think it very likely, that if a beam of plane-polarized light were reflected under proper conditions from the surface of intensely magnetized iron, it would have its plane of polarization turned through a sensible angle in the process of reflection." - John Kerr<ref>J.Kerr, Philosophical Magazine 3 (1877) p.312.</ref></blockquote><br />
<br />
Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| 2x Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
<br />
<b>Magnetic domains</b><br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains. When multiple magnetic domains are formed, magnetostatic energy in the system decreases as the net magnetization of the system is reduced. Common instances where we can find magnetic domains would be in the random arrangement of magnetic domains on fridge magnets, and in magnetic recording devices such as Video Home System (VHS) tapes. Several domain imaging techniques can be used to observe and study these magnetic domains, and the most inexpensive, time saving, and least intrusive method would be through Magneto-optical Kerr Effect imaging technique. <br />
<br />
<b>Magneto-Optical Kerr Effect (MOKE)</b><br />
<br />
Conceived by John Kerr in 1877<ref>P.Weinberger writes about Kerr's famous communications to the Philosophical Magazine - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>, the magneto-optic Kerr effect (MOKE) describes the rotation of light polarization when reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
<b>MOKE microscopy</b><br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED connected in conjunction with a current limiting resistor. The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole after Mirror 2.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
* Added micrometer screw translation stage for sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the <i>tube length</i>, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>. A second parameter that must be kept in mind is the <i>working distance</i>, which is the distance that the sample must be placed in front of the objective. For the 10x and 60x objective, these are 1.5mm and 0.15mm respectively<ref>10x Objective - [https://www.edmundoptics.com/p/10x-din-plan-commercial-grade-objective/5386/| Edmund Optics]</ref><ref>60x Objective - [https://www.edmundoptics.com.sg/p/60x-din-achromactic-finite-intl-standard-objective/3137/| Edmund Optics]</ref>. Hence, when using the 60x objective, the sample is practically kissing the objective.<br />
<br />
The use of a micrometer screw translation stage allowed for finer control over the position of our sample to a precision of <math>\pm 10</math> microns (5 micron contribution from both ends of the measurement).<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample. Although its specific composition is unknown, it is expected that the overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<br />
(@Joel)<br />
<br />
<br />
<br />
<b>Series of permanent magnets</b> <br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. This is as described in <b> insert figure ref</b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. In this set of data, we observed the following... and chose the ideal polarization angle at an arbitrary rotation degree of ... After gleaning these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of ten microns. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2457Kerr Microscope2022-04-29T04:49:16Z<p>MayInn: /* Theory */</p>
<hr />
<div>Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| 2x Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
<br />
<b>Magnetic domains</b><br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains. When multiple magnetic domains are formed, magnetostatic energy in the system decreases as the net magnetization of the system is reduced. Common instances where we can find magnetic domains would be in the random arrangement of magnetic domains on fridge magnets, and in magnetic recording devices such as Video Home System (VHS) tapes. Several domain imaging techniques can be used to observe and study these magnetic domains, and the most inexpensive, time saving, and least intrusive method would be through Magneto-optical Kerr Effect imaging technique. <br />
<br />
<b>Magneto-Optical Kerr Effect (MOKE)</b><br />
<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
Conceived by John Kerr in 1877<ref>P.Weinberger writes about Kerr's famous communications to the Philosophical Magazine - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>, the magneto-optic Kerr effect (MOKE) describes the rotation of light polarization when reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
<b>MOKE microscopy</b><br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED connected in conjunction with a current limiting resistor. The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole after Mirror 2.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
* Added micrometer screw translation stage for sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the <i>tube length</i>, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>. A second parameter that must be kept in mind is the <i>working distance</i>, which is the distance that the sample must be placed in front of the objective. For the 10x and 60x objective, these are 1.5mm and 0.15mm respectively<ref>10x Objective - [https://www.edmundoptics.com/p/10x-din-plan-commercial-grade-objective/5386/| Edmund Optics]</ref><ref>60x Objective - [https://www.edmundoptics.com.sg/p/60x-din-achromactic-finite-intl-standard-objective/3137/| Edmund Optics]</ref>. Hence, when using the 60x objective, the sample is practically kissing the objective.<br />
<br />
The use of a micrometer screw translation stage allowed for finer control over the position of our sample to a precision of <math>\pm 10</math> microns (5 micron contribution from both ends of the measurement).<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample. Although its specific composition is unknown, it is expected that the overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<br />
(@Joel)<br />
<br />
<br />
<br />
<b>Series of permanent magnets</b> <br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. This is as described in <b> insert figure ref</b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. In this set of data, we observed the following... and chose the ideal polarization angle at an arbitrary rotation degree of ... After gleaning these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of ten microns. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2456Kerr Microscope2022-04-29T04:48:23Z<p>MayInn: /* Theory */</p>
<hr />
<div>Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| 2x Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
<br />
<b>Magnetic domains</b><br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains. When multiple magnetic domains are formed, magnetostatic energy in the system decreases as the net magnetization of the system is reduced. Common instances where we can find magnetic domains would be in the random arrangement of magnetic domains on fridge magnets, and in magnetic recording devices such as Video Home System (VHS) tapes. Several domain imaging techniques can be used to observe and study these magnetic domains, and the most inexpensive, time saving, and least intrusive method would be through Magneto-optical Kerr Effect imaging technique. <br />
<br />
<b>Magneto-Optical Kerr Effect (MOKE)</b><br />
<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
Conceived by John Kerr in 1877<ref>P.Weinberger writes about Kerr's famous communications to the Philosophical Magazine - [https://web.archive.org/web/20110718214456/http://www.computational-nanoscience.de/Weinberger/Famous-Papers/PML-2008.pdf| Wayback Machine].</ref>, the magneto-optic Kerr effect (MOKE) describes the rotation of light polarization when reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED connected in conjunction with a current limiting resistor. The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole after Mirror 2.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
* Added micrometer screw translation stage for sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the <i>tube length</i>, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>. A second parameter that must be kept in mind is the <i>working distance</i>, which is the distance that the sample must be placed in front of the objective. For the 10x and 60x objective, these are 1.5mm and 0.15mm respectively<ref>10x Objective - [https://www.edmundoptics.com/p/10x-din-plan-commercial-grade-objective/5386/| Edmund Optics]</ref><ref>60x Objective - [https://www.edmundoptics.com.sg/p/60x-din-achromactic-finite-intl-standard-objective/3137/| Edmund Optics]</ref>. Hence, when using the 60x objective, the sample is practically kissing the objective.<br />
<br />
The use of a micrometer screw translation stage allowed for finer control over the position of our sample to a precision of <math>\pm 10</math> microns (5 micron contribution from both ends of the measurement).<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample. Although its specific composition is unknown, it is expected that the overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<br />
(@Joel)<br />
<br />
<br />
<br />
<b>Series of permanent magnets</b> <br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. This is as described in <b> insert figure ref</b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. In this set of data, we observed the following... and chose the ideal polarization angle at an arbitrary rotation degree of ... After gleaning these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of ten microns. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2438Kerr Microscope2022-04-29T04:14:54Z<p>MayInn: /* Theory */</p>
<hr />
<div>Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
Conceived by John Kerr in the 1980s, the magneto-optic Kerr effect (MOKE) describes the changes to light reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
<b>MOKE imaging modes</b><br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
<br />
<b>Magnetic domains</b><br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the tube length, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>.<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample. Although its specific composition is unknown, it is expected that the overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<br />
(@Joel)<br />
<br />
<br />
<br />
<b>Series of permanent magnets</b> <br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. This is as described in <b> insert figure ref</b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. In this set of data, we observed the following... and chose the ideal polarization angle at an arbitrary rotation degree of ... After gleaning these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of half a millimeter. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2436Kerr Microscope2022-04-29T04:13:55Z<p>MayInn: /* Theory */</p>
<hr />
<div>Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
Conceived by John Kerr in the 1980s, the magneto-optic Kerr effect (MOKE) describes the changes to light reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
<br />
<b>Magnetic domains</b><br />
<br />
In magnetic materials, there exists magnetic dipoles wherein their magnetic interactions with each other are called dipolar interactions which are related to their separation. Such interactions result in the formation of regions of uniform magnetization, also known as magnetic domains.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the tube length, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>DIN Standard Microscope Objective Lenses - [https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable| Microscope World].</ref>.<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample. Although its specific composition is unknown, it is expected that the overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<br />
(@Joel)<br />
<br />
<br />
<br />
<b>Series of permanent magnets</b> <br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. This is as described in <b> insert figure ref</b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. In this set of data, we observed the following... and chose the ideal polarization angle at an arbitrary rotation degree of ... After gleaning these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of half a millimeter. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2427Kerr Microscope2022-04-29T03:58:30Z<p>MayInn: /* Improvements and Reflections */</p>
<hr />
<div>Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
Conceived by John Kerr in the 1980s, the magneto-optic Kerr effect (MOKE) describes the changes to light reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the tube length, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>{{Cite web<br />
| title = DIN Standard Microscope Objective Lenses<br />
| author = MicroscopeWorld<br />
| work = nabis.fisi.polimi.it<br />
| date = 24 September 2012<br />
| access-date = 31 March 2022<br />
| url = https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable.<br />
| quote = <br />
}}</ref>.<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample. Although its specific composition is unknown, it is expected that the overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<br />
(@Joel)<br />
<br />
<br />
<br />
<b>Series of permanent magnets</b> <br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. This is as described in <b> insert figure ref</b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. In this set of data, we observed the following... and chose the ideal polarization angle at an arbitrary rotation degree of ... After gleaning these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Surface conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of half a millimeter. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2426Kerr Microscope2022-04-29T03:58:14Z<p>MayInn: /* Improvements and Reflections */</p>
<hr />
<div>Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
Conceived by John Kerr in the 1980s, the magneto-optic Kerr effect (MOKE) describes the changes to light reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Angled Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|200px|right|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
[[File: Red laser pointer dirty.jpeg|200px|right|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. <br />
<br />
[[File: Kerr mirror near.jpeg|250px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|250px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup. The two mirrors facilitate beam alignment.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the tube length, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>{{Cite web<br />
| title = DIN Standard Microscope Objective Lenses<br />
| author = MicroscopeWorld<br />
| work = nabis.fisi.polimi.it<br />
| date = 24 September 2012<br />
| access-date = 31 March 2022<br />
| url = https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable.<br />
| quote = <br />
}}</ref>.<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample. Although its specific composition is unknown, it is expected that the overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<br />
(@Joel)<br />
<br />
<br />
<br />
<b>Series of permanent magnets</b> <br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. This is as described in <b> insert figure ref</b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. In this set of data, we observed the following... and chose the ideal polarization angle at an arbitrary rotation degree of ... After gleaning these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
<b>Sample conditions</b> - The samples used did not exactly have perfectly smooth surfaces which could have contributed to the scattering observed. For instance, the Si/SiO2 empty substrate had scratches on it, likely due to inadequate handling, and extra efforts had to be implemented to avoid such regions. Better handling and care for the surfaces would be recommended as MOKE is a surface related technique.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of half a millimeter. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2423Kerr Microscope2022-04-29T03:54:35Z<p>MayInn: /* Setup 2: Microscope Setup */</p>
<hr />
<div>Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. [[Kerr Microscope#Setup 1: Angled Setup|Setup 1]] reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). [[Kerr Microscope#Setup 2: Microscope Setup|Setup 2]] more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Microscope Setup 10x Objective<br />
|-<br />
! scope=row | 11<br />
| Lab Magnetic Sample and VFL light source<br />
|-<br />
! scope=row | 12<br />
| 60x Objective<br />
|-<br />
! scope=row | 13<br />
| Final Experimental Readings<br />
|}<br />
<br />
==Theory==<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material. Therefore, light entering the material would be slowed by different amounts depending on its polarization.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
Conceived by John Kerr in the 1980s, the magneto-optic Kerr effect (MOKE) describes the changes to light reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Experimental Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Location: S11-02-04<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
* Magnetic samples<br />
** Steel sheet wound with copper wire<br />
** Magnetic tape from floppy disk & cassette tape<br />
** Magnetic film on Si/SiO2 substrate (lab sample)<br />
<br />
This section details the two main iterations of our experimental setup.<br />
<br />
===Setup 1: Angled Setup===<br />
<br />
As a first observation of the MOKE, we utilized a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|thumb|Setup 1. The laser pointer is mounted on an acrylic stand shown in bottom left of image.]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. [[File: Red laser pointer dirty.jpeg|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
[[File: Kerr mirror near.jpeg|300px|left|thumb|Mirror 1]]<br />
[[File: Kerr mirror far.jpeg|300px|left|thumb|Mirror 2]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain a good image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
===Setup 2: Microscope Setup===<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a single mode fiber for an even cleaner light source.<br />
* Swapped to a lab prepared magnetic sample.<br />
<br />
When working with microscope objectives, it is important to be aware of the tube length, which is the distance between the objective and the image produced by the objective. We used an objective that was manufactured according to the DIN standard, which specifies a 160mm tube length. Hence, we positioned our CCD array 160mm away from the objective to capture the image. If working with an RMS objective, the tube length is 170mm instead<ref>{{Cite web<br />
| title = DIN Standard Microscope Objective Lenses<br />
| author = MicroscopeWorld<br />
| work = nabis.fisi.polimi.it<br />
| date = 24 September 2012<br />
| access-date = 31 March 2022<br />
| url = https://blog.microscopeworld.com/2012/09/din-standard-microscope-objective-lenses.html#:~:text=A%20typical%20DIN%20standard%20microscope,Most%20DIN%20optics%20are%20interchangeable.<br />
| quote = <br />
}}</ref>.<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard empty Si/SiO2 substrate as a control sample. And next, we have a magnetic sample. Although its specific composition is unknown, it is expected that the overall intensity garnered from the setup is to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<br />
(@Joel)<br />
<br />
<br />
<br />
<b>Series of permanent magnets</b> <br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. This is as described in <b> insert figure ref</b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes locating the ideal polarization angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. In this set of data, we observed the following... and chose the ideal polarization angle at an arbitrary rotation degree of ... After gleaning these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realized that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
==Improvements and Reflections==<br />
<br />
This section contains our reflections on the experiment and some thoughts on how we, or anyone else wishing to reproduce and improve, could have done better.<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting, deburring and so forth. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample was the most costly in terms of experimental runtime. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for holding our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
<b>Less monochromatic/coherent light source</b> - Using a red laser gave unwanted interference patterns when illuminating our light source. This made it difficult to discern MOKE effects. Using the laser diode (not the laser pen), we attempted a workaround where we reduced the voltage supply to just below the lasing threshold. In this regime, the diode cannot lase and behaves closer to an LED with a broader bandwidth. However, this also reduced the intensity of the light hitting the sample to the point where we could barely see it. If tasked to redo the experiment, we would put more care into sourcing an appropriate light source for our needs.<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of half a millimeter. However, we could not directly observe the magnetization characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2195Kerr Microscope2022-04-28T13:01:54Z<p>MayInn: /* Results and Analysis */</p>
<hr />
<div>Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. Setup 1 reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). Setup 2 more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Troubleshooting at NPBS interface<br />
|-<br />
! scope=row | 11<br />
| New Magnetic Sample and Light Source<br />
|-<br />
! scope=row | 12<br />
| 60x<br />
|-<br />
! scope=row | 13<br />
| Final Setup<br />
|}<br />
<br />
==Setup==<br />
Location: S11-02-04<br />
<br />
==Theory==<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
Conceived by John Kerr in the 1980s, the magneto-optic Kerr effect (MOKE) describes the changes to light reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Setup 1: Angled Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
<br />
As a first observation of the MOKE, we utilised a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|thumb|]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. [[File: Red laser pointer dirty.jpeg|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain an image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
==Setup 2: Microscope Setup==<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a fiber for an even cleaner light source.<br />
* Swapped to a magnetic sample <br />
<br />
Samples:<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard Si/SiO2 substrate as a control sample. And next, we have a a magnetic film sample on top of a Si/SiO2 substrate. Although its specific composition is unknown, the magnetic sample is know to magnetically saturate at a field of about 0.1 T. From 0 T to 0.1 T, the domain density is known to decrease for this sample and correspondingly, we would expect the overall intensity garnered from the setup to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<br />
(@Joel)<br />
<br />
<br />
<br />
<b>Series of permanent magnets</b> <br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be -0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from -0.473 T to -0.005 T. This is as described in <b> insert figure ref</b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes the locating of the ideal polarisation angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. In this set of data, we observed the following... and chose the ideal polarization angle at an arbitrary rotation degree of ... After gleaning these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realised that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of half a millimeter. However, we could not directly observe the magnetisation characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Improvements and Reflections==<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting and deburring. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample also took a substantial amount of time. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2194Kerr Microscope2022-04-28T13:01:31Z<p>MayInn: /* Results and Analysis */</p>
<hr />
<div>Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. Setup 1 reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). Setup 2 more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Troubleshooting at NPBS interface<br />
|-<br />
! scope=row | 11<br />
| New Magnetic Sample and Light Source<br />
|-<br />
! scope=row | 12<br />
| 60x<br />
|-<br />
! scope=row | 13<br />
| Final Setup<br />
|}<br />
<br />
==Setup==<br />
Location: S11-02-04<br />
<br />
==Theory==<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
Conceived by John Kerr in the 1980s, the magneto-optic Kerr effect (MOKE) describes the changes to light reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Setup 1: Angled Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
<br />
As a first observation of the MOKE, we utilised a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|thumb|]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. [[File: Red laser pointer dirty.jpeg|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain an image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
==Setup 2: Microscope Setup==<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a fiber for an even cleaner light source.<br />
* Swapped to a magnetic sample <br />
<br />
Samples:<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard Si/SiO2 substrate as a control sample. And next, we have a a magnetic film sample on top of a Si/SiO2 substrate. Although its specific composition is unknown, the magnetic sample is know to magnetically saturate at a field of about 0.1 T. From 0 T to 0.1 T, the domain density is known to decrease for this sample and correspondingly, we would expect the overall intensity garnered from the setup to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<br />
(@Joel)<br />
<br />
<br />
<br />
<b>Series of permanent magnets</b> <br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be 0.473 T. By varying the separation between the magnet stack surface and the probe from 0 to 40 mm, we measured the external field to vary from 0.473 T to 0.005 T. This is as described in <b> insert figure ref</b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes the locating of the ideal polarisation angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. In this set of data, we observed the following... and chose the ideal polarization angle at an arbitrary rotation degree of ... After gleaning these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realised that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of half a millimeter. However, we could not directly observe the magnetisation characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Improvements and Reflections==<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting and deburring. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample also took a substantial amount of time. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2192Kerr Microscope2022-04-28T12:58:15Z<p>MayInn: /* Results and Analysis */</p>
<hr />
<div>Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. Setup 1 reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). Setup 2 more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Troubleshooting at NPBS interface<br />
|-<br />
! scope=row | 11<br />
| New Magnetic Sample and Light Source<br />
|-<br />
! scope=row | 12<br />
| 60x<br />
|-<br />
! scope=row | 13<br />
| Final Setup<br />
|}<br />
<br />
==Setup==<br />
Location: S11-02-04<br />
<br />
==Theory==<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
Conceived by John Kerr in the 1980s, the magneto-optic Kerr effect (MOKE) describes the changes to light reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Setup 1: Angled Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
<br />
As a first observation of the MOKE, we utilised a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|thumb|]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. [[File: Red laser pointer dirty.jpeg|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain an image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
==Setup 2: Microscope Setup==<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a fiber for an even cleaner light source.<br />
* Swapped to a magnetic sample <br />
<br />
Samples:<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard Si/SiO2 substrate as a control sample. And next, we have a a magnetic film sample on top of a Si/SiO2 substrate. Although its specific composition is unknown, the magnetic sample is know to magnetically saturate at a field of about 0.1 T. From 0 T to 0.1 T, the domain density is known to decrease for this sample and correspondingly, we would expect the overall intensity garnered from the setup to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<br />
(@Joel)<br />
<br />
<br />
<br />
<b>Series of permanent magnets</b> <br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be 0.473 T. By varying the separation between the magnet stack surface and the probe, we were able to obtain the relationship between the external field and separation from the magnet surface. This is as described in <b> insert figure ref</b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes the locating of the ideal polarisation angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. In this set of data, we observed the following... and chose the ideal polarization angle at an arbitrary rotation degree of ... After gleaning these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realised that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of half a millimeter. However, we could not directly observe the magnetisation characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Improvements and Reflections==<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting and deburring. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample also took a substantial amount of time. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2191Kerr Microscope2022-04-28T12:57:58Z<p>MayInn: /* Results and Analysis */</p>
<hr />
<div>Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. Setup 1 reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). Setup 2 more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Troubleshooting at NPBS interface<br />
|-<br />
! scope=row | 11<br />
| New Magnetic Sample and Light Source<br />
|-<br />
! scope=row | 12<br />
| 60x<br />
|-<br />
! scope=row | 13<br />
| Final Setup<br />
|}<br />
<br />
==Setup==<br />
Location: S11-02-04<br />
<br />
==Theory==<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
Conceived by John Kerr in the 1980s, the magneto-optic Kerr effect (MOKE) describes the changes to light reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Setup 1: Angled Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
<br />
As a first observation of the MOKE, we utilised a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|thumb|]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. [[File: Red laser pointer dirty.jpeg|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain an image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
==Setup 2: Microscope Setup==<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a fiber for an even cleaner light source.<br />
* Swapped to a magnetic sample <br />
<br />
Samples:<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard Si/SiO2 substrate as a control sample. And next, we have a a magnetic film sample on top of a Si/SiO2 substrate. Although its specific composition is unknown, the magnetic sample is know to magnetically saturate at a field of about 0.1 T. From 0 T to 0.1 T, the domain density is known to decrease for this sample and correspondingly, we would expect the overall intensity garnered from the setup to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<br />
(@Joel)<br />
<br />
<br />
<br />
<b>Series of permanent magnets</b> <br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be 473 mT. By varying the separation between the magnet stack surface and the probe, we were able to obtain the relationship between the external field and separation from the magnet surface. This is as described in <b> insert figure ref</b><br />
<br />
<b>Polarization dependent intensity changes</b><br />
<br />
In commercial MOKE microscopy systems, the very first few steps often includes the locating of the ideal polarisation angle which works with the specific sample. In this light, we had performed polarization angle dependent intensity studies to verify this point, without an external field provided by the magnet stack. We could determine which polarization angle (1) works best with our setup and camera, as well as (2) gives us decent signal to be able to observe changes in intensity. The former ensures that the camera is operational and not oversaturated during the data collection process. In this set of data, we observed the following... and chose the ideal polarization angle at an arbitrary rotation degree of ... After gleaning these insights on selection of polarization angle, we then proceed with measurements with the specific polarization angle. We also had realised that additional adjustments was necessary to our second polarizer so as to extinguish more of the intensity that the camera was picking up, as it was saturating too much. <br />
<br />
<b> Field dependent intensity changes</b><br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of half a millimeter. However, we could not directly observe the magnetisation characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Improvements and Reflections==<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting and deburring. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample also took a substantial amount of time. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2187Kerr Microscope2022-04-28T12:41:59Z<p>MayInn: /* Results and Analysis */</p>
<hr />
<div>Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. Setup 1 reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). Setup 2 more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Troubleshooting at NPBS interface<br />
|-<br />
! scope=row | 11<br />
| New Magnetic Sample and Light Source<br />
|-<br />
! scope=row | 12<br />
| 60x<br />
|-<br />
! scope=row | 13<br />
| Final Setup<br />
|}<br />
<br />
==Setup==<br />
Location: S11-02-04<br />
<br />
==Theory==<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
Conceived by John Kerr in the 1980s, the magneto-optic Kerr effect (MOKE) describes the changes to light reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Setup 1: Angled Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
<br />
As a first observation of the MOKE, we utilised a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|thumb|]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. [[File: Red laser pointer dirty.jpeg|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain an image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
==Setup 2: Microscope Setup==<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a fiber for an even cleaner light source.<br />
* Swapped to a magnetic sample <br />
<br />
Samples:<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard Si/SiO2 substrate as a control sample. And next, we have a a magnetic film sample on top of a Si/SiO2 substrate. Although its specific composition is unknown, the magnetic sample is know to magnetically saturate at a field of about 0.1 T. From 0 T to 0.1 T, the domain density is known to decrease for this sample and correspondingly, we would expect the overall intensity garnered from the setup to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<br />
(@Joel)<br />
<br />
<br />
<b>Series of permanent magnets</b> <br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be 473 mT. By varying the separation between the magnet stack surface and the probe, we were able to obtain the relationship between the external field and separation from the magnet surface. This is as described in <b> insert figure ref</b><br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of half a millimeter. However, we could not directly observe the magnetisation characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Improvements and Reflections==<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting and deburring. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample also took a substantial amount of time. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2186Kerr Microscope2022-04-28T12:41:41Z<p>MayInn: /* Results and Analysis */</p>
<hr />
<div>Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. Setup 1 reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). Setup 2 more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Troubleshooting at NPBS interface<br />
|-<br />
! scope=row | 11<br />
| New Magnetic Sample and Light Source<br />
|-<br />
! scope=row | 12<br />
| 60x<br />
|-<br />
! scope=row | 13<br />
| Final Setup<br />
|}<br />
<br />
==Setup==<br />
Location: S11-02-04<br />
<br />
==Theory==<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
Conceived by John Kerr in the 1980s, the magneto-optic Kerr effect (MOKE) describes the changes to light reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Setup 1: Angled Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
<br />
As a first observation of the MOKE, we utilised a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|thumb|]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. [[File: Red laser pointer dirty.jpeg|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain an image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
==Setup 2: Microscope Setup==<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a fiber for an even cleaner light source.<br />
* Swapped to a magnetic sample <br />
<br />
Samples:<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard Si/SiO2 substrate as a control sample. And next, we have a a magnetic film sample on top of a Si/SiO2 substrate. Although its specific composition is unknown, the magnetic sample is know to magnetically saturate at a field of about 0.1 T. From 0 T to 0.1 T, the domain density is known to decrease for this sample and correspondingly, we would expect the overall intensity garnered from the setup to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<br />
(@Joel)<br />
<br />
<br />
<b>Series of permanent magnets</b> <br />
<br />
In this project, we were provided with numerous tiny disc magnets. By stacking these disc magnets one on top of the other, we were able to enhance the overall magnetic field of the tiny disc magnets, such that this stack now works as a much bigger stronger magnet as a whole. After dismantling the setup, the magnet stack was removed and brought to a lab to check out the external field with a Hall metre. The maximum field at the surface of the magnet, in contact with the back of the sample was measured to be 473 mT. By varying the separation between the magnet stack surface and the probe, we were able to obtain the relationship between the external field and separation from the magnet surface. This is as described in <hl> insert figure ref</hl><br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of half a millimeter. However, we could not directly observe the magnetisation characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Improvements and Reflections==<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting and deburring. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample also took a substantial amount of time. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2185Kerr Microscope2022-04-28T12:22:57Z<p>MayInn: /* Improvements and Reflections */</p>
<hr />
<div>Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Broadly, our goals are:<br />
# Build an imaging setup<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. Setup 1 reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). Setup 2 more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Troubleshooting at NPBS interface<br />
|-<br />
! scope=row | 11<br />
| New Magnetic Sample and Light Source<br />
|-<br />
! scope=row | 12<br />
| 60x<br />
|-<br />
! scope=row | 13<br />
| Final Setup<br />
|}<br />
<br />
==Setup==<br />
Location: S11-02-04<br />
<br />
==Theory==<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
Conceived by John Kerr in the 1980s, the magneto-optic Kerr effect (MOKE) describes the changes to light reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Setup 1: Angled Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
<br />
As a first observation of the MOKE, we utilised a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|thumb|]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. [[File: Red laser pointer dirty.jpeg|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain an image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
==Setup 2: Microscope Setup==<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a fiber for an even cleaner light source.<br />
* Swapped to a magnetic sample <br />
<br />
Samples:<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard Si/SiO2 substrate as a control sample. And next, we have a a magnetic film sample on top of a Si/SiO2 substrate. Although its specific composition is unknown, the magnetic sample is know to magnetically saturate at a field of about 0.1 T. From 0 T to 0.1 T, the domain density is known to decrease for this sample and correspondingly, we would expect the overall intensity garnered from the setup to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<br />
(@Joel)<br />
<br />
==Summary==<br />
<br />
Goals (as at top of page):<br />
# Build an imaging setup (eg. Microscope)<br />
# Image a magnetic sample<br />
# Automate scanning of sample<br />
<br />
In view of our stated goals, we were successful in the first, halfway towards accomplishing the second and completely whiffed on the third. We built a working 10x/60x microscope with a sample stage that could be translated with a precision of half a millimeter. However, we could not directly observe the magnetisation characteristics of our sample on the computer screen and some post processing of our images was required.<br />
<br />
==Improvements and Reflections==<br />
<br />
<b>Making our own experimental parts</b> - For our group members, it was the first time soldering, cutting and deburring. We tinkered with our light source and also made our own magnetic sample. This was fresh and fun, although surprisingly time consuming.<br />
<br />
<b>Aligning</b> - Realigning our optical setup each time we modified our light source or sample also took a substantial amount of time. This got better over time as we got more familiar with our setup and had a better feel of how to tune certain parts. The addition of the double mirrors for beam alignment as well as an xyz-translation stage for our sample also streamlined the alignment process. In hindsight however, we should have taken more time to consider each change we wished to make before actually implementing it.<br />
<br />
<b>Managing the external fields from magnets</b> - The first improvement we would like to implement would be to collect data from the magnetic sample at lower external magnetic fields, where the magnets are much further away from the sample surface. As the sample saturates at about 0.1 T, we would not be able to observe the changes in domains at fields higher than 0.1 T. It would be great for us to have a Hall meter on hand such that we could measure the external field provided by the series of magnets at the varying separation from the sample. <br />
<br />
<b>Lock-in-amplifier</b> - The data that we have collected thus far could have been pointing towards the low signals collected, such that no to low observable changes were captured by the camera. When low signals are concerned, lock-in-amplifiers come to mind. We could implement a lock-in-amplifier in the setup, possibly with a chopper as well to send pulsed signals to the sample. With this, even minute changes in intensity could be detected. However, instead of MOKE microscope, our setup would be more of a spectroscope!<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2174Kerr Microscope2022-04-28T12:00:18Z<p>MayInn: /* Setup 2: Microscope Setup */</p>
<hr />
<div>Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Some useful search keywords are:<br />
* Faraday rotation<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. Setup 1 reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). Setup 2 more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Troubleshooting at NPBS interface<br />
|-<br />
! scope=row | 11<br />
| New Magnetic Sample and Light Source<br />
|-<br />
! scope=row | 12<br />
| 60x<br />
|-<br />
! scope=row | 13<br />
| Final Setup<br />
|}<br />
<br />
==Setup==<br />
Location: S11-02-04<br />
<br />
==Theory==<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
Conceived by John Kerr in the 1980s, the magneto-optic Kerr effect (MOKE) describes the changes to light reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Setup 1: Angled Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
<br />
As a first observation of the MOKE, we utilised a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|thumb|]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. [[File: Red laser pointer dirty.jpeg|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain an image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
==Setup 2: Microscope Setup==<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a fiber for an even cleaner light source.<br />
* Swapped to a magnetic sample <br />
<br />
Samples:<br />
<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard Si/SiO2 substrate as a control sample. And next, we have a a magnetic film sample on top of a Si/SiO2 substrate. Although its specific composition is unknown, the magnetic sample is know to magnetically saturate at a field of about 0.1 T. From 0 T to 0.1 T, the domain density is known to decrease for this sample and correspondingly, we would expect the overall intensity garnered from the setup to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<br />
(@Joel)<br />
<br />
==Improvements and General Reflections==<br />
<br />
==Summary==<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=2173Kerr Microscope2022-04-28T12:00:05Z<p>MayInn: /* Setup 2: Microscope Setup */</p>
<hr />
<div>Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect (MOKE) describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Some useful search keywords are:<br />
* Faraday rotation<br />
<br />
==Overview==<br />
This section contains a bird's eye view of our experimental time line. We began the experiment in week 5 of the semester and ended in week 13. In our attempt to observe the [https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect Magneto-Optic Kerr effect], we tinkered with two different optical setups. Setup 1 reflects a beam of linearly polarizer light off a magnetic sample which we then pass through an analyzer and capture on our CCD (webcam). Setup 2 more closely resembles a microscope.<br />
<br />
{| class="wikitable plainrowheaders"<br />
|+ Timeline<br />
! scope=col | Week<br />
! scope=col | Milestone<br />
|-<br />
! scope=row | 5<br />
| Gathering and Initial Setup<br />
|-<br />
! scope=row | 6<br />
| Machining and Setup Design<br />
|-<br />
! scope=row | 7<br />
| Angled Setup<br />
|-<br />
! scope=row | 8<br />
| -<br />
|-<br />
! scope=row | 9<br />
| Mirror Alignment<br />
|-<br />
! scope=row | 10<br />
| Troubleshooting at NPBS interface<br />
|-<br />
! scope=row | 11<br />
| New Magnetic Sample and Light Source<br />
|-<br />
! scope=row | 12<br />
| 60x<br />
|-<br />
! scope=row | 13<br />
| Final Setup<br />
|}<br />
<br />
==Setup==<br />
Location: S11-02-04<br />
<br />
==Theory==<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that the permittivity depends on the direction. The permittivity affects speed of light in a material.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
Conceived by John Kerr in the 1980s, the magneto-optic Kerr effect (MOKE) describes the changes to light reflected from a magnetized surface. MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation.<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Setup 1: Angled Setup==<br />
<br />
[[File: Angled schematic kerr.png|thumb| Setup Schematic. A polarized light source is reflected off our sample at an angle, passed through an analyzer and finally recorded on our CCD array.]]<br />
<br />
Equipment:<br />
* Power Supply<br />
* Red LED<br />
* Pinhole Aperture<br />
* Plano-convex lens (100mm)<br />
* Steel sheet & Copper Wire<br />
* Sheet Polarizer x2<br />
* CCD Array (Webcam)<br />
<br />
As a first observation of the MOKE, we utilised a basic setup that reflected a linearly polarized light source off our sample - an electromagnet that consists of a steel sheet wrapped with copper wire. The light source is a LED The reflected beam is focused by a plano-convex lens and passed through an analyzer before it is finally captured on our CCD array (webcam). The open source video capture software [https://obsproject.com| OBS] was used to display the captured image.<br />
<br />
[[File: Kerr setup angled initial.jpeg|thumb|]]<br />
<br />
The intention with this setup is that if we align the axes of the polarizer and analyzer, the beam would be completely extinguished for a non-magnetic sample. Then, regardless of which of the three MOKE effects were at play, a magnetic sample would alter the polarization of the reflected beam, causing it to only be partially extinguished by the analyzer. In practice, since we are working with non-ideal polarizers that have high extinction ratios (but not 100%), the image of a non-magnetic sample would have been used as a baseline for comparison with a magnetic sample. By exporting image captures from the OBS software and isolating the pixel intensities, a study could have been done by taking the differences in pixel intensities between the two images.<br />
<br />
Alas, while the experimental setup was simple, the greatest stumbling block proved to be the very first step - capturing an image. Aligning all the optical components proved to be challenging and time consuming, particularly when shifting the webcam back and forth in an attempt to focus the image since this meant unscrewing the base, adjusting the position of the webcam, and tilting the base at an angle to fit a screw back into the optical table. On the suggestion of Prof. Christian, we cobbled together a crude z-translation stage which used two additional base holders to 'lock' onto the base of the webcam from either side and allow movement only along the optical axis. This did not solve the alignment issue directly, but it did allow us to identify another problem that we ought to tackle first. <br />
<br />
The laser pointer casing was slightly bulbous toward the front end. This meant that when it was mounted onto the acrylic holder (see image), it was tilted up slightly, and thus the plane in which the light beam travelled was not parallel to the optical table but tilted upward. Consequently, for every shift of our webcam along the z-axis, a corresponding change in height would have to be made. At this juncture, a decision was made to modify the light source before proceeding with imaging.<br />
<br />
===Setup 1.1: Double Mirror Alignment===<br />
<br />
Main Change: <br />
* Added two mirrors attached to adjustable mounts.<br />
<br />
Other Minor Changes:<br />
* Added a second lens to focus an image onto the CCD array, rather than the beam itself.<br />
* Swapped to a sample with a smoother surface to reduce diffuse reflection - the magnetic tape of a floppy disk.<br />
* Swapped to a 650nm laser diode ([[Media: Laser diode kerr.pdf|Datasheet]]) as the red laser pointer produced a rather 'dirty' beam with various artifacts. [[File: Red laser pointer dirty.jpeg|thumb| Not all laser pointers are equal. The first laser pointer we used turned out to have a rather dirty beam. The pinhole aperture might have helped to remove some of these artifacts, but to be sure we decided to switch to a laser diode that produced a cleaner beam.]]<br />
<br />
The usage of the mirrors for alignment is as follows:<br />
# Place a pinhole aperture near the second mirror and turn the knobs on the <i>first</i> mirror to adjust the pitch and yaw until the laser beam is centered on the pinhole.<br />
# Swap the pinhole to a location farther down the beam path. Tune the knobs on the <i>second</i> mirror until the beam is centered.<br />
# Repeat steps 1 and 2, continuously swapping the pinhole between the near and far locations until the beam passes through the pinhole at both locations.<br />
<br />
Result: Still unable to obtain an image of our sample. Our beam does not cover a large enough region of our CCD array and the majority of what we are imaging is likely from ambient light sources. Alignment also proves difficult as it is sometimes hard to discern the light that originates from our light source. At this juncture, a decision was made to modify the rest of the optical setup to increase magnification.<br />
<br />
==Setup 2: Microscope Setup==<br />
<br />
[[File: Kerr microscope schematic.png|thumb| Schematic of microscope setup.]]<br />
<br />
[[File:Microscope setup kerr.jpeg|thumb| Microscope setup, sans pinhole.]]<br />
<br />
Main change: <br />
* Revamped optical setup to resemble that of a microscope.<br />
<br />
Other minor changes:<br />
* Switched light source once more to a laser pen (aka Visual Fault Locator) coupled to a fiber for an even cleaner light source.<br />
* Swapped to a magnetic sample <br />
<br />
Samples:<br />
To test the iterated setups, two main samples were used, in addition to a series of permanent magnets. The two samples were firstly, a standard Si/SiO2 substrate as a control sample. And next, we have a a magnetic film sample on top of a Si/SiO2 substrate. Although its specific composition is unknown, the magnetic sample is know to magnetically saturate at a field of about 0.1 T. From 0 T to 0.1 T, the domain density is known to decrease for this sample and correspondingly, we would expect the overall intensity garnered from the setup to decrease as the field increases if stripe domains are the brighter features, vice versa.<br />
<br />
In this final iteration, imaging was a success. We had successfully built a microscope. Now for the Kerr part...<br />
<br />
==Results and Analysis==<br />
<br />
(@Joel)<br />
<br />
==Improvements and General Reflections==<br />
<br />
==Summary==<br />
<br />
==Lab Session Logs==<br />
<br />
<span style="color:red">To be deleted once relevant info has been filtered out.</span><br />
<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=581Kerr Microscope2022-03-22T13:18:58Z<p>MayInn: /* Theory */</p>
<hr />
<div>Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Some useful search keywords are:<br />
* Faraday rotation<br />
<br />
==Setup==<br />
Location: S11-02-04<br />
<br />
==Theory==<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that it depends on the direction. The permittivity affects speed of light in a material.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
Conceived by the John Kerr in the 1980s during in his advent into magneto-optics, the magneto-optic Kerr effect (MOKE) describes the changes to light reflected from a magnetized surface. MOKE can be further MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation (what's this?).<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Lab Session Logs==<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=579Kerr Microscope2022-03-22T08:22:57Z<p>MayInn: /* Team members */</p>
<hr />
<div>Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
* Sim May Inn<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Some useful search keywords are:<br />
* Faraday rotation<br />
<br />
==Setup==<br />
Location: S11-02-04<br />
<br />
==Theory==<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that it depends on the direction. The permittivity affects speed of light in a material.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
The magneto-optic Kerr effect (MOKE) describes the changes to light reflected from a magnetized surface. MOKE can be further MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation (what's this?).<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Lab Session Logs==<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=578Kerr Microscope2022-03-22T08:19:43Z<p>MayInn: /* 18 Mar 2022 */</p>
<hr />
<div>Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Some useful search keywords are:<br />
* Faraday rotation<br />
<br />
==Setup==<br />
Location: S11-02-04<br />
<br />
==Theory==<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that it depends on the direction. The permittivity affects speed of light in a material.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
The magneto-optic Kerr effect (MOKE) describes the changes to light reflected from a magnetized surface. MOKE can be further MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation (what's this?).<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Lab Session Logs==<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented yet.<br />
# Remove the blue LED about the camera which was initially there for simply aesthetics. Soldering was utilised to remove the relevant circuits and parts from the board.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if LED and lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=577Kerr Microscope2022-03-22T08:17:25Z<p>MayInn: /* 18 Mar 2022 */</p>
<hr />
<div>Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Some useful search keywords are:<br />
* Faraday rotation<br />
<br />
==Setup==<br />
Location: S11-02-04<br />
<br />
==Theory==<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that it depends on the direction. The permittivity affects speed of light in a material.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
The magneto-optic Kerr effect (MOKE) describes the changes to light reflected from a magnetized surface. MOKE can be further MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation (what's this?).<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Lab Session Logs==<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
# If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
# Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
# Propose the use of a beam expander before the camera - was not implemented.<br />
# Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=576Kerr Microscope2022-03-22T08:16:59Z<p>MayInn: /* 18 Mar 2022 */</p>
<hr />
<div>Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Some useful search keywords are:<br />
* Faraday rotation<br />
<br />
==Setup==<br />
Location: S11-02-04<br />
<br />
==Theory==<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that it depends on the direction. The permittivity affects speed of light in a material.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
The magneto-optic Kerr effect (MOKE) describes the changes to light reflected from a magnetized surface. MOKE can be further MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation (what's this?).<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Lab Session Logs==<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
Field of view provided by the webcam was too wide in comparison to the laser spot. Ideally, the image should be less than the diameter of the laser spot. A few methods were proposed in response to this:<br />
1) If we were unable to see the actual area, we should minimally be able to see a change in intensity of the imaged spot, with and without a magnetic sample. However, we were not able to observe such variation in intensity, probably due to the small size of the area of interest, and the weak signals we were receiving.<br />
2) Attempted to block out all ambient light, and isolate only the signals from the laser spot, but again, we were not able to observe any obvious variation in intensity.<br />
3) Propose the use of a beam expander before the camera - was not implemented.<br />
4) Removed the lens in the camera, which causes the wide view.<br />
<br />
Next: to test if lens removal helped ease the situation.<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Kerr_Microscope&diff=575Kerr Microscope2022-03-22T04:11:21Z<p>MayInn: /* Lab Session Logs */</p>
<hr />
<div>Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
Through Kerr microscopy, we aim to characterize the relative changes in magnetization across a magnetic sample.<br />
<br />
==Team members==<br />
* Joel Yeo<br />
* Gan Jun Herng<br />
(Feel free to edit this page or email me at joelyeo@u.nus.edu if you would like to join.)<br />
<br />
==Idea==<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. A magnetic sample can be borrowed from a team member's research lab. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
Some useful search keywords are:<br />
* Faraday rotation<br />
<br />
==Setup==<br />
Location: S11-02-04<br />
<br />
==Theory==<br />
Light reflected from a magnetized surface may change both polarization and reflected intensity. This comes about because the magneto-optic material has an <i>anisotropic</i> permittivity, meaning that it depends on the direction. The permittivity affects speed of light in a material.<br />
<br />
<math> v_p = \frac{1}{\epsilon\mu} </math><br />
<br />
The magneto-optic Kerr effect (MOKE) describes the changes to light reflected from a magnetized surface. MOKE can be further MOKE can be further categorized depending on the relative orientations of the reflecting plane to the magnetic field.<br />
<br />
[[File:MOKE geometry.png|thumb|MOKE geometry.]]<br />
<br />
Experiments involving the different MOKE orientations are typically carried out in the following manner.<br />
# Polar MOKE -- Near normal incidence to avoid Kerr rotation (what's this?).<br />
# Longitudinal MOKE -- Incidence at an angle to surface, parallel to <math> \vec{B} </math> field. Linearly polarized light becomes elliptically polarized. The change in polarization is directly proportional to <math> \vec{B} </math>.<br />
# Transverse MOKE -- Incidence at an angle to surface, perpendicular to <math> \vec{B} </math> field. This affects reflectivity <math> r </math>. Viewed from the source, if <math> \vec{B} </math> points to the right of the incident plane, the Kerr vector adds to the Fresnel amplitude vector and the intensity of the reflected light is <math> \|r + k\|^2 </math>. If it points to the left, it is <math> \|r - k\|^2 </math>.<br />
<br />
==Lab Session Logs==<br />
===10 Feb 2022===<br />
[[File:Kerr-microscope.png|thumb|Schematic for Kerr microscopy.]]<br />
Hunted down the required parts. We used the diagram from NaBiS from Politechnico di Milano’s physics department as our guide <ref>{{Cite web<br />
| title = Kerr microscope – NaBiS<br />
| author = <br />
| work = nabis.fisi.polimi.it<br />
| date = <br />
| access-date = 28 February 2022<br />
| url = http://nabis.fisi.polimi.it/equipments/kerr-microscope/<br />
| quote = <br />
}}</ref>.<br />
<br />
Linear polarizer was found, but extremely dirty. First rinsed with water<br />
than finished cleaning with isopropanol (IPA). IPA available in S11-02-04 room<br />
cupboard.<br />
Discussed some other things between us and with TAs:<br />
* Stage requires mm precision<br />
* blah<br />
<br />
[[File:MOKE power supply.jpeg|thumb|Power supply for LED.]]<br />
<br />
We only used the two knobs on the right hand side. First turn the voltage up slightly to set a limiting voltage, then slowly turn on the current till the LED turns on. Observation shows LED tends to turn on at about 2V. The positive end (red) should be connected to the positive (anode) side of the LED. This can be seen as the longer leg of the LED.<br />
<br />
===15 Feb 2022===<br />
From the info on the NaBiS page, we may need to characterise the material in all three MOKE orientations to get a full 3D image. But we’ll think about that later.<br />
<br />
Today was more parts gathering, a little machining to make the parts that we need.<br />
<br />
Magnetic material - Got a bunch of steel sheets from CK, and some copper wire to wrap around it like a mini solenoid. The sheets are a soft magnetic material (<math> \mu </math> = 5000 − 7000 according to CK) and will be magnetised when a current runs through. CK suggested we set it up so we can change the direction of the magnetic field by switching ??? (not exactly sure, will need to figure out later), and not manually moving the sample. Might be easier down the road. <br />
<br />
The steel sheet was cut with some steel cutting scissors, and de-burred with sandpaper. The copper wires had to be stripped for connection. This was done with some Stanley blades and polished with some sandpaper. Not the cleanest, but as long as we get a current it’s fine.<br />
<br />
===18 Mar 2022===<br />
<br />
==Measurements==<br />
<br />
==References==<br />
{{reflist}}</div>MayInnhttps://pc5214.org/index.php?title=Main_Page&diff=574Main Page2022-03-22T04:10:37Z<p>MayInn: </p>
<hr />
<div><strong>MediaWiki has been installed.</strong><br />
Welcome to the main page for the PC5214 graduate module AY2122, Sem2</strong>.<br />
Here, we leave project descriptions, literature references, and other collateral information. You will need to create an account in class to obtain write access.<br />
<br />
Actual lecture locations will be placed here until we have reached a stable state. If you are interested and have not been able to register, please send me an email (if you have not done so already) to [mailto:phyck@nus.edu.sg phyck@nus.edu.sg].<br />
<br />
'''<span style="color:#ff0000">For the session on 14 Jan and some following sessions, we were given LT29.</span>'''<br />
<br />
Cheers, Christian<br />
<br />
This page is currently set up.<br />
<br />
==Lab spaces==<br />
* '''S11-02-04''' (next to physics dept resource room). This is where most optics-related projects should go.<br />
* '''S12 level 4''', "year 1 teaching lab", back room, "vanderGraff lab". This is perhaps were non-optics related projects would fit.<br />
* '''S13-01-??''' (blue door: former accoustics lab). Not sure yet who could go there, but it is a really really quiet place!<br />
* anything else you have access to<br />
<br />
==Projects==<br />
Please leave a a link to your project page (or pages) here, and leave a short description what this is about. Write the '''stuff you need''' under the description too.<br />
<br />
===[[Project 1 (example)]]===<br />
Keep a very brief description of a project or even a suggestion here, and perhaps the names of the team members, or who to contact if there is interest to join.<br />
<br />
===[[Confocal Microscopy]]===<br />
Team Members: Wang Tingyu, Xue Rui, Yang Hengxing<br />
<br />
A Confocal Microscopy or Confocal Laser Scanning Microscopy (CLSM) uses pinhole to block out all out of focus light to enhance optical resolution, very different from traditional wide-field fluorescence microscopes. To offset the block of out of focus lights, the light intensity is detected by a photomultiplier tube or avalanche photodiode, which transforms the light signal into an electrical one. We will try to build a Setup like this to enhance optical resolution and maybe get profile information about the sample.<br />
<br />
===[[An interferometric method for measuring the resonance frequency of vibrating system]]===<br />
<br />
Members: [[User:Nakarin|Nakarin Jayjong]], Joel Auccapuclla, [[User:Xiaoyu|Xiaoyu Nie]], [[User:Haotian|Haotian Song]].<br />
<br />
In this project, the resonance frequency of the vibrating system namely the vibration transducer is measured using a Michelson interferometer.<br />
<br />
===[[Homodyne detection]]===<br />
Proposed By: [[User:Johnkhootf|John Khoo]]<br />
<br />
[https://en.wikipedia.org/wiki/Homodyne_detection ''Optical'' homodyne detection] is a method for detecting messages transmitted in optical signals, where a frequency or phase modulated signal is compared to what is misleadingly called the "local oscillator" (LO) signal, which is generated from the same source but not modulated with the message. In order to probe quantum effects, it is important to bring the noise of the detector down to the [https://en.wikipedia.org/wiki/Shot_noise ''shot-noise limit''], where the only fluctuations observed arise from the discrete nature of photons, which can be theoretically modelled as the vacuum-state fluctuations of the quantised electromagnetic field. This project's first objective is to build a homodyne detector from scratch.<br />
<br />
'''Stuff we need''': Acousto-optical modulator, electro-optical modulator, transformer to control EOM, photodiodes, current-to-voltage converter (I'm not sure what this is - can we just use a resistor connected to the ground and measure the voltage?), Raspberry Pi (I hope the ADC is good enough for this), mirrors and beamsplitters<br />
<br />
===[[Laser Microphone]]===<br />
Team Members: Nicholas Chong Jia Le, Marcus Low Zuo Wu<br />
<br />
A laser spot illuminating a vibrating surface should move along with it, and tracking the motion of the spot should theoretically allow us to retrieve some of the information regarding the vibrations of the surface. If a loud enough sound causes the surface to vibrate, this should theoretically be enough for the transmission of audio information through visual means. We have a few different methods through which we will attempt to realise this.<br />
<br />
===[[Plasma emission spectroscopy]]===<br />
Proposed By: Park Kun Hee<br />
<br />
Pulsed plasma in partial vacuum is characterised, by analysing [https://en.wikipedia.org/wiki/Spectral_line_ratios line intensity ratios] to determine its temperature and density.<br />
<br />
===[[Characterization of Single Photon Counters]]===<br />
Proposed By: Yeo Zhen Yuan (Looking for Teammates!)<br />
<br />
The project is to characterize an Avalanche PhotoDiode (APD) and compare its efficiency with commercial counterparts like [https://www.digikey.com/en/products/detail/excelitas-technologies/SPCM-AQRH-10-FC/6235280 this device]. It works based on the photoelectric effect to turn incident photon into photoelectron. This photoelectron is then accelerated in an electric field to produce cascading electrons and this "electron avalanche" is detected as a spike in the current. Analog signals will need to be processed via custom electronics and ultimately provide a digital readout. Current commercial detectors boast 50% Photon Detector Efficiency (PDE) at room temperature and that will be our goal. They typically cost $2000-$5000 which seems over-priced and ready for disruption. Liquid nitrogen temperatures may be needed to see how large a PDE we can get.<br />
<br />
What is SPCM good for? Copied from the datasheet/brochure: LIDAR, Quantum Cryptography, Photon correlation spectroscopy, Astronomical observation, Optical range finding, Adaptive optics, Ultra-sensitive fluorescence, Particle sizing, Microscopy. So maybe this would become a toy/tool for next year's students.<br />
<br />
===[[Kerr Microscope]]===<br />
Proposed By: Sim May Inn (write up by Joel Yeo)<br />
<br />
'''Team members: Gan Jun Herng, Joel Yeo, Sim May Inn'''<br />
<br />
'''Project Location: S11-02-04'''<br />
<br />
Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
<br />
In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. An example of a magnetic sample is the magnetic tape from an old school cassette tape. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
<br />
'''Items needed (as of 28 Feb 2022):'''<br />
* Light source (visibile wavelength): <s> Laser, LED </s>, laser diode<br />
* <s> Linear polarizer (sheet) x 2Camera (CCD/CMOS) </s><br />
* <s> Non-polarizing beam splitter </s><br />
* <s> Camera (CCD/CMOS) </s><br />
* <s> Pinhole/aperture </s><br />
* <s> Magnetic samples for Kerr microscopy (eg. Magnetic film, magnets, ferromagnetic materials) </s><br />
* Arduino<br />
* Microscope stage<br />
* Piezoelectrics (?) for moving stage<br />
<br />
===[[Electron Gun]]===<br />
Team Members: Aliki Sofia Rotelli, Lai Tian Hao, Lim En Liang Irvin, Tan Chuan Jie <br />
<br />
The purpose of this project is to design and build an electron gun from the initial concept in order to create a detectable electron beam through the use of a phosphor-coated screen. Additionally, the beam current will be examined in order to better define the devices' capabilities. Mass spectrometry, x-ray production for linear accelerators, and electron-beam lithography are just a few of the applications for electron gun technology.<br />
<br />
===[[Smoke detection in air]]===<br />
Team Members: Cheng De Hao, Huang Hai Tao, Wang Zheng Yu <br />
<br />
Using detector to detect the scattering light and amplify the signal by using the lock-in amplifier.<br />
<br />
===[[Anti-glare LCD]]===<br />
Team members: Zhang Yuanyuan, Ming Xiaohan, Han Shixin<br />
<br />
As s bad lighting phenomenon, glare phonomenon brings inconvenience to all aspects of human life, especially people's access to information on instruments. In order to suppress glare effectively, anti-glare film is put into research. The common anti-glare film in the market is an optical film using the principle of optical scattering, but it can not adapt to the change of light environment in time, which has some limitations in practical application. In this study, a two-dimensional barcode micro-region orientation structure, based on the characteristics of liquid crystal, namely a random grating structure, was designed by simulation in the lab and using MATLAB software, and its optional parameters were searched.<br />
<br />
===[[Custom atomic beam source]]===<br />
Team Members: Lu Tiangao, Li Putian<br />
<br />
===[[Schlieren imaging]]===<br />
Team members: Zhang Xingjian, Du Jinyi<br />
<br />
===[[Contactless Conductivity Measurement]]===<br />
<br />
Team members: Chen Guohao, Jiang Luwen<br />
<br />
The purpose of this project is to measure the conductivity of materials without having to make electrical contact with them. Specifically, we make use of the eddy-current induced in the materials to calculate the conductivity.<br />
<br />
===[[Quantum Random Number Generator]]===<br />
<br />
Team members: Wang Yang, Xiao Yucan, Zhang Munan<br />
<br />
Random numbers are a fundamental resource in science and engineering with important applications in simulation and cryptography. The inherent randomness at the core of quantum mechanics makes quantum systems a perfect source of entropy. Quantum random number generation is one of the most mature quantum technologies with many alternative generation methods. The purpose of our project is to build a simple optics-based QRNG. We will also collect the random number generated by our device and use some methods to check the randomness.<br />
<br />
===[[Orbits of the Galilean Moons]]===<br />
<br />
<br />
Team Members: [[User:Matthew|Matthew Wee]]<br />
<br />
Galileo Galilei’s discovery of celestial bodies that orbit something other than the Earth marked the beginning of the end of the geocentric model of the universe. In this project, we will perform the same observations on those moons as Galileo did 400 years ago.<br />
<br />
===[[Capacitor array based ADC]]===<br />
<br />
<br />
Team Members: Zhang Chengyue, Yang Ningli, Guo Diandian, Chen Jiayu<br />
<br />
In this project, we are going to make an ADC.<br />
<br />
==Resources==<br />
===[[Recorded sessions]]===<br />
Some of the sessions will be recorded and uploaded to youtube. Find a description on the [[Recorded sessions]] page.<br />
<br />
===Devices and material===<br />
Apart form all the stuff in the teaching lab, we have a few resources you may want to consider for your project<br />
*...<br />
<br />
'''Books:'''<br />
* P.R. Bevington, D.K. Robinson: Data Reduction and Error Analysis for the Physical Sciences, 3rd edition. McGrawHill, ISBN0-07-119926-8. A very good book containing all the questions you never allowed yourself to ask about error treatment, statistics, fitting of data to models etc.<br />
* Horrowitz/Hill: The Art of Electronics<br />
* C.H. Moore, C.C. Davis, M.A. Coplan: Building Scientific Apparatus. 2nd or higher edition. Perseus Books, ISBN0-201-13189-7. A very comprehensive book about many dirty details in experimental physics, and ways to get simple problems solved. Appears a bit dated, but is a good start for many experimental projects up to this day!<br />
* Christopher C. Davis: Laser and Electro-optics. Useful as a general introduction to many contemporary aspects you come across when working with lasers, with a reasonable introduction of the theory. Very practical for optics.<br />
<br />
'''Software:'''<br />
Some of the more common data processing tools used in experimental physics:<br />
* [http://www.gnuplot.info/ '''Gnuplot''']: A free and very mature data display tool that works on just about any platform used that produces excellent publication-grade eps and pdf figures. Can be also used in scripts. Open source and completely free.<br />
* Various '''Python''' extensions. [http://www.python.org/ Python] is a very powerful free programming language that runs on just about any computer platform. It is open source and completely free.<br />
* '''Matlab''': Very common, good toolset also for formal mathematics, good graphics. Expensive. We may have a site license, but I am not sure how painful it is for us to get a license for this course. Ask me if interested.<br />
* '''Mathematica''': More common among theroetical physicists, very good in formal maths, now with better numerics. Graphs are ok but can be a pain to make looking good. As with Matlab, we do have a campus license but an increasingly painful licensing ritual. Ask me if interested or follow the instruction to install the software in your desktop.<br />
* '''Origin''': Very widespread data processing software with a complete graphical user interface, integrates well into a Windows environment. Most likely available in your research labs, not sure if NUS has a site license.<br />
* '''Labview''': Many of you may have seen this in your labs, but I am not too familiar with it, and chances are it is too resource-hungry to run on the machines we have there. It keeps its promise of a fast learning curve if you want to do simple things but it can get a REAL pain if you want to do subtle things, or want to do things fast, or want to debug code. Expensive and resource-hungry, but comes with good integration of also expensive hardware. May not be worth it if you know any programming language.<br />
* [https://www.circuitlab.com/ '''Circuit Lab''']: a convenient software to design and simulate electrical circuits directly at your browser. I think Flash is required. It works well in Chrome.<br />
<br />
===[[Acronym database]]===<br />
This is an attempt to clarify the countless acronyms we use in our sub-communities (follow headline link)<br />
<br />
<br />
===[[Gnuplot tricks]]===<br />
Follow the headline link for some of the random questions that came up with gnuplot.<br />
<br />
== Previous PC5214 wikis ==<br />
* [http://pc5214.org/AY1819S2 AY2018/19 Sem2]<br />
* [http://pc5214.org/AY1415S1 AY2014/15 Sem1]<br />
* [http://pc5214.org/AY1314S1 AY2013/14 Sem1]<br />
* [http://pc5214.org/AY1213S1 AY2012/13 Sem1]<br />
* [http://pc5214.org/AY1112S1 AY2011/12 Sem1]<br />
* [http://pc5214.org/AY1011S1 AY2010/11 Sem1]<br />
<br />
== Some wiki reference materials==<br />
Consult the [//meta.wikimedia.org/wiki/Help:Contents User's Guide] for information on using the wiki software. Other sources:<br />
<br />
* [//www.mediawiki.org/wiki/Manual:FAQ MediaWiki FAQ]<br />
* [[Writing mathematical expressions]]<br />
* [[Uploading images]]</div>MayInnhttps://pc5214.org/index.php?title=In-situ_magnetic_imaging_and_Hall_detection&diff=573In-situ magnetic imaging and Hall detection2022-03-22T04:10:17Z<p>MayInn: Blanked the page</p>
<hr />
<div></div>MayInnhttps://pc5214.org/index.php?title=Main_Page&diff=572Main Page2022-03-22T04:09:38Z<p>MayInn: </p>
<hr />
<div><strong>MediaWiki has been installed.</strong><br />
Welcome to the main page for the PC5214 graduate module AY2122, Sem2</strong>.<br />
Here, we leave project descriptions, literature references, and other collateral information. You will need to create an account in class to obtain write access.<br />
<br />
Actual lecture locations will be placed here until we have reached a stable state. If you are interested and have not been able to register, please send me an email (if you have not done so already) to [mailto:phyck@nus.edu.sg phyck@nus.edu.sg].<br />
<br />
'''<span style="color:#ff0000">For the session on 14 Jan and some following sessions, we were given LT29.</span>'''<br />
<br />
Cheers, Christian<br />
<br />
This page is currently set up.<br />
<br />
==Lab spaces==<br />
* '''S11-02-04''' (next to physics dept resource room). This is where most optics-related projects should go.<br />
* '''S12 level 4''', "year 1 teaching lab", back room, "vanderGraff lab". This is perhaps were non-optics related projects would fit.<br />
* '''S13-01-??''' (blue door: former accoustics lab). Not sure yet who could go there, but it is a really really quiet place!<br />
* anything else you have access to<br />
<br />
==Projects==<br />
Please leave a a link to your project page (or pages) here, and leave a short description what this is about. Write the '''stuff you need''' under the description too.<br />
<br />
===[[Project 1 (example)]]===<br />
Keep a very brief description of a project or even a suggestion here, and perhaps the names of the team members, or who to contact if there is interest to join.<br />
<br />
===[[Confocal Microscopy]]===<br />
Team Members: Wang Tingyu, Xue Rui, Yang Hengxing<br />
<br />
A Confocal Microscopy or Confocal Laser Scanning Microscopy (CLSM) uses pinhole to block out all out of focus light to enhance optical resolution, very different from traditional wide-field fluorescence microscopes. To offset the block of out of focus lights, the light intensity is detected by a photomultiplier tube or avalanche photodiode, which transforms the light signal into an electrical one. We will try to build a Setup like this to enhance optical resolution and maybe get profile information about the sample.<br />
<br />
===[[An interferometric method for measuring the resonance frequency of vibrating system]]===<br />
<br />
Members: [[User:Nakarin|Nakarin Jayjong]], Joel Auccapuclla, [[User:Xiaoyu|Xiaoyu Nie]], [[User:Haotian|Haotian Song]].<br />
<br />
In this project, the resonance frequency of the vibrating system namely the vibration transducer is measured using a Michelson interferometer.<br />
<br />
===[[Homodyne detection]]===<br />
Proposed By: [[User:Johnkhootf|John Khoo]]<br />
<br />
[https://en.wikipedia.org/wiki/Homodyne_detection ''Optical'' homodyne detection] is a method for detecting messages transmitted in optical signals, where a frequency or phase modulated signal is compared to what is misleadingly called the "local oscillator" (LO) signal, which is generated from the same source but not modulated with the message. In order to probe quantum effects, it is important to bring the noise of the detector down to the [https://en.wikipedia.org/wiki/Shot_noise ''shot-noise limit''], where the only fluctuations observed arise from the discrete nature of photons, which can be theoretically modelled as the vacuum-state fluctuations of the quantised electromagnetic field. This project's first objective is to build a homodyne detector from scratch.<br />
<br />
'''Stuff we need''': Acousto-optical modulator, electro-optical modulator, transformer to control EOM, photodiodes, current-to-voltage converter (I'm not sure what this is - can we just use a resistor connected to the ground and measure the voltage?), Raspberry Pi (I hope the ADC is good enough for this), mirrors and beamsplitters<br />
<br />
===[[Laser Microphone]]===<br />
Team Members: Nicholas Chong Jia Le, Marcus Low Zuo Wu<br />
<br />
A laser spot illuminating a vibrating surface should move along with it, and tracking the motion of the spot should theoretically allow us to retrieve some of the information regarding the vibrations of the surface. If a loud enough sound causes the surface to vibrate, this should theoretically be enough for the transmission of audio information through visual means. We have a few different methods through which we will attempt to realise this.<br />
<br />
===[[Plasma emission spectroscopy]]===<br />
Proposed By: Park Kun Hee<br />
<br />
Pulsed plasma in partial vacuum is characterised, by analysing [https://en.wikipedia.org/wiki/Spectral_line_ratios line intensity ratios] to determine its temperature and density.<br />
<br />
===[[Characterization of Single Photon Counters]]===<br />
Proposed By: Yeo Zhen Yuan (Looking for Teammates!)<br />
<br />
The project is to characterize an Avalanche PhotoDiode (APD) and compare its efficiency with commercial counterparts like [https://www.digikey.com/en/products/detail/excelitas-technologies/SPCM-AQRH-10-FC/6235280 this device]. It works based on the photoelectric effect to turn incident photon into photoelectron. This photoelectron is then accelerated in an electric field to produce cascading electrons and this "electron avalanche" is detected as a spike in the current. Analog signals will need to be processed via custom electronics and ultimately provide a digital readout. Current commercial detectors boast 50% Photon Detector Efficiency (PDE) at room temperature and that will be our goal. They typically cost $2000-$5000 which seems over-priced and ready for disruption. Liquid nitrogen temperatures may be needed to see how large a PDE we can get.<br />
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What is SPCM good for? Copied from the datasheet/brochure: LIDAR, Quantum Cryptography, Photon correlation spectroscopy, Astronomical observation, Optical range finding, Adaptive optics, Ultra-sensitive fluorescence, Particle sizing, Microscopy. So maybe this would become a toy/tool for next year's students.<br />
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===[[Kerr Microscope]]===<br />
Proposed By: Sim May Inn (write up by Joel Yeo)<br />
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'''Team members: Gan Jun Herng, Joel Yeo, Sim May Inn'''<br />
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'''Project Location: S11-02-04'''<br />
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Imaging a sample can be done in many ways, depending on the light-matter interaction we are interested in observing. The magneto-optic Kerr effect describes the change in polarization and intensity of incident light when it impinges on the surface of a magnetic material. The resultant reflected light can then form an image through focusing optics which provides high contrast between areas of different magnetization.<br />
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In this project, we will be aiming to build a basic Kerr microscope using off-the-shelf polarizers, objectives, detectors and laser source. An example of a magnetic sample is the magnetic tape from an old school cassette tape. To increase the field of view, we also plan to incorporate automatic raster scanning of the sample through means of an Arduino-controlled sample stage.<br />
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'''Items needed (as of 28 Feb 2022):'''<br />
* Light source (visibile wavelength): <s> Laser, LED </s>, laser diode<br />
* <s> Linear polarizer (sheet) x 2Camera (CCD/CMOS) </s><br />
* <s> Non-polarizing beam splitter </s><br />
* <s> Camera (CCD/CMOS) </s><br />
* <s> Pinhole/aperture </s><br />
* <s> Magnetic samples for Kerr microscopy (eg. Magnetic film, magnets, ferromagnetic materials) </s><br />
* Arduino<br />
* Microscope stage<br />
* Piezoelectrics (?) for moving stage<br />
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===[[Electron Gun]]===<br />
Team Members: Aliki Sofia Rotelli, Lai Tian Hao, Lim En Liang Irvin, Tan Chuan Jie <br />
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The purpose of this project is to design and build an electron gun from the initial concept in order to create a detectable electron beam through the use of a phosphor-coated screen. Additionally, the beam current will be examined in order to better define the devices' capabilities. Mass spectrometry, x-ray production for linear accelerators, and electron-beam lithography are just a few of the applications for electron gun technology.<br />
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===[[Smoke detection in air]]===<br />
Team Members: Cheng De Hao, Huang Hai Tao, Wang Zheng Yu <br />
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Using detector to detect the scattering light and amplify the signal by using the lock-in amplifier.<br />
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===[[In-situ magnetic imaging and Hall detection]]===<br />
Proposed by Sim May Inn<br />
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In computing and memory device architectures, propagation of magnetic domains play an essential role in the encoding and transport of information. Magnetic domain imaging is often employed to unveil such propagation dynamics. Additionally, other than observation and studies of their dynamics, means of detection are also of interest, and this can be achieved through the read-outs of Hall effect signals. As such, this project aims to design and modify an existing low temperature setup to be able to concurrently perform both magnetic domain imaging and Hall signal detection.<br />
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===[[Anti-glare LCD]]===<br />
Team members: Zhang Yuanyuan, Ming Xiaohan, Han Shixin<br />
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As s bad lighting phenomenon, glare phonomenon brings inconvenience to all aspects of human life, especially people's access to information on instruments. In order to suppress glare effectively, anti-glare film is put into research. The common anti-glare film in the market is an optical film using the principle of optical scattering, but it can not adapt to the change of light environment in time, which has some limitations in practical application. In this study, a two-dimensional barcode micro-region orientation structure, based on the characteristics of liquid crystal, namely a random grating structure, was designed by simulation in the lab and using MATLAB software, and its optional parameters were searched.<br />
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===[[Custom atomic beam source]]===<br />
Team Members: Lu Tiangao, Li Putian<br />
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===[[Schlieren imaging]]===<br />
Team members: Zhang Xingjian, Du Jinyi<br />
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===[[Contactless Conductivity Measurement]]===<br />
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Team members: Chen Guohao, Jiang Luwen<br />
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The purpose of this project is to measure the conductivity of materials without having to make electrical contact with them. Specifically, we make use of the eddy-current induced in the materials to calculate the conductivity.<br />
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===[[Quantum Random Number Generator]]===<br />
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Team members: Wang Yang, Xiao Yucan, Zhang Munan<br />
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Random numbers are a fundamental resource in science and engineering with important applications in simulation and cryptography. The inherent randomness at the core of quantum mechanics makes quantum systems a perfect source of entropy. Quantum random number generation is one of the most mature quantum technologies with many alternative generation methods. The purpose of our project is to build a simple optics-based QRNG. We will also collect the random number generated by our device and use some methods to check the randomness.<br />
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===[[Orbits of the Galilean Moons]]===<br />
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Team Members: [[User:Matthew|Matthew Wee]]<br />
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Galileo Galilei’s discovery of celestial bodies that orbit something other than the Earth marked the beginning of the end of the geocentric model of the universe. In this project, we will perform the same observations on those moons as Galileo did 400 years ago.<br />
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===[[Capacitor array based ADC]]===<br />
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Team Members: Zhang Chengyue, Yang Ningli, Guo Diandian, Chen Jiayu<br />
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In this project, we are going to make an ADC.<br />
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==Resources==<br />
===[[Recorded sessions]]===<br />
Some of the sessions will be recorded and uploaded to youtube. Find a description on the [[Recorded sessions]] page.<br />
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===Devices and material===<br />
Apart form all the stuff in the teaching lab, we have a few resources you may want to consider for your project<br />
*...<br />
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'''Books:'''<br />
* P.R. Bevington, D.K. Robinson: Data Reduction and Error Analysis for the Physical Sciences, 3rd edition. McGrawHill, ISBN0-07-119926-8. A very good book containing all the questions you never allowed yourself to ask about error treatment, statistics, fitting of data to models etc.<br />
* Horrowitz/Hill: The Art of Electronics<br />
* C.H. Moore, C.C. Davis, M.A. Coplan: Building Scientific Apparatus. 2nd or higher edition. Perseus Books, ISBN0-201-13189-7. A very comprehensive book about many dirty details in experimental physics, and ways to get simple problems solved. Appears a bit dated, but is a good start for many experimental projects up to this day!<br />
* Christopher C. Davis: Laser and Electro-optics. Useful as a general introduction to many contemporary aspects you come across when working with lasers, with a reasonable introduction of the theory. Very practical for optics.<br />
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'''Software:'''<br />
Some of the more common data processing tools used in experimental physics:<br />
* [http://www.gnuplot.info/ '''Gnuplot''']: A free and very mature data display tool that works on just about any platform used that produces excellent publication-grade eps and pdf figures. Can be also used in scripts. Open source and completely free.<br />
* Various '''Python''' extensions. [http://www.python.org/ Python] is a very powerful free programming language that runs on just about any computer platform. It is open source and completely free.<br />
* '''Matlab''': Very common, good toolset also for formal mathematics, good graphics. Expensive. We may have a site license, but I am not sure how painful it is for us to get a license for this course. Ask me if interested.<br />
* '''Mathematica''': More common among theroetical physicists, very good in formal maths, now with better numerics. Graphs are ok but can be a pain to make looking good. As with Matlab, we do have a campus license but an increasingly painful licensing ritual. Ask me if interested or follow the instruction to install the software in your desktop.<br />
* '''Origin''': Very widespread data processing software with a complete graphical user interface, integrates well into a Windows environment. Most likely available in your research labs, not sure if NUS has a site license.<br />
* '''Labview''': Many of you may have seen this in your labs, but I am not too familiar with it, and chances are it is too resource-hungry to run on the machines we have there. It keeps its promise of a fast learning curve if you want to do simple things but it can get a REAL pain if you want to do subtle things, or want to do things fast, or want to debug code. Expensive and resource-hungry, but comes with good integration of also expensive hardware. May not be worth it if you know any programming language.<br />
* [https://www.circuitlab.com/ '''Circuit Lab''']: a convenient software to design and simulate electrical circuits directly at your browser. I think Flash is required. It works well in Chrome.<br />
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===[[Acronym database]]===<br />
This is an attempt to clarify the countless acronyms we use in our sub-communities (follow headline link)<br />
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===[[Gnuplot tricks]]===<br />
Follow the headline link for some of the random questions that came up with gnuplot.<br />
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== Previous PC5214 wikis ==<br />
* [http://pc5214.org/AY1819S2 AY2018/19 Sem2]<br />
* [http://pc5214.org/AY1415S1 AY2014/15 Sem1]<br />
* [http://pc5214.org/AY1314S1 AY2013/14 Sem1]<br />
* [http://pc5214.org/AY1213S1 AY2012/13 Sem1]<br />
* [http://pc5214.org/AY1112S1 AY2011/12 Sem1]<br />
* [http://pc5214.org/AY1011S1 AY2010/11 Sem1]<br />
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== Some wiki reference materials==<br />
Consult the [//meta.wikimedia.org/wiki/Help:Contents User's Guide] for information on using the wiki software. Other sources:<br />
<br />
* [//www.mediawiki.org/wiki/Manual:FAQ MediaWiki FAQ]<br />
* [[Writing mathematical expressions]]<br />
* [[Uploading images]]</div>MayInnhttps://pc5214.org/index.php?title=In-situ_magnetic_imaging_and_Hall_detection&diff=153In-situ magnetic imaging and Hall detection2022-02-07T10:00:45Z<p>MayInn: /* Idea */</p>
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<div>In computing and memory device architectures, propagation of magnetic domains play an essential role in the encoding and transport of information. Magnetic domain imaging is often employed to unveil such propagation dynamics. Additionally, other than observation and studies of their dynamics, means of detection are also of interest, and this can be achieved through the read-outs of Hall effect signals. As such, this project aims to design and modify an existing low temperature setup to be able to concurrently perform both magnetic domain imaging and Hall signal detection.<br />
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==Team members==<br />
* Sim May Inn<br />
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==Idea==<br />
Modifications are to be implemented onto a low temperature magnetic imaging system which consists of a cryostat, to allow for in-situ device characterisation. This project aims to apply the knowledge gleaned from this module, from cryostats, to pumps, to LIAs, and more if possible.<br />
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==Setup==<br />
Outside of campus.<br />
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==Measurements==</div>MayInn