First Day of the Program
From Left to Right: Alex Sarracino, Michele Kelley, Kathleen Bolan, Zack Li, Angela Ludvigsen, Kevin Montes, Katherine De Los Santos, Daniel Hickox-Young, Stephanie Tietz, Alexander Lidiak, Britta Gorman, Paul Scott
Last Night of the Program
Alex Sarracino: LED Efficiency
This summer I was assigned to Dr. Jacques Peretti’s group at PMC in Ecole Polytechnique. I worked for the most part with Graduate student Petr Polovodov, and Dr. Yves Lassailly, and initially with fellow REU student Zequn Li. The initial goal for my project was to see whether inhomogeneity in semiconductor LED samples were one of the causes for loss of efficiency at higher currents in LEDs.
The project at the beginning was to put a GaN sample in the STM, and scan it with a platinum tip. This caused the semiconductor sample to emit photons, which were captured in a photomultiplier. We scanned an area so we got a 2D image of the luminescence at each point. Since the STM also gives us a 3D image of the topology, we were able to match the topology with the luminescence and see if there was any overlap in the areas with inhomogeneity and places with more or less luminescence.
Afterwards, we wanted to study an interesting result of using high currents when scanning the sample, which is that a modification of up to ~70nm in depth can be created. What I did was that I made multiple modifications with different parameters and scan times, calculated the depth, and plotted the results into a graph to see how the depth changes depending on how long you scan the sample for. We were also interested in how the modification process itself worked, so I made a grid pattern with different parameters, and used an SEM to do chemical analysis of the sample.
Another interesting aspect of this modification was that they can also work as gratings. So anther thing I worked on is trying to make the modifications as small and as close to each other as possible, to see how precise our modification procedure can be. One of the ideas is that if we can make gratings small enough and in the semi-conductor, we can then modify the light while still in the LED. The modifications were then viewed with an SEM to make sure that the grid pattern was correct.
But modifications affect the luminescence of the LED, and it was not good that we were modifying the surface as we were scanning it, so we wanted to stop it. We put a 2nm layer of Palladium on the sample, to see if this would stop the modification. Palladium was used because it was readily available and it is very transparent to electrons, so it wouldn’t cause too much of a drop in efficiency.
The last week of the program I stopped studying the modifications and attempted to get the luminescence signal from a GaAs sample. This proved to be hard since GaAs is not transparent (which made the alignment tough) and had issues during the approach of the STM tip.
Katherine De Los Santos: Solidification of Marbles and the Search for Second Harmonic Generation
A marble is a liquid droplet coated in hydrophobic or hydrophilic molecules called surfactants; surfactants allow the marbles to maintain its spherical shape by lowering the surface tension between two liquids. The droplets consisted of NOA, liquid crystals, and magnetic particles. The carrier phase consisted of fluorinated oil and different types of surfactants of varying ratios. Our goal was to determine the best “concoction” that will yield the most stable marbles.
After collecting liquid marbles made in the microfluidic device, we exposed them to uv light in order to solidify them. All the marbles were composed in part of NOA (Norland Optical Adhesive), a liquid photopolymer that solidifies when exposed to UV light.
We viewed the solidified marbles for 4 of the trials with a scanning electron microscope. We saw that many of the marbles ended up collapsing from their spherical shape, some had a distinct phase separation, and others were stable and sturdy.
The second goal of the experiment was to try to detect second harmonic generation (SHG) in the marbles with magnetic particles called maghemite. In linear optics, the polarization, which is the density of permanent or induced electric dipole, is proportional to the applied electric field. This relationship completely describes the response of the optical medium. The polarization can be expanded as a Taylor series. When the electric field is small the effect of the higher order terms is negligible. They are important, however, at high intensities and it’s where we can detect non-linear phenomena such as SHG. SHG results from the contribution of the second term, the square of the electric field, where emitted light of twice the frequency and half the wavelength of the incident light can be detected.
We probed the marbles with the maghemite particles using two-photon scanning microscopy at a wavelength of 850 nm. We looked for SHG, so emission of violet light at 425 nm. We were unsuccessful in our efforts generate SHG with maghemite. All scans came up blank. But prospective work on this project can test out different particles, and improve upon our most promising marble concoction.
Living and working in Paris for two months has changed my perception of what a European metropolitan city is and what being a scientist abroad might entail. Applying for graduate school abroad does not seem as difficult as I once thought. Any scientific community in France has links or collaborators in the United States. No group is an isolated island. I knew Paris was a beautiful city, but I didn’t know it was also diverse and dynamic, an ideal environment for a young scientist.
Daniel Hickox-Young: Emission Patterns of Optical Microcavities
I spent this summer working at École Normale Supériore de Cachan in the Laboratoire de Photonique Quantique et Moléculaire under professor Mélanie Lebental and with the assistance of Léonard Milliet-Treboux. My research focused on analyzing the spectra and patterns of light emitted from optical microcavities with a variety of geometries and pumping techniques. Optical microcavities have applications in a variety of fields, including data transfer and chemical/biosensing. Our goal was not to prepare samples for use in any particular field but rather to better understand the emission characteristics of a few cavities, which could lead to further innovations in the future.
The cavities I studied are composed of PMMA (polymer, known commercially as “plexi-glass”) doped with DCM (laser dye). We worked with both 2D and 3D samples. The 2D samples are about 0.6 microns thick, which is approximately the same as the wavelength of the emitted light, so oscillations out of the plane are unlikely when observing light emitted in the plane of the sample. We pump the cavities from above with a 532nm pulsed laser and observe the emission spectra in the form of a wavelength vs intensity plot using a spectrometer. Much of my initial work involved writing computer programs in Python in order to plot and analyze the spectra information.
Some samples produce a highly directional emission pattern. A former member of the group, Stefan Bittner, recently proposed a model predicting oscillations in intensity near these maximal emission angles for the 2D square. No one had observed these oscillations previously due to the high resolution required. However, by introducing a vertical slit between the cavity and the spectrometer we were able to increase our resolution. I performed experiments with multiple samples of various sizes, aligning the sample with the pump beam and taking data with the spectrometer at optimal pumping power. I wrote some additional programs in order to isolate the data for certain wavelengths and plot the changes in intensity as a function of viewing angle. The data and analysis confirmed the predicted oscillations with very good agreement.
I also had the opportunity to do some research in two relatively new areas: 3D cavities and pumping with an SLM. The 3D cavities were analyzed in a manner very similar to the 2D cavities, only we needed to acquire and analyze data taken in three dimensions instead of two. 3D cavities are of interest because the new dimension should allow for new and potentially interesting optical paths. The SLM is a Spatial Light Modulator which another member of the program, Paul Scott, configured to allow us to change the shape of the pumping beam. This should allow us to pump different parts of the cavity, ideally allowing us to pump as efficiently as possible. We only really began to study these two areas, so while what we’ve found is interesting we aren’t able to say anything conclusive yet.
In summary, I had a unique opportunity this summer to explore previously undiscovered territory, both in probing the precise emission patterns of the well-studied 2D square, and in beginning to characterize the novel 3D cavities and SLM pumping techniques. Perhaps more importantly, I gained a lot of experience with designing and carrying out experiments. I was also able to practice programming and in particular to utilize my own computer programs in a data analysis context. It was an honor to work in such an excellent group and to learn about an interesting field. I’d like to thank Dr. Mélanie Lebental and Léonard Milliet-Treboux once again for welcoming me this summer, and for their invaluable guidance and assistance throughout the experience.
Britta Gorman: Role of external electric fields on the domain microstructures of ferroelectric materials
For the 2015 summer iREU Optics in the City of Light program I worked at the Université Paris-Sud with Laurent Daniel. My lab is investigating the effects of external electric fields on the domain microstructures of ferroelectric materials. The purpose is to learn the optimal region of use and how different materials react in extreme conditions. The team previously performed in situ x-ray diffraction experiments to measure the variations in atomic distances when the applied electric field is changed. This is a very common measurement method in which x-rays are diffracted through the various atomic planes in a sample and interfere to make circular diffraction patterns. Each circle indicates diffraction through a different atomic plane, the diffraction angle is related to the atomic distance by Bragg’s law.
My job was to adjust a preexisting Matlab code to analyze these x-ray diffraction data of various ferroelectric materials. During our experiment, we utilized shear mode elongation, therefore the external electric field was applied perpendicular to the spontaneous polarization of the sample. The Matlab code analyzes the change in intensity and position of the diffraction rings when the electric field is changed.
Our sample contained tetragonal unit cells, therefore, for certain atomic planes the atomic distance varies slightly due to different orientations of the cell. This leads to two slightly different angles of diffraction and thus two very close diffraction rings instead of one. The ratio between these two diffraction patterns can yield information about the relative orientation of the unit cells in the sample and thus the polarization.
The initial Matlab code was a rough outline of what was the end goal for my project. Many changes had to be made to produce a more rigorous analysis and more consistent results. Many graphs were added to the double-peak subroutine to further analyze the existing data. The program also analyzed the shift in peak position for single-peak data. This shift is a result of a change in the atomic distance and thus a compression or elongation of the unit cell perpendicular to that plane.
My results were consistent with what was expected and with the theory. My adjusted code will be used to analyze x-ray diffraction data of many different materials to investigate the responses of these samples under specific conditions and their inevitable repolarization.
Stephanie Tietz: Analysis of Marbles in Microfluidic Devices
This summer I worked alongside my fellow REU researcher, Katherine de los Santos, in the lab of Professor Abdel el Abed at ENS-Cachan. We began the research by fabricating microfluidic devices in a clean room. Once we had made devices, we used them to make microdroplets inside the inner channels. This can be done through special geometries, like a flow-focusing channel which pinches the solution until a droplet is formed. When we were able to successfully and consistently make droplets, we added magnetic particles to them. Our goal was to see what concentration was needed to get aggregation when a magnetic field was applied.
With a known magnetic particle concentration in mind, we worked on solidifying those droplets by using a negative photoresist material to carry the magnetic particles. When UV light is applied to this material, it solidifies which allowed us to make “marbles” – solid microdroplets. These droplets were then used in our second-harmonic generation analysis to determine if maghemite particles exhibited this optical phenomenon.
Even though we didn't end up finding second-harmonic generation with the maghemite particles, I feel that having the opportunity to get into a lab and perform experiments was invaluable. Because of this research experience, I will be more comfortable in labs in the future and it solidified my plan to pursue experimental research in graduate school. I was also excited to discover that the same lab machines we used to fabricate the microfluidic devices will be used to make metamaterials in my upcoming research.
I want to take the time to thank the University of Michigan, ENS-Cachan, Dr. Abdel el Abed, and the NSF for their time and support in my research endeavor this summer.
Michele Kelley: LymanAlpha Project
Over the course of this summer I worked under the guidance of Jacques Robert at Laboratoire Aimé Cotton (LAC), Université Paris-Sud as a part of the LymanAlpha project. LymanAlpha has the objective of creating an ultra-cold atomic hydrogen beam for a many facet purpose; high precision metrology on ultra-cold hydrogen, molecular physics with cold hydrogen, cold collisions, and gravity measurements on anti-hydrogen for equivalence principle testing once the experiment is transferred to CERN.
Essentially, the creation of an ultra-cold hydrogen beam can be broken down into four major steps. First diatomic, hydrogen is pumped into a vacuumed and water cooled glass tube system. It is then dissociated into atomic hydrogen by a resonance and load coil antenna system at 27 MHz, a radio frequency. The beam of atom hydrogen then undergoes cryogenic cooling by liquid helium, which will lower its temperature to approximately 10 K, leaving the atoms with an average velocity of 400 m/s. At this point, the beam of hydrogen is cold enough to be super cooled by lasing cooling. This will be achieved with a quasi continuous laser lasing at the Lyman alpha line of hydrogen, which is 121.6 nm. The team at LAC is constructing this ground breaking laser.
My project was to create the atomic hydrogen beam using radio frequency dissociation. This requires a two coil system; a resonance coil, also called an antenna coil, and the load coil that is connected directly to the radio frequency source. The dissociation occurs because free electrons oscillate in the field created by the coils. This causes inelastic collisions between the electrons and the diatomic hydrogen. Through several processes, this leads to the production of atomic hydrogen at a high percentage, approximately 98%. One of the most notable and visually recognizable processes is the creation of photons as electrons fall down in energy levels as a result of these collisions. For hydrogen, this is the pinkish glow of the alpha Balmer line at 656.3 nm.
Since this process of dissociation by radio frequency has been studied for several decades, I mostly did background research during the course of the first month of my internship. In this way, I was able to refine my skills in garnering ideas and designs from past experiments in various articles. I was thus able to build upon them in hopes of creating a superior system. Additionally, deviling so deep into the theory behind the dissociation process and the mechanism that induces it was critical to my ability to identify and fix problems as they occurred during the second month when the system was being constructed.
Foremost for my overall experience, I was fortunate enough to have daily interaction with my mentor, Jacques Robert. It was important to him that I was able to see how he as an experimental physicist thinks and attacks a problem. My mentor wanted to ensure that not only was I fully involved in the process of creating the atomic hydrogen beam, but also that I learned how to be a great scientist in a scope that was outside of my project. For this, I am ever thankful as it has surely impacted, not only this summer, but my entire career as a physicist.
As a whole, I expanded my knowledge of atomic beams and refined my laboratory and research skills. However, more importantly, I gained a perspective on the process of conducting research; from the theory, to the construction, to the problem solving skill necessary to be successful. Before this experience, I was quite sure that I wanted to work in the international field and this summer has helped affirm this. I am ever grateful to the National Science Foundation for supporting opportunities that allow students, such as myself, to conduct research in a cross-cultural setting.
Angela Ludvigsen: Ultra-cold hydrogen beams
My name is Angela Ludvigsen and I am an undergraduate at the University of Wisconsin – River Falls. During the summer of 2015, I had the opportunity to participate in the Optics in the City of Light iREU hosted by the University of Michigan – Ann Arbor and sponsored by the National Science Foundation. My research advisor was Professor Jacques Robert of the Laboratoire Aimé Cotton.
Professor Robert is engaged in research on ultra-cold hydrogen beams. At a pressure of about 10^-6 torr, gaseous H2 molecules undergo a radio frequency discharge resulting in the creation of electrons and H atoms. The H atoms are directed into a chamber and cooled to 10 Kelvin using liquid He cryogenic cooling. This allows the atoms to gain a density factor of about 5 with respect to ambient temperature and reduce their velocity to 400m/s. Once available, a 121.6nm Quasi-Continuous-Wave laser will be used to slow the 400m/s H atoms to 10m/s in about 50cm within 200μs. The laser will reduce the temperature of the H atoms to about 10mK. This will result is a slower beam of H atoms with a well characterized velocity distribution.
I came into the iREU thinking that I would work on an aspect of Prof Robert’s project and have deliverables to show for my time. To my surprise, my first requirement was "to learn," and not "to do." I spent many hours reading research papers and learning about my mentor’s research in ultra-cold hydrogen beams. I discovered that the purpose of the iREU was to help me to understand what it was like to be a scientific researcher, devoted to learning and to research. I was not in the iREU to learn to be a lab technician. I took advantage of opportunities to visit Laboratoire Aimé Cotton‘s physicists' labs and to learn about their research. I visited the machine shop and made friends with the gifted technicians there. I was in the audience for two different PhD candidates presenting and defending their research and ultimately, being awarded their degrees. My adviser included me in a meeting he had with other researchers collaborating on what processes would be better than calculating the diabatization, by hand, for H+H- and H(1s)+H(4s). It was quite evident that professional researchers were willing to share their expertise to help other researchers solve problems.
When my advisor judged me sufficiently ready, he gave me a challenge--to build a helical copper coil and antenna with these properties: it must use 2mm copper wire, resonate at a radio frequency of 27MHz, and be contained in a shield of constant size. I referred to some research papers, checked our lab for materials readily available, and worked out the math for the coil. My advisor approved the design and gave me permission to build it.
Once it was built, I tested the helical coil. I found that my coil achieved the highest power with a triple loop antenna. This made sense, since with a higher power, a higher discharge is produced.
Under my advisor's guidance, I integrated the helical coil and antenna into the experimental system. The helical coil and an antenna were wrapped around two concentric glass tubes, the inner tube filled with gaseous H2 molecules. When switched on and working properly, the helical coil and antenna would create a discharge of H2 molecules, producing H atoms and electrons. When I tried to create the H2 discharge, problems arose. There was no experimental procedure to follow. I would work with my coil and the equipment until something broke, then figure out how to fix it or develop a plan "B." When results didn't emerge as expected, I would try to figure out how to resolve it. I encountered blown fuses, broken glass tubes, ill-fitting replacement tubes, missing tesla coils, contamination, weak light sources, and the helical coil and antenna not being powerful enough to create a discharge. To solve these problems, I would have to refer back to the research papers, search online for equipment instruction manuals, talk with other researchers, and visit the machine shop to change parameters.
By the end, the helical coil and antenna were able to achieve a discharge with H2. Spectra were collected to verify that the discharge was indeed Hydrogen.
The learning I have now with the helical resonator will be of great benefit in my future. My advisor at my home university, Professor Lowell McCann shared that the ideas of resonance, impedance, and trying to produce a particular inductance crop up a lot in research.
This experience has been terrific. I have become a better problem solver and learned that I must be a superlative collaborator. While a researcher works alone on learning from research that has gone on before and developing and modifying experimental procedure, an awesome researcher also forms relationships with other physicists and technical support people to collaborate on problem solving and learning.
After researching with Professor Robert this summer, I have learned that it is important to have a strong hold on a goal and know that there are an infinite number of paths to reaching that goal.
Alexander Lidiak: Plasmonic nano-structure fabrication
During my summer 2015 research project I worked with Thanh Do Minh, an ENS Cachan graduate student, on a plasmonic nano-structure fabrication method involving two-beam laser interference lithography and thermal annealing. We were both under the supervision of Associate Professor Ngoc Diep Lai.
Surface plasmon resonation occurs in a metal if the frequency of the incident light matches the natural frequency of the metal surface electrons oscillating against the restoring force of their positive nuclei. This resonance enhances the absorption and scattering of the particles as well as the electric field surrounding the particles. Many researchers have demonstrated the enhancement of plasmonic effects when patterned arrays of plasmonic particles are used as opposed to a single plasmonic particle. Consequently, periodically arranged plasmonic particles have become coveted for applications such as bio-sensing, photovoltaic cells, 2-D optics, and optical filtration. Due to the high demand for them, many means of fabricating plasmonic structures have been proposed, but current methods are either high-cost and low throughput or have complicated additional processes.
The first major step we used in fabricating the nano-structures was the formation of the photoresist template by using two beam laser interference lithography. Atop a thin glass substrate is a polymer that becomes soluble when exposed to light, called positive photoresist (PR), which was spin coated at a thickness of either 1200nm or 600nm. After spin coasting, baking was used to improve adhesion between the glass and PR and the sample would be kept in a black box and away from light before exposure. The optical setup we used was fairly simple and consisted of a 532nm beam which went through a beam expander then an iris before the beam was split at a beam splitter after which one of the beams was redirected to overlap with the other atop the sample’s surface. The overlapping of the two beams created an interference pattern. When the sample is developed, the intensity pattern of the interfering lasers is imprinted upon the photoresist atop the substrate. I did many exposures varying duration, interference period, and the angle of rotation (if a 2-D structure was desired, changing the angle would alter the resulting shape). I also performed equally many subsequent development processes and analyses of the resulting nano-structures by microscope. The goal of this step was to obtain sturdy, and preferably large, photoresist pillars on the substrate.
Once the photoresist structures were created, magnetron sputtering is used to coat the surface with a nano-layer of gold. This involved us going to the clean room and waiting as the magnetron sputtering machine magically dusted the sample with gold. The sample was then analyzed more precisely by SEM and if the structure looked good, we would then thermally annealed it at 500°C for 30 minutes. During annealing the core PR template evaporates, and the molten gold material falls and coalesces into a periodic structure and voila a periodic plasmonic nano-structure is the result. This particular method was meant to be and is a low-cost, simple, tunable, and high throughput method to fabricate large areas of periodic plasmonic nano-structures, which makes it especially pertinent to research and application purposes.
Kevin Montes: Fabrication of photonic crystals
This summer I worked with Dr. Ngoc Diep Lai and his group in the Quantum and Molecular Photonics Laboratory at École Normale Supérieur de Cachan. The group’s research is focused on developing and optimizing methods for fabrication of photonic crystals. Recently, the group has developed a direct laser writing technique to fabricate 2D and 3D sub-micrometric structures in a simple, low cost way. The method, known as Low One-Photon Absorption (LOPA), employs only a continuous and low power laser emitting at 532nm, giving it an advantage over the traditional Two Photon Absorption technique for 3D fabrication that uses a high power pulsed laser. It has already been demonstrated that LOPA can be used to embed single gold nanoparticles into polymeric structures and thereby increase the nanoparticles’ fluorescence after coupling. Due to their fluorescence properties, these coupled gold nanoparticles can be applied for use in biological imaging and solar cell technology, as well as potentially even optical computing. Under the guidance of Ph.D. student Trang Nguyen, I investigated the effect of the size of the gold nanoparticles on their coupling with an SU-8 photoresist structure using LOPA.
Trang and I first prepared samples for fabrication in the clean room by spin-coating a layer of SU-8, a thin layer of gold nanoparticle solution, and another SU-8 layer on each glass substrate. After each SU-8 layer, we baked the sample on a hot plate in order to remove the residual solvents. We repeated this process several times with different nanoparticle solutions to produce several samples with layers of 10nm, 30nm, 50nm, 80nm, and 100nm diameter gold nanoparticles.
After preparing our samples, we fabricated and analyzed them using the 532 nm laser and optical setup in our lab. For each individual sample, we first located the gold nanoparticles to a 25nm precision using the laser at low power (microwatt range) and a photodiode to collect emitted photons from the nanoparticles’ fluorescence. For this step, the laser power was weak enough so as not induce any polymerization in the photoresist. Once the positions of the nanoparticles were known, we fabricated a target structure around them using the same laser at high power (milliwatt range). The target structure consisted of a hexagonal 2D photonic structure of micropillars, with a micropillar fabricated around the gold nanoparticle in the center. Afterwards, we developed the samples in an SU-8 developer solution to wash away the monomers and leave our fabricated structure.
I then characterized each sample by recording its fluorescence after coupling with the photodiode and laser at low power. Using an analysis program called Igor, I imported this data and analyzed it by comparing the nanoparticles fluorescence intensity both before and after coupling. I found that the 10nm diameter set had the highest average gain in fluorescence, while the average gain reached a minimum at the 50nm diameter. In the future these results can be compared with the Lumerical simulation used in our lab to see whether or not the theory predicts these findings.
During the fabrication process, a microsphere was also observed to form around the gold nanoparticle. This is due to the fact that when the nanoparticle absorbs laser light it dissipates energy as heat in a spherically symmetric fashion, causing the polymerization rate to increase where the temperature exceeds a threshold and form a polymeric sphere. Using a scanning electron microscope, I measured the diameter of the microspheres that remained intact after fabrication and compared them to the corresponding nanoparticles’ peak fluorescence intensity after coupling. It appeared that there was a positive correlation between the microsphere size and fluorescence, but it is hard to make a precise conclusion until more data is collected.
Before the end of the program, I gave an oral presentation on my project to the lab, summarizing the work I did. Overall, my experience with ENS Cachan and the lab was a very positive one. The students and researchers were very helpful, and I learned so much about photonics during my work with them.
Kathleen Bolan: Method for using nanoparticles to detect biomolecules
Whiteboard Video: Not permitted to display
I worked in the Laboratoire d’Optique et Biosciences at École Polytechnique with Antigoni Alexandrou, Max Richly, and Pascal Preira. They are working on developing a method of using nanoparticles to detect biomolecules in solution, which could potentially be more sensitive than current techniques. It is similar to the enzyme-linked immunosorbent assay (ELISA), but uses nanoparticles instead of enzymes.
One of the main challenges in this project is preventing nonspecific binding of the nanoparticles to the glass slides. I was mainly focused on testing the methods of passivating the slides. The two main methods of passivation were epoxidation and silanisation. We passivated slides and prepared them with differing concentrations of the test biomolecule, and then observed how much of a signal was present compared to a negative slide with none of the biomolecule present. We also tried using wells made by attaching a PDMS with a hole in it to the glass slide, rather than just the flat slide, which affected the passivation.
I also worked on some of the tests of functionalizing the slides to create an even layer of antibodies on the slides. The factors that need to be considered include which molecules are used to attach the antibodies to the slides, their concentrations, incubation times, and how each layer of molecules is added to the slide.
Paul Scott: Computer generated phase patterns
We were purposed with the task of developing a method to produce computer generated phase patterns that could be used for beam shaping applications. A liquid crystal spatial light modulator (LCSLM) was configured with the phase pattern to produce a kinoform that only modulates the phase of the incident beam into a desired pattern at the image plane. The algorithm used to produce the phase pattern was improved by implementing an iterative algorithm that increased the reconstructed image fidelity. Additionally, we overlaid grid patterns on top of the initial image to further increase convergence. Analysis was performed to assess the number of iterations required to produce a satisfactory image. The various applications of beam shaping include laser marking, optical traps, and video/image projection.
Zack Li: LED Droop
LED lights last longer and work more efficiently compared to incandescent and fluorescent lamps. The high efficiency has a catch – it’s only possible at low current densities. Thanks to a still poorly understood process called LED droop, LED lamps must contain large semiconductor surfaces in order to maintain a low current density but run at high currents. This raises the cost of these devices, and the expense of LED lamps remains the major roadblock for widespread adoption.
The primary culprit for LED droop is the Auger effect, in which the orderly recombination of electrons and holes in a semiconductor is disrupted when there are high current densities. Electrons begin to interact with each other, and the end result is that energy goes into the kinetic energy of electrons instead of the production of photons. Instead of light, the diode emits hot electrons. My research group at École Polytechnique had done some of the pioneering work on this subject, and my summer project focused on a specific aspect of this process.
With Lucio Martinelli and Jacques Peretti, I investigated the connection between inhomogeneity of semiconductor material and LED droop. Inhomogeneity results in localization effects where electrons tend to go through only parts of the material, raising the current density in those places of the material. To investigate this, my first two weeks were spent working with a Scanning Tunneling Microscope (STM), which tunnels electrons into the semiconductor material in order to understand the landscapes within the diodes. I also worked on connecting our measurements to the ABC model, a model of droop which separates the main physical effects into three parameters.
With Lucio, I then dove into some electrical engineering, putting together a pulsed current source that could reach high currents without damaging the experimental samples. After two weeks, my knowledge of electrical circuits, and amplifiers in particular, had greatly improved. I then used this current source to reproduce known measurements of LED droop and the characteristic curves of our LED samples, as well as taking optical spectra at various currents. Here, we found our main result of this project – when one varies the indium content of the diodes, and thus the inhomogeneity, this also affects the shape of the optical spectra.
Unsure of whether this change in the spectral shape was due to electrical fields within the semiconductor, or the Auger effect, we then moved to experimenting within high vacuum and doing electron spectroscopy. We could then peer at the hot electrons being produced, ostensibly, by the Auger effect.