First Week of the Program
Top row, left to right: Rishabh Kothari, Madelyn Johnson, Alondra Stafford, Victoria Adebayo, Brittany Karki
Lower row: Jordan Coney, Mel Tamhane, Alejandro Garcia
Dinner in Paris
2022 Projects
Alejandro Garcia: Spin dependent recombination
Whiteboard Video:
This summer I worked at École Polytechnique in the condensed matter physics lab. I worked along with fellow REU student Rishabh Kothari. In June we worked under Alistair Rowe and also with Agatha Ulibarri, a previous REU student that was in the same program as us back in 2017. With Alistair we looked at a ternary alloy semiconductor called InGaAs and studied a property known as spin dependent recombination or SDR. We worked on an optical setup and took measurements by shining polarized light at the InGaAs sample and measuring the photoluminescence or PL emitted by the sample at different polarizations. The laser excitation causes electrons in the valence band of the semiconductor to be excited to the conduction band and under recombination when the electron falls back to the valence band it emits PL which we can measure. The special thing about the sample we looked at is that it contained Ga2+ defects which can accept electrons from the conduction band before they reach the valence band and cause no PL to be emitted since that pathway for recombination is non-radiative. The interesting thing is that due to certain spin selection rules, circularly polarized light or sigma light causes the electrons in the valence band to be promoted to the conduction band with a preference towards one spin orientation. This does not occur with linearly polarized light or pi light as the electrons promoted are of a random spin orientation. So having a preference towards one spin orientation causes the electrons that occupy the defects to also have a preference towards that same spin orientation meaning that there is less room for the conduction band electrons to move to the defects due to the Pauli exclusion principle. So more electrons move directly to the valence band and recombine radiatively when excited with sigma light compared to with pi light. This can be measured as a higher intensity of PL when exciting with sigma light versus with pi light. The ratio of the polarization of PL measured when exciting with sigma light versus the polarization of PL measured when exciting with pi light is the SDR ratio. This is what we were trying to study and see if we could observe. We tried at different temperatures, at different laser excitation powers, and at different doses of Ga2+ since these are all factors that can affect SDR. We had a cryocooler that we could pump and cool down to temperatures of 20K. In addition we worked with a Raman spectrometer and took maps of a sample of InGaN where the Raman spectrometer would take Raman spectra at many different points on the sample in a given region. Through this and the help of a previous paper we were able to determine nitrogen concentrations present at different areas on the sample by analyzing the area under a certain peak in the Raman spectra. Rishabh wrote Matlab code that was able to do this very effectively and present a nice color map of the sample corresponding to nitrogen concentrations. The work we did on this Raman map is actually going to be put into a paper written by one of the PhD students at the lab. In July we worked under Fabian Cadiz where we did several experiments studying different properties of other semiconductors. We did more PL measurements where we studied the PL emitted by exciton recombination in WSe2. Excitons are electron-hole quasiparticles that can recombine and emit PL. Also we studied the effects of magnetic fields on the polarization of PL emitted by a GaAs sample. With the help of another researcher, Alain, we installed a magnetic field normal to the sample surface and later we installed an oscillating magnetic field that acts parallel to the sample surface. We could change the strength of the normal magnetic field with an electromagnet by changing the current. We could achieve strengths of up to about 70 Gauss. By taking certain fits of data acquired in these experiments we could, through analysis of parameters in the fit, obtain information about things like the electron spin lifetimes in the sample, the temperature of the electrons, and the fermi level of the sample. Overall, I learned many skills regarding optics such as alignment of lasers, laser safety, setting up optical setups, and theory and notation on optics and on semiconductors. Also useful to learn were skills about using software for spectrometers and using Origin, a very useful data analysis and graphing software.
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Alondra Stafford: Optical tweezers for biomedical applications
Whiteboard Video:
My name is Alondra Stafford and I am a chemistry major and math minor at Spelman College in Atlanta, Georgia. This summer I participated in the Optics in the City of Lights Summer Research Experience for Undergraduates through the University of Michigan. During this experience, I studied optical tweezers for biomedical applications at the Institut d'Optique Graduate School at Université de Paris-Saclay under the supervision of Dr. Nathalie Westbrook.
Dr. Nathalie Westbrook's research is based in biophotonics and uses microrheology to study the causes of Thrombosis, which is a health condition that occurs when a formation of blood clots blocks the veins or arteries. This blockage causes an obstruction to the blood flow in the circulatory system and causes serious complications to the brain and lungs. Thrombosis can affect people of any age, race, gender, and ethnicity.
To study the mechanics of thrombosis, this research lab uses optical tweezers. Optical tweezers are highly focused laser beams that hold and move microscopic objects like atoms, molecules, and living cells. Optical tweezers influence the motion of the microscopic object in a non-contact manner by only using light. Optical tweezers are used in this lab to trap silicon microbeads attached to blood cells. Using the laser beam, force and stress can be applied to the blood cells. The microbeads will be used as markers to study displacement caused by the brownian motion, which is the random movement of microscopic objects in a fluid resulting from the impact of surrounding forces. The goal of the lab is to develop an automated measuring instrument to study the mechanical forces of blood clots using optical tweezers so that doctors may be able to treat patients more readily and efficiently.
To approach this goal, my tasks this summer were to design a phantom blood clot that was transparent and resembled the mechanical properties of a human blood clot, differentiate attachment methods for cell to glass surface attachment and cell to microbead attachment, and analyze resultant brownian motion and viscosity vs elasticity graphs using a Matlab program. Over the course of 9 weeks, I have gained knowledge in biophotonics, optical microscopy, laser safety, gel formation, and microbead experimentation. I’m happy to have had this experience because I was able to witness the interdisciplinary works of this lab and how physics affects medicine. Furthermore, I was able to see how I, as a chemistry student, could play a role in the study of biophotonics.
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Brittany Karki: High order mode conversion for nonlinear optics experiments in micro structured fibers
Whiteboard Video:
The campus I worked at was Institut d’Optique Graduate School (IOGS) in Palaiseau. I was in the Labratoire Charles Fabry working in the Groupe Photonique Non Lineaire under the supervision of Philipe Delaye. The project I was assigned was high order mode conversion for nonlinear optics experiments in micro structured fibers. The objective of this experiment was to assemble a system that allows for selectivity of optical modes. This purpose was to test this system which has been used by other experimental groups, to see of it would be a viable inclusion in my advisors’ larger experiments in generating pairs of single photons.
We started an empty optics table first set up a HeNe laser with mirrors and an objective lens This was just to get the beam into a fiber so that initial fiber could be attached to a filter later. Next thing we built a piezo system that applied the acoustic perturbation. This system consisted of an aluminum cone glued to a piezo electric wafer. This was then charachterized by heterodyne sensor to know how much physical displacement of the piezo took place at each particular applied frequency. This was done several times, by reflecting the beam off the point of the aluminum cone, the soldering and the surface of the wafer.
Next, the filter for the mode conversion was fabricated. This was done by pulling a multi-mode optical fiber to a specific diameter, so it could act as a single mode fiber by filtering out higher order modes. After optimizing the system with a single filter to achieve the highest power output and verify that the exiting beam was in the single fundamental mode, that first filter was replaced with an optical fiber that was a double filter. This fiber had been pulled in two regions. The first was to filter out any high order mode initial beam. There was then a longer middle section that was of the fiber’s original diameter where the acoustic perturbation would be applied. In that perturbation would introduce higher order modes back into the fiber. The second filter would serve to filter that signal to a achieve the single fundamental mode. The purpose of this is to visualize the signal in the time domain to look for a loss in power when no perturbation is taking place. When there is a drop in the signal, that means that for that particular frequency, high order modes were created.
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Jordan Coney: Scanning Tunneling Luminescent Microscopy of Silicon Carbide
Whiteboard Video:
I worked under Dr. Alistar Rowe and Dr. Natalia Alyabyeva at École Polytechnique in the Physique de la Matière Condensée (PMC) lab. The goal of my project this summer was to interrogate the semiconductor Silicon Carbide (SiC) using a Scanning Tunneling Luminescent Microscope (STLM) and to find a) the electroluminescent properties of SiC, b) measure and record the topography of the SiC sample, and c) see if there is a specific mode of emission from the SiC sample.
The SiC sample that we use is commercially made. Through a process where amorphous Si is grafted onto the SiC sample and then melted at high temperatures and etched to remove the solid Si, our sample can oxidize and grow defects. From the etching that was done to our sample, SiC is arranged in Macrosteps and Microsteps. A macrostep is composed of one long terrace and many small step structures called risers (each macrostep making an edge-like structure). A microstep would be each riser in the macrostep.
Experiments using photoluminescence on this sample have occurred in the past. This setup looks like a laser of a certain energy level (high enough to excite the electrons from the valence band of the SiC to the conduction band) being used to probe the surface of our SiC sample. This process creates an electron-hole pairing where through the relaxation and recombination process, luminescence can occur. There was greater light intensity observed in the risers from our sample. This implies that there might be a greater defect intensity in our risers in comparison to the terraces.
My project for this summer was to use electroluminescence to analyze our SiC sample. To do this we needed to use the STLM. The STLM is a machine that operates in an ultra-high vacuum and uses a metallic tip (usually tungsten) to probe our sample by sending electrons through a potential barrier to the sample’s surface. Getting the electrons from the tip to the sample through a potential barrier is quantum tunneling. The tip from the STLM is microns away from the sample and is given an electric current and then we apply a voltage to our sample. From this process, we get electroluminescence where electrons from the tip are sent to the conduction band of our sample and fall into the valence band, and this produces a photon. The electroluminescent emissions from the SiC sample are then directed to a lens that is set up at the bottom of the STLM and are collimated into another lens under the STLM setup. From there, we set up an optical fiber to collect the emissions and send them to a spectrometer to analyze our emissions. SiC without defects emits emissions in the blue. The defects that are found in SiC are producing emissions in the green.
We were able to successfully measure and record the topography of our SiC sample along with being able to use the STLM to probe the SiC sample for electroluminescence (where we then used python code to analyze and graph our results).
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Malhar Tamhane: Simulating Optical Response of Bragg Mirrors for Gold Nanoparticles
Whiteboard Video:
This summer I worked in the LuMIN lab under Bruno Palpant at Central Supélec at the Université de Paris Saclay. I researched in the field of nanophotonics, specifically in regard to gold nanoparticles and their optical properties.
Gold nanoparticle surface plasmons exhibit several interesting properties when irradiated at their resonant frequency. More specifically, they can act as fast yet miniscule heat sources. In biomedical applications, this property, coupled with biochemical delivery, enables the development of treatments to eradicate cancer cells. While much of the field of nano photonics developed in the past century, humans have included gold nanoparticles in artwork and craft for thousands of years because of their aesthetic (and optical) properties.
My project this summer involved building a simulation to help optimize the fabrication of a Bragg mirror setup for a defect layer of gold nanoparticles. Bragg mirrors are thin films that have very little to no absorbance. When stacked in bilayers with two films of low and high refractive index, the mirrors can act as band pass filters for certain wavelengths of light. This wavelength is determined by the size of the films which enables design of the wavelength that the Bragg mirrors permit to pass through. I modeled the propagation of light through stacks of these Bragg mirrors using the transfer matrix method which is based on the interfaces between thin films and the propagation in each film. The calculated reflectance and transmittance values result from values in the final transfer matrix. The defect layer that contains the gold nanoparticles is also modeled in a similar manner. However, because of the gold nanoparticle inclusions, the Yamaguchi effective medium theory is needed to mathematically represent the cavity since it consists of multiple materials. This theory was not included in the final simulations I ran but I did write code for its calculation.
Finally, I modeled the influence of a finite substrate of either silicon of BK7 glass. The challenge with the substrate was two-fold. First, substrates like silicon absorb light which needs to be included in the calculation. Second, with the size of the substrates, the light is no longer coherent which requires different methods for calculation. With some alterations to the transfer matrix method, I was successfully able to model both of these caveats that come with a finite substrate.
This summer helped me further develop my research skills and taught me quite a bit about theoretical modeling. I also was lucky enough to pickup some French along the way and spend time with the amazing people at the LuMIN lab at Centrale Supélec.
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Victoria Adebayo: Probing local recombination properties in nitride semiconductor quantum heterostructures
Whiteboard Video:
This summer I worked on “Probing local recombination properties in nitride semiconductor quantum heterostructures” at Ecole Polytechnique’s Laboratoire de Physique de la Matière Condensée (PMC). Nitrides have many uses such as for blue and white commercial LEDs (light emitting diodes) and for UV water purification applications. However, LEDs often use ternary alloys like InGaN and AlGaN, whose properties are still poorly known, particularly the role of defects and alloy disorder on the recombination properties. Thus, our goal through this project was to better understand the electronic processes in these nitrides. In my project, we probed a sample of an InGaN quantum well that contained defects. To examine these defects, we used a scanning tunneling microscope in order to probe the energy states in the sample while imaging the surface. The STM uses a sharp metal tip made either mechanically from platinum or chemically from tungsten using chemical etching. The tip is brought as close to the surface of the sample as possible without actually touching it. We then apply a positive voltage which injects electrons from the tip into our sample. We probed the sample at both room temperature (300K) and cooled down to 100K using liquid nitrogen. First, we conducted topography measurements in order to image the surface and find the area of the sample with the defects in order to zoom and examine the area further. We then conducted spectroscopy measurements on a 32 by 32 grid to probe the light emission near the defects. At room temperature, we found that light emission was higher near the defects at lower energy levels and at 100K we found two spectra contributions. I learned a lot this summer and I had an amazing experience learning and exploring in Paris.
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Madelyn Johnson: Reading and Writing Fluorescent Micropatterns onto a Surface with Adaptive Optics
Whiteboard Video:
This summer I worked on a project at the Photophysique et photochimie supramoléculaires et macromoléculaires (PPSM) laboratory with Jeff Audibert and Vitor Brasiliense at ENS Paris-Saclay. My project focused on building an optical set-up that allows switching between a confocal path and a TIRF path for writing and reading patterns on a surface.
Writing consists of photo-grafting a molecule to a glass substrate. This occurs when a solution containing a photo-sensitive molecule is excited by the focused laser (470 nm) in an inverted microscope. The molecules form covalent bonds with the substrate and polymerize with already grafted molecules, creating a “pile.” The samples I mostly worked with this summer were diazonium salts. We used two different methods to find the refractive index of the sample. The first utilized quantitative phase imaging using a diffuser to calculate the optical path difference (OPD). The sample was then analyzed using Atomic Force Microscopy (AFM). Using the tapping mode in AFM, the precise thickness (L) of each grafted object was determined, and along with the refractive index for water, the medium, the refractive index of the sample was found using this equation: OPD = L (nsample - nmedium). The second method for determining the refractive index of our sample involved matching the refractive index with another fluid. When a fluid of equal refractive index is placed on top of the grafted object, it is no longer visible. I took pictures of my object in fluids of varying refractive indices and compared these pictures to determine which refractive index the object most closely matched, concluding that this refractive index must then be the refractive index of my sample. I was also able to help with a project that looked at the effect of laser power on the grafting rate. In these experiments, we were able to monitor the grafting in real time, allowing us to better understand the surface modification kinetics. Another way to do this is with fluorescence, using TIRF.
TIRF-M, or total internal reflection fluorescence microscopy, refers to the reading portion of the set-up. This method utilizes objective based TIRF to look at objects that are close to the surface. With Wide-Field illumination, a plane wave excites an entire volume, not just the part near the surface, resulting in a low signal-to-noise ratio (SNR) with axial resolution in the range of 500 nm when the fluorescent objects at the vicinity of the surface are imaged. For TIRF illumination, the light is directed at the surface at an angle by translating the laser focus at the back focal plane. If this angle is greater than the critical angle, the light is completely reflected. This creates an evanescent wave that is transmitted at the surface medium interface. The intensity of this evanescent wave decreases exponentially in the z-direction, only exciting the part of the fluorescent object that is closest to the surface, therefore decreasing the optical sectioning below 100 nm, resulting in the enhancement of the SNR and of the axial resolution. For flat field TIRF, two Axicons, a lens, and the 470 nm laser are used to create a Bessel beam, or ring of light, which is then focused at the back focal plane of the objective. Changing the distance between the lens and the second Axicon changes the diameter of the ring focus which in turn changes the angle of incidence at the object plane. This allows for a homogeneous excitation profile and a wider field of view, compared to the previous set-up (one side illumination). The reading part can be used as variable angle TIRF (VA-TIRF) or variable conical illumination.
The new set-up can easily switch between writing and reading so that in the future an object can be grafted and then analyzed in TIRF. Potential applications for this include tomography where they hope to be able to create 3D images of the grafted objects.
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Rishabh Kothari: Spin-dependent recombination (SDR) in ternary semiconductors
Whiteboard Video:
This summer, I worked with Alejandro Garcia of the iREU program on two main projects utilizing micro-photoluminescence (μ-PL) to probe the optical properties of semiconductors at the Laboratoire de Physique de la Matière Condensé (LPMC) at Ecolé Polytechnique.
Our setup consisted of a Ti:Sapphire continuous-wave laser normally incident on a sample inside a chamber that can achieve a vacuum of 10-6 mbar and can be cooled to about 20 K. This excitation path allowed us to select the incident laser power and polarization, allowing for polarization-resolved PL. A quarter-wave plate was used to switch between linearly polarized laser light to circularly polarized light. The resulting PL from the sample was incident on a spectrometer to record spectra.
We first worked under Dr. Alistair Rowe and Agatha Ulibarri to investigate spin-dependent recombination (SDR) in ternary semiconductors, specifically Ga-ion implanted In0.09Ga0.91As. We measured SDR by dividing the integrated intensity over the relevant PL peak for circularly polarized excitation by that of linearly polarized excitation. SDR is observed in Ga-ion implanted InGaAs due to the stabilization of deep paramagnetic Ga2+ interstitial defect states in the material, which can, through σ excitation of mostly spin up or spin down electrons, become spin polarized [1]. Our results did not show a clear optimum power and Ga-ion implantation dose that maximized SDR as has been previously observed, but we were able to inform future experiments in InGaAs. Spin-filtering, as described, will be important to the development of spintronics and its applications to quantum computing as well as other quantum correlation devices
We also got the opportunity to use Raman spectroscopy to map changes in N content across a GaAs1-xNx wafer, with x nominally equal to 2%, which also shows SDR. By identifying relevant features in the spectra and creating spectra data analysis tools, we produced useful maps of the N content on the surface that will inform further study on the role of N content on the optical properties of GaAs1-xNx.
For our second project, we worked with Dr. Fabian Cadiz to verify experimental methods of measuring spin relaxation times in semiconductors at low temperatures. Our first goal was to observe variation in the polarization of PL from n-doped GaAs with an in-plane static magnetic field. The resulting curve is a Hanle curve, and the width of the curve can be related to the spin relaxation time at a given excitation power [2]. This informed our study of the same sample subject to an in-plane oscillating field with an out-of-plane static field. By varying the out-of- plane field at a given power and in-plane field frequency, we expect to observe a drop in PL polarization to zero at some out-of-plane field strength, with a graph shape similar to that with the in-plane static field at the same power. In the future, this setup will be used to probe electron properties in 2D semiconductors to explore their fundamental physics and inform future device development.
Thanks to all our mentors this summer for their guidance and patience, Dr. Sangjun Park for his help running our experiments, and everyone at LPMC for being welcoming and making our experience fun and engaging.
[1] C.T. Nguyen et al., Applied Physics Letters 103, 052403 (2013)
[2] R. I. Dzhioev et al., Physical Review B 66, 245204 (2002)
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Whiteboard Video:
This summer I worked at École Polytechnique in the condensed matter physics lab. I worked along with fellow REU student Rishabh Kothari. In June we worked under Alistair Rowe and also with Agatha Ulibarri, a previous REU student that was in the same program as us back in 2017. With Alistair we looked at a ternary alloy semiconductor called InGaAs and studied a property known as spin dependent recombination or SDR. We worked on an optical setup and took measurements by shining polarized light at the InGaAs sample and measuring the photoluminescence or PL emitted by the sample at different polarizations. The laser excitation causes electrons in the valence band of the semiconductor to be excited to the conduction band and under recombination when the electron falls back to the valence band it emits PL which we can measure. The special thing about the sample we looked at is that it contained Ga2+ defects which can accept electrons from the conduction band before they reach the valence band and cause no PL to be emitted since that pathway for recombination is non-radiative. The interesting thing is that due to certain spin selection rules, circularly polarized light or sigma light causes the electrons in the valence band to be promoted to the conduction band with a preference towards one spin orientation. This does not occur with linearly polarized light or pi light as the electrons promoted are of a random spin orientation. So having a preference towards one spin orientation causes the electrons that occupy the defects to also have a preference towards that same spin orientation meaning that there is less room for the conduction band electrons to move to the defects due to the Pauli exclusion principle. So more electrons move directly to the valence band and recombine radiatively when excited with sigma light compared to with pi light. This can be measured as a higher intensity of PL when exciting with sigma light versus with pi light. The ratio of the polarization of PL measured when exciting with sigma light versus the polarization of PL measured when exciting with pi light is the SDR ratio. This is what we were trying to study and see if we could observe. We tried at different temperatures, at different laser excitation powers, and at different doses of Ga2+ since these are all factors that can affect SDR. We had a cryocooler that we could pump and cool down to temperatures of 20K. In addition we worked with a Raman spectrometer and took maps of a sample of InGaN where the Raman spectrometer would take Raman spectra at many different points on the sample in a given region. Through this and the help of a previous paper we were able to determine nitrogen concentrations present at different areas on the sample by analyzing the area under a certain peak in the Raman spectra. Rishabh wrote Matlab code that was able to do this very effectively and present a nice color map of the sample corresponding to nitrogen concentrations. The work we did on this Raman map is actually going to be put into a paper written by one of the PhD students at the lab. In July we worked under Fabian Cadiz where we did several experiments studying different properties of other semiconductors. We did more PL measurements where we studied the PL emitted by exciton recombination in WSe2. Excitons are electron-hole quasiparticles that can recombine and emit PL. Also we studied the effects of magnetic fields on the polarization of PL emitted by a GaAs sample. With the help of another researcher, Alain, we installed a magnetic field normal to the sample surface and later we installed an oscillating magnetic field that acts parallel to the sample surface. We could change the strength of the normal magnetic field with an electromagnet by changing the current. We could achieve strengths of up to about 70 Gauss. By taking certain fits of data acquired in these experiments we could, through analysis of parameters in the fit, obtain information about things like the electron spin lifetimes in the sample, the temperature of the electrons, and the fermi level of the sample. Overall, I learned many skills regarding optics such as alignment of lasers, laser safety, setting up optical setups, and theory and notation on optics and on semiconductors. Also useful to learn were skills about using software for spectrometers and using Origin, a very useful data analysis and graphing software.
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Alondra Stafford: Optical tweezers for biomedical applications
Whiteboard Video:
My name is Alondra Stafford and I am a chemistry major and math minor at Spelman College in Atlanta, Georgia. This summer I participated in the Optics in the City of Lights Summer Research Experience for Undergraduates through the University of Michigan. During this experience, I studied optical tweezers for biomedical applications at the Institut d'Optique Graduate School at Université de Paris-Saclay under the supervision of Dr. Nathalie Westbrook.
Dr. Nathalie Westbrook's research is based in biophotonics and uses microrheology to study the causes of Thrombosis, which is a health condition that occurs when a formation of blood clots blocks the veins or arteries. This blockage causes an obstruction to the blood flow in the circulatory system and causes serious complications to the brain and lungs. Thrombosis can affect people of any age, race, gender, and ethnicity.
To study the mechanics of thrombosis, this research lab uses optical tweezers. Optical tweezers are highly focused laser beams that hold and move microscopic objects like atoms, molecules, and living cells. Optical tweezers influence the motion of the microscopic object in a non-contact manner by only using light. Optical tweezers are used in this lab to trap silicon microbeads attached to blood cells. Using the laser beam, force and stress can be applied to the blood cells. The microbeads will be used as markers to study displacement caused by the brownian motion, which is the random movement of microscopic objects in a fluid resulting from the impact of surrounding forces. The goal of the lab is to develop an automated measuring instrument to study the mechanical forces of blood clots using optical tweezers so that doctors may be able to treat patients more readily and efficiently.
To approach this goal, my tasks this summer were to design a phantom blood clot that was transparent and resembled the mechanical properties of a human blood clot, differentiate attachment methods for cell to glass surface attachment and cell to microbead attachment, and analyze resultant brownian motion and viscosity vs elasticity graphs using a Matlab program. Over the course of 9 weeks, I have gained knowledge in biophotonics, optical microscopy, laser safety, gel formation, and microbead experimentation. I’m happy to have had this experience because I was able to witness the interdisciplinary works of this lab and how physics affects medicine. Furthermore, I was able to see how I, as a chemistry student, could play a role in the study of biophotonics.
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Brittany Karki: High order mode conversion for nonlinear optics experiments in micro structured fibers
Whiteboard Video:
The campus I worked at was Institut d’Optique Graduate School (IOGS) in Palaiseau. I was in the Labratoire Charles Fabry working in the Groupe Photonique Non Lineaire under the supervision of Philipe Delaye. The project I was assigned was high order mode conversion for nonlinear optics experiments in micro structured fibers. The objective of this experiment was to assemble a system that allows for selectivity of optical modes. This purpose was to test this system which has been used by other experimental groups, to see of it would be a viable inclusion in my advisors’ larger experiments in generating pairs of single photons.
We started an empty optics table first set up a HeNe laser with mirrors and an objective lens This was just to get the beam into a fiber so that initial fiber could be attached to a filter later. Next thing we built a piezo system that applied the acoustic perturbation. This system consisted of an aluminum cone glued to a piezo electric wafer. This was then charachterized by heterodyne sensor to know how much physical displacement of the piezo took place at each particular applied frequency. This was done several times, by reflecting the beam off the point of the aluminum cone, the soldering and the surface of the wafer.
Next, the filter for the mode conversion was fabricated. This was done by pulling a multi-mode optical fiber to a specific diameter, so it could act as a single mode fiber by filtering out higher order modes. After optimizing the system with a single filter to achieve the highest power output and verify that the exiting beam was in the single fundamental mode, that first filter was replaced with an optical fiber that was a double filter. This fiber had been pulled in two regions. The first was to filter out any high order mode initial beam. There was then a longer middle section that was of the fiber’s original diameter where the acoustic perturbation would be applied. In that perturbation would introduce higher order modes back into the fiber. The second filter would serve to filter that signal to a achieve the single fundamental mode. The purpose of this is to visualize the signal in the time domain to look for a loss in power when no perturbation is taking place. When there is a drop in the signal, that means that for that particular frequency, high order modes were created.
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Jordan Coney: Scanning Tunneling Luminescent Microscopy of Silicon Carbide
Whiteboard Video:
I worked under Dr. Alistar Rowe and Dr. Natalia Alyabyeva at École Polytechnique in the Physique de la Matière Condensée (PMC) lab. The goal of my project this summer was to interrogate the semiconductor Silicon Carbide (SiC) using a Scanning Tunneling Luminescent Microscope (STLM) and to find a) the electroluminescent properties of SiC, b) measure and record the topography of the SiC sample, and c) see if there is a specific mode of emission from the SiC sample.
The SiC sample that we use is commercially made. Through a process where amorphous Si is grafted onto the SiC sample and then melted at high temperatures and etched to remove the solid Si, our sample can oxidize and grow defects. From the etching that was done to our sample, SiC is arranged in Macrosteps and Microsteps. A macrostep is composed of one long terrace and many small step structures called risers (each macrostep making an edge-like structure). A microstep would be each riser in the macrostep.
Experiments using photoluminescence on this sample have occurred in the past. This setup looks like a laser of a certain energy level (high enough to excite the electrons from the valence band of the SiC to the conduction band) being used to probe the surface of our SiC sample. This process creates an electron-hole pairing where through the relaxation and recombination process, luminescence can occur. There was greater light intensity observed in the risers from our sample. This implies that there might be a greater defect intensity in our risers in comparison to the terraces.
My project for this summer was to use electroluminescence to analyze our SiC sample. To do this we needed to use the STLM. The STLM is a machine that operates in an ultra-high vacuum and uses a metallic tip (usually tungsten) to probe our sample by sending electrons through a potential barrier to the sample’s surface. Getting the electrons from the tip to the sample through a potential barrier is quantum tunneling. The tip from the STLM is microns away from the sample and is given an electric current and then we apply a voltage to our sample. From this process, we get electroluminescence where electrons from the tip are sent to the conduction band of our sample and fall into the valence band, and this produces a photon. The electroluminescent emissions from the SiC sample are then directed to a lens that is set up at the bottom of the STLM and are collimated into another lens under the STLM setup. From there, we set up an optical fiber to collect the emissions and send them to a spectrometer to analyze our emissions. SiC without defects emits emissions in the blue. The defects that are found in SiC are producing emissions in the green.
We were able to successfully measure and record the topography of our SiC sample along with being able to use the STLM to probe the SiC sample for electroluminescence (where we then used python code to analyze and graph our results).
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Malhar Tamhane: Simulating Optical Response of Bragg Mirrors for Gold Nanoparticles
Whiteboard Video:
This summer I worked in the LuMIN lab under Bruno Palpant at Central Supélec at the Université de Paris Saclay. I researched in the field of nanophotonics, specifically in regard to gold nanoparticles and their optical properties.
Gold nanoparticle surface plasmons exhibit several interesting properties when irradiated at their resonant frequency. More specifically, they can act as fast yet miniscule heat sources. In biomedical applications, this property, coupled with biochemical delivery, enables the development of treatments to eradicate cancer cells. While much of the field of nano photonics developed in the past century, humans have included gold nanoparticles in artwork and craft for thousands of years because of their aesthetic (and optical) properties.
My project this summer involved building a simulation to help optimize the fabrication of a Bragg mirror setup for a defect layer of gold nanoparticles. Bragg mirrors are thin films that have very little to no absorbance. When stacked in bilayers with two films of low and high refractive index, the mirrors can act as band pass filters for certain wavelengths of light. This wavelength is determined by the size of the films which enables design of the wavelength that the Bragg mirrors permit to pass through. I modeled the propagation of light through stacks of these Bragg mirrors using the transfer matrix method which is based on the interfaces between thin films and the propagation in each film. The calculated reflectance and transmittance values result from values in the final transfer matrix. The defect layer that contains the gold nanoparticles is also modeled in a similar manner. However, because of the gold nanoparticle inclusions, the Yamaguchi effective medium theory is needed to mathematically represent the cavity since it consists of multiple materials. This theory was not included in the final simulations I ran but I did write code for its calculation.
Finally, I modeled the influence of a finite substrate of either silicon of BK7 glass. The challenge with the substrate was two-fold. First, substrates like silicon absorb light which needs to be included in the calculation. Second, with the size of the substrates, the light is no longer coherent which requires different methods for calculation. With some alterations to the transfer matrix method, I was successfully able to model both of these caveats that come with a finite substrate.
This summer helped me further develop my research skills and taught me quite a bit about theoretical modeling. I also was lucky enough to pickup some French along the way and spend time with the amazing people at the LuMIN lab at Centrale Supélec.
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Victoria Adebayo: Probing local recombination properties in nitride semiconductor quantum heterostructures
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This summer I worked on “Probing local recombination properties in nitride semiconductor quantum heterostructures” at Ecole Polytechnique’s Laboratoire de Physique de la Matière Condensée (PMC). Nitrides have many uses such as for blue and white commercial LEDs (light emitting diodes) and for UV water purification applications. However, LEDs often use ternary alloys like InGaN and AlGaN, whose properties are still poorly known, particularly the role of defects and alloy disorder on the recombination properties. Thus, our goal through this project was to better understand the electronic processes in these nitrides. In my project, we probed a sample of an InGaN quantum well that contained defects. To examine these defects, we used a scanning tunneling microscope in order to probe the energy states in the sample while imaging the surface. The STM uses a sharp metal tip made either mechanically from platinum or chemically from tungsten using chemical etching. The tip is brought as close to the surface of the sample as possible without actually touching it. We then apply a positive voltage which injects electrons from the tip into our sample. We probed the sample at both room temperature (300K) and cooled down to 100K using liquid nitrogen. First, we conducted topography measurements in order to image the surface and find the area of the sample with the defects in order to zoom and examine the area further. We then conducted spectroscopy measurements on a 32 by 32 grid to probe the light emission near the defects. At room temperature, we found that light emission was higher near the defects at lower energy levels and at 100K we found two spectra contributions. I learned a lot this summer and I had an amazing experience learning and exploring in Paris.
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Madelyn Johnson: Reading and Writing Fluorescent Micropatterns onto a Surface with Adaptive Optics
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This summer I worked on a project at the Photophysique et photochimie supramoléculaires et macromoléculaires (PPSM) laboratory with Jeff Audibert and Vitor Brasiliense at ENS Paris-Saclay. My project focused on building an optical set-up that allows switching between a confocal path and a TIRF path for writing and reading patterns on a surface.
Writing consists of photo-grafting a molecule to a glass substrate. This occurs when a solution containing a photo-sensitive molecule is excited by the focused laser (470 nm) in an inverted microscope. The molecules form covalent bonds with the substrate and polymerize with already grafted molecules, creating a “pile.” The samples I mostly worked with this summer were diazonium salts. We used two different methods to find the refractive index of the sample. The first utilized quantitative phase imaging using a diffuser to calculate the optical path difference (OPD). The sample was then analyzed using Atomic Force Microscopy (AFM). Using the tapping mode in AFM, the precise thickness (L) of each grafted object was determined, and along with the refractive index for water, the medium, the refractive index of the sample was found using this equation: OPD = L (nsample - nmedium). The second method for determining the refractive index of our sample involved matching the refractive index with another fluid. When a fluid of equal refractive index is placed on top of the grafted object, it is no longer visible. I took pictures of my object in fluids of varying refractive indices and compared these pictures to determine which refractive index the object most closely matched, concluding that this refractive index must then be the refractive index of my sample. I was also able to help with a project that looked at the effect of laser power on the grafting rate. In these experiments, we were able to monitor the grafting in real time, allowing us to better understand the surface modification kinetics. Another way to do this is with fluorescence, using TIRF.
TIRF-M, or total internal reflection fluorescence microscopy, refers to the reading portion of the set-up. This method utilizes objective based TIRF to look at objects that are close to the surface. With Wide-Field illumination, a plane wave excites an entire volume, not just the part near the surface, resulting in a low signal-to-noise ratio (SNR) with axial resolution in the range of 500 nm when the fluorescent objects at the vicinity of the surface are imaged. For TIRF illumination, the light is directed at the surface at an angle by translating the laser focus at the back focal plane. If this angle is greater than the critical angle, the light is completely reflected. This creates an evanescent wave that is transmitted at the surface medium interface. The intensity of this evanescent wave decreases exponentially in the z-direction, only exciting the part of the fluorescent object that is closest to the surface, therefore decreasing the optical sectioning below 100 nm, resulting in the enhancement of the SNR and of the axial resolution. For flat field TIRF, two Axicons, a lens, and the 470 nm laser are used to create a Bessel beam, or ring of light, which is then focused at the back focal plane of the objective. Changing the distance between the lens and the second Axicon changes the diameter of the ring focus which in turn changes the angle of incidence at the object plane. This allows for a homogeneous excitation profile and a wider field of view, compared to the previous set-up (one side illumination). The reading part can be used as variable angle TIRF (VA-TIRF) or variable conical illumination.
The new set-up can easily switch between writing and reading so that in the future an object can be grafted and then analyzed in TIRF. Potential applications for this include tomography where they hope to be able to create 3D images of the grafted objects.
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Rishabh Kothari: Spin-dependent recombination (SDR) in ternary semiconductors
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This summer, I worked with Alejandro Garcia of the iREU program on two main projects utilizing micro-photoluminescence (μ-PL) to probe the optical properties of semiconductors at the Laboratoire de Physique de la Matière Condensé (LPMC) at Ecolé Polytechnique.
Our setup consisted of a Ti:Sapphire continuous-wave laser normally incident on a sample inside a chamber that can achieve a vacuum of 10-6 mbar and can be cooled to about 20 K. This excitation path allowed us to select the incident laser power and polarization, allowing for polarization-resolved PL. A quarter-wave plate was used to switch between linearly polarized laser light to circularly polarized light. The resulting PL from the sample was incident on a spectrometer to record spectra.
We first worked under Dr. Alistair Rowe and Agatha Ulibarri to investigate spin-dependent recombination (SDR) in ternary semiconductors, specifically Ga-ion implanted In0.09Ga0.91As. We measured SDR by dividing the integrated intensity over the relevant PL peak for circularly polarized excitation by that of linearly polarized excitation. SDR is observed in Ga-ion implanted InGaAs due to the stabilization of deep paramagnetic Ga2+ interstitial defect states in the material, which can, through σ excitation of mostly spin up or spin down electrons, become spin polarized [1]. Our results did not show a clear optimum power and Ga-ion implantation dose that maximized SDR as has been previously observed, but we were able to inform future experiments in InGaAs. Spin-filtering, as described, will be important to the development of spintronics and its applications to quantum computing as well as other quantum correlation devices
We also got the opportunity to use Raman spectroscopy to map changes in N content across a GaAs1-xNx wafer, with x nominally equal to 2%, which also shows SDR. By identifying relevant features in the spectra and creating spectra data analysis tools, we produced useful maps of the N content on the surface that will inform further study on the role of N content on the optical properties of GaAs1-xNx.
For our second project, we worked with Dr. Fabian Cadiz to verify experimental methods of measuring spin relaxation times in semiconductors at low temperatures. Our first goal was to observe variation in the polarization of PL from n-doped GaAs with an in-plane static magnetic field. The resulting curve is a Hanle curve, and the width of the curve can be related to the spin relaxation time at a given excitation power [2]. This informed our study of the same sample subject to an in-plane oscillating field with an out-of-plane static field. By varying the out-of- plane field at a given power and in-plane field frequency, we expect to observe a drop in PL polarization to zero at some out-of-plane field strength, with a graph shape similar to that with the in-plane static field at the same power. In the future, this setup will be used to probe electron properties in 2D semiconductors to explore their fundamental physics and inform future device development.
Thanks to all our mentors this summer for their guidance and patience, Dr. Sangjun Park for his help running our experiments, and everyone at LPMC for being welcoming and making our experience fun and engaging.
[1] C.T. Nguyen et al., Applied Physics Letters 103, 052403 (2013)
[2] R. I. Dzhioev et al., Physical Review B 66, 245204 (2002)
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