Optics in the City of Light

2014 Projects

Ronald Hobson: Modeling and fabrication of photonic crystals with Two-Beam Interference

This summer I worked with Ngoc Diep Lai and Alexandra Jurgens at Ecole Normale Suprieure de Cachan on the modeling and fabrication of photonic crystals with Two-Beam Interference. Photonic crystals (PC) are periodic dielectric micro- or nano-structures that affect the propagation of electromagnetic waves. In comparison to most fabrication techniques of PCs, holographic lithography (HL) is an inexpensive technique that promises large-area and defect-free PCs. HL also allows for the fabrication of high-level symmetric structures known as quasi-periodic crystals. Despite current research and development procedures for the fabrication of PCs, the simplest and most efficient method to fabricate desired 2D and 3D photonic crystals is by using a two-beam interference technique. Indeed, by employing multi-exposure of two-beam interference pattern.

This summer I had two main goals. One is to use Matlab to simulate the intensity of the two interference beams as it passes through a epoxy-based negative photoresist called SU-8. I started by writing my own code in C++ because this was the only code I had available at the time. That did not work so Ill so I used another code that had be previously written to simulate the intensity of the interference pattern. Eventually I was able to download Matlab and write my own program. The program would display the intensity in a 3D plot.

After simulating the intensity pattern, I would then fabricate the 2D and 3D photonic crystals. To do this I would go to the cleanroom and spin coat a thin layer of about 2µm of SU-8 onto a glass substrate. Since SU-8 is photosensitive, I would have to transport the samples from the cleanroom to the optics room in a special black box that would greatly reduce the light exposure of our PCs. I would then expose the SU-8 with a pulsed UV laser with a wavelength of 355nm. The beam was split into two identical beams that Ire monochromatic and had the same intensities and polarization. The fabrication time is only a few second and the structures are quite uniform for a large area of about 1 cm. The fabricated structures possess a typical lattice constant of about 1µm, but I Ire trying to fabricate PCs with a lattice as small as half of excitation wavelength. After UV exposure, the PCs go through a development process. I soaked the PCs in SU-8 developer, isopropanol, and DI water. The development process washes away any excess SU-8 monomers that are still in the PC structures.

Our first batch of photonic crystals had good structures, but many of the PCs were washed away during the development process. To combat this problem, I would spin coat another layer of SU-8 of about 100-500nm thick and develop that layer before spin coating the 2µm layer of SU-8. This helped keep the PCs from washing away during the development process.

After fabricating the photonic crystals, I would then characterize them by three different methods. The simplest method was by shining a red laser through the PCs. This would result in a diffraction pattern that corresponds to the type of structure we fabricated; 4 dots represented a square structure, 6 dots represented a hexagonal structure, and 12 dots represented a 12 fold symmetry quasi periodic structure.

Another way to characterize PCs was by using an optical microscope and a scanning electron microscope. With the optical microscope, I would see the uniformity of the structures, however because of the low magnification, I could only observe the type of structure and the cleanliness of the structure. Some of the structures were not so clean because some previous structures during development would wash away in the solution and derby would attached to the next structure.

The SEM was the best way to observe the PCs. by using the SEM, I could determine the size of the lattice, solidity of the structure and any other major distinctions. Further analysis showed a modulation in intensity. One reason behind the modulation in intensity could be from the nonuniform pulsed laser, unstable stage, or a reflection from the stage. Due to the lack of time and change in setup by Alex, I was not able to fabricate any more PCs or correct the setup.


Dana Kralicek: Rare-earth doped nanoparticles for biomedical imaging applications

My research took place in the Laboratoire d’Optique et Biosciences at l’École Polytechnique under the guidance of Max Richly and Dr. Antigoni Alexandrou. The lab is currently investigating the many uses of rare-earth doped nanoparticles for biomedical imaging applications. Using these nanoparticles, they have developed a method of measuring the dissociation kinetics between two biomolecules. Although methods for this measurement already exist, like laser plasmon resonance, proteins with exceptionally high affinities can take entire days to observe. The method proposed would dramatically decrease the observation time which will be highly valuable to the medical community, as many drugs have very high affinity. My research this summer focused on developing this technique and validating previous results.

After reading the previous work published by the laboratory, I researched the existing methods of measuring dissociations times and their limitations. Our method is intriguing because it uses and external force to lower the energy barrier of the protein-ligand bond, allowing it to dissociate much quicker. I was able to focus on the streptavidin-biotin conjugation and to calculate a basis for the amount of force needed in the experiment. I presented my finding to the laboratory group, and then I began my experimentation.

I worked on a series of experiments testing the validity of using bovine serum albumin (BSA) as a pacification agent on Glycidoxypropyltrimethoxysilane (glymo), an epoxy glue. It was concluded that our methods were sound, and BSA adequately passivated the surface, allowing us to be sure that the nanoparticles we were imaging were specifically bound to the protein we were targeting.

After these results, I was able to move on to using Polydimethylsiloxane (PDMS) microchannels to conduct dissociation experiments between proteins and ligands with high affinity. I created the channels and used a plasma cleaner to prepare the surfaces. Then I injected the various layers for the experiment I was preparing. An example of this: first a layer of glymo, then a layer of BSA mixed with biotinylated BSA, and then a layer of nanoparticles. After the channels had incubated, I was able to image the nanoparticles using an argon laser in the near ultraviolet region. I then applied a flow to the channel using the input and output ports to observe the time scale at which the nanoparticles attached to the ligands of the protein-ligand interaction dissociated. This was observed by the nanoparticle disappearing from the screen, as the flow rates were quite high. Using this method of collecting data, we will be able to measure the amount of time the ligands take to dissociate at varying flow rates. Fitting this data to an exponential curve, we will be able to infer the dissociation rate under normal conditions with no flow.


Alexandra Jurgens: Fabrication and modeling of photonic crystals using a multi-exposure two-beam interference technique

For the summer of 2014 iREU Optics in the City of Light I worked at École Normale Supérieure de Cachan in the Laboratoire de photonique quantique et moléculaire, or the Quantum and Molecular Photonics Laboratory. I worked under Assistant Professor Diep Lai to study photonic crystals. Photonic crystals are periodic dielectric micro- or nano-structures that affect the propagation of electromagnetic waves. The periodicity of the structure produces a photonic band gap, which refers to a region in the electromagnetic spectrum which is unable to pass through the crystal. This region exists at wavelengths on the same order as the periodicity length. This means to study photonic crystals with a photonic band gap in the visible range, which is the region of interest, micro- and nano-structures must be created with high precision. Holographic lithography is an inexpensive technique that uses photoresist material and laser interference to create large-area and defect-free photonic crystals. We studied this technique, focusing on two-beam interference with multi-exposures. Interference patterns were created with a UV laser, which was then shone onto a photoresist sample. Once the photoresist was developed, this created a physical reproduction of the interference pattern. These photoresist reproductions are effective photonic crystals. By exposing the same sample of photoresist with an interference pattern multiple times, more complex photonic crystals are created.

We studied the characteristics and production of photonic crystals using computational methods. Using MATLAB, we modeled various types of photonic crystals that can be created with the two-beam multi-exposure technique. This demonstrated theoretically that it is possible to realize any type of periodic, defect-free structures using this technique. We also modeled computationally the absorption effect of employed photoresist, which limits the thickness of photonic crystal structures. This effect refers to the tendency of the UV light to be completely absorbed into the photoresist at short distances from the surface, which creates an uneven exposure in the z-axis (through the photoresist). This is problematic for creation of photonic crystals thicker than ~2 microns, which is desirable to make photonic crystals with noticeable periodicity in three dimensions. Through modeling techniques we were able to better pin point the problems that need to be solved with experimental fabrication techniques.

Originally we attempted to solve the absorption effect issue by shifting the exposure wavelength to the visible range, which has a lower absorption coefficient and would therefore penetrate more deeply into the photoresist. However, time constraints meant we instead focused on studying the two-beam interference technique and production of photonic quasi-crystals. Photonic quasi-crystals are ordered structures that can only be created with the two-beam multi-exposure technique and are highly symmetric. Only with photonic quasi-crystals has symmetry of 6-fold, 10-fold and even 60-fold been achieved experimentally. We confirmed the usefulness of this technique, and proved that it accurately and consistently creates high quality quasi-crystals.

In conclusion, this summer was a research experience that was unique and a wonderful learning experience. There is much more research to be done concerning photonic crystals, and it is likely that the original goal of moving the exposure wavelength into the visible range must be completed before the absorption effect problem can be fully solved. Furthermore, it is likely that with improved optical set up, such as the introduction of a fiber optic, the two-beam interference technique can be pushed to create ever smaller and more complex structures.


Susannah Betts: Search for the cause of the drop in efficiency of blue GaN LEDs that occurs at high injected current densities

I was assigned to work in Jacques Peretti’s Electron, Photons & Surfaces group at the Condensed Matter Physics laboratory at École Polytechnique. Kevin and I primarily worked with one of his graduate students, Marco Piccardo. Our goal was to contribute to the search for the cause of the drop in efficiency of blue GaN LEDs that occurs at high injected current densities.

Our main projects consisted of measuring electroluminescence and photoluminescence spectra produced by different LED samples and in different experimental conditions. Our LED samples were a mixture of commercial and scientific grade LEDs provided to us through collaboration with researchers at UCSB. In the process of setting up these experiments we learned to align our optics table, optimize our Ti:Sapphire laser, use a second and third-harmonic generator, design a sample holder for our LEDs, solder wires, install and use a cryostat to cool our samples, piranha etch our samples, and more. We also learned through experience and asking for lots of help how to troubleshoot spectroscopic experiments, from being able to identify diffraction and interference patterns to choosing the correct lens for the wavelength of emission.

In analyzing our data and participating in designing our experiments, we learned about the useful interaction between experimentation, modelling, calculation, and reference to scientific publications and textbooks. If we had a theory for why we observed a diffraction pattern reflected from the LED when photoexcited at a certain angle of incidence, our mentors insisted that we mathematically and then computationally model the predicted results to confirm a sufficiently reasonable match. We did our graphing and data analysis in Kaleidagraph and our computational work with Mathematica and MatLAB.

Our results were specific to the characterization of the samples we used. However, the principles at work in our experiments and in the LEDs were general physical principles from quantum mechanics, thermal and statistical mechanics, optics, and solid-state physics. We discovered this summer that studying LEDs requires a broad spectrum of knowledge about physics, optical equipment, and research methods.

We finished our summer research with a 10-minute presentation to the graduate students at our laboratory. Throughout the summer, the graduate students and researchers at PMC were incredibly helpful—answering questions, lending us equipment, taking time to explain concepts, suggesting profitable avenues of research, and pushing us to be able to understand and communicate our own research.


Lewis Jones: Realization of a sensor for biochemical species using a polymer optical microresonator.

This summer I worked under the supervision my advisor Dr. Nguyen and Post Doctor Camille Delezoide along with Ph.D. student David Chauvin investigating the topic of optical microring resonators for applications in biological sensing.

Specifically, my role in this research project focused mainly with the fabrication and optimization for the performance of these devices, including the development of a more cost efficient optofludic cell. Advancements within this technology will hopefully be used for further implementations within of studies of biology and chemical based interactions.

For the first three weeks of my research program I became acquainted with my research topic by shadowing my supervising Ph.D. student David Chauvin. During this time I was able to gain valuable insight on how he ran his biological experiments using the fiber optic setup that I would eventually use to characterize my own sensing devices. I also learned quite a bit about of the procedures, equipment, and theory behind my research project in addition with reading an extensive amount of articles about label-free biological sensors.

By the forth week of the program I began to fabricate my own microring resonators which would be used to test the improvements of our sensor technology. These actions were carried out in a cleanroom environment, and where I used tools such as photolithography and profilometer machine.

The devices that I created were new in the aspect that we aimed to change the height of our waveguides and remove the cytop layer in our microresonator, and replace it with a less expensive polymer coating.

During the first week of July I began to characterize my devices in a separate, but similar optical setup. I was able to take wave sweeps in the near infrared range to measure the spectral intensity of the transverse magnetic and transverse electric fields. More importantly, I was able to analyze the resonance peaks of the coupled infrared light that came out of our microring resonators; through doing so I was able to calculate the extinction ratio in decibels and contrast ratio of our recorded resonance peaks.

For the following time during my internship, I was able to discuss my results with my advisor, and compare the performance of my device to the older devices, which contained a cytop layer. Overall, the accuracy of new devices preformed worse than the older ones, with both the extinction ratio and contrast ratio proving to have lower averages. Looking back, my international research experience has given me a much broader outlook on graduate school possibilities, as well as an invaluable cultural experience that I will remember for the rest of my life.


Richard Williams: Terahertz Spectroscopy Using Total Internal Reflection

This summer I worked with Guilhem Gallot at Ecole Polytechnique. We used a femtosecond, pulsed laser to generate terahertz waves. Generation happens with two electrodes placed on a semiconducting sheet held at a potential difference. The laser creates charge carriers in the semiconductor, which accelerate from the two electrodes to produce the THz radiation. We then used the THz radiation to study properties of water such as index of refraction and permittivity using total internal reflection spectroscopy and a phenomenon known as the evanescent wave. Terahertz waves are particularly useful in studying water and biological systems because of their sensitivity to matter with energy profiles in the meV range.

My portion of the project included some theoretical and experimental work. The first half of the summer I spent reading publications, developing an understanding of the setup, and creating a theoretical model based on work done by other groups that have performed these measurements in the past. I used MATLAB to develop a code that modeled the permittivity of water based on a Double Debye model and parameters that were experimentally determined by Tanaka and his cooperatives.

After I successfully created and tested my model, I created another piece of code to analyze future data. This code took the raw data from the computer, converted the delay line position to a time profile, ran the temporal data through a Fourier Transform to view the data in the frequency domain, and through various other calculations produced the final permittivity of the water. This took a lot of debugging because of issues with the square root function in MATLAB and issues with dephasing. I faced similar issues with the theoretical model.

The last three weeks or so in the lab I spent taking data. The protocol is as follows. After calibrating the laser, we would place a patch on the silicon prism. We would then take measurements with nothing on the patch as a reference. Calculations for the permittivity involve a ratio of air and water measurements to remove dependency on a variable that we cannot accurately measure. After the air measurements, we add water to the patch and perform the measurements again. All of these measurements are performed under Nitrogen to reduce the humidity in the air and as a result noise in our measurements.


Norquata Allen: Optimization and Characterization of Optical Sectioning in a Structured Illumination Microscope

The research presented in this paper was conducted under to supervision of Guillaume Dupuis of the Centre de Photonique Biomédicale (CPBM). CPBM is a core facility of the Fédération Lumière Matière (LUMAT) within the Université de Paris-Sud (Orsay). This laboratory specializes in designing and creating optical microscopes that are used for biomedical studies of cells. The research I conducted this summer was centered on finding or creating a better method of acquiring images that produces the most optimized view of a cell or substance through an optical microscope. The techniques used in this imaging process are structured illumination, optical sectioning and HiLo microscopy.

Our imaging process is a three-step process that starts with acquiring two separate images of a cell. The first image is called the mirror image. The mirror image is a standard illumination image that blurs the out of focus parts of the image. The blurriness can be removed by using a high pass filter. The second image is a fringe image. The fringe image is a structured illumination image of a particular fringe size. This image is only in focus through the depth of field of the objective of the microscope. Any part of the image that is out of focus in the fringe image can be removed using a low pass filter. In order to produce the most optimized image, we merge the mirror and fringe image through a process called HiLo microscopy. HiLo is an algorithm that employs the respective cut off frequencies of the two images during the merging process and produces the most in focus part of the image. In the end, we have a more resolute image that is very detailed and can be used by biologists to discover in depth characteristics about the cell in question. These HiLo images produce a particular part of the cell known as an optical section. The optical section is a thin slice of the image in which the signal has been isolated. This process is important because it can be done without physically cutting and/or damaging the cell. The point at which the optical section signal is isolated is called the z-depth.

At the start of the research, I was tasked with creating new fringe sizes and orientations to be tested against the existing fringes on the cells in order to determine the most optimal fringe image to create the most resolute HiLo image. I acquired images and analyzed the data using the besselJ function within MatLab and an FFT function that is based on the Fourier transform. In the end I had normalized plots in which I compared the actual data to an ideal case to determine the precision of each fringe size and orientation.

My conclusion was that the vertical orientation of fringes, an orientation I created during my research, was the most optimal orientation for imaging. In the future, the lab will use these fringe images to go forward with creating more optimized images as well as some of the code I produced to test even more orientations and fringe patterns.

Katie Hamann: Optically sectioned fluorescence microscopy using HiLo microscopy

This summer as an intern in the University of Michigan’s Optics in the City of Light iREU program, I did research in Optics under the supervision of Guillaume Dupuis of Université Paris-Sud, located in Orsay, France. The project I worked on was concerned with optically sectioned fluorescence microscopy using a technique called HiLo microscopy [1]. Whereas traditional confocal microscopy techniques require scanning the entire sample pixel by pixel, this HiLo technique requires only two images: one with standard illumination, and another with structured illumination. The structured image is produced by imposing a pattern on the laser that then appears on the sample. These two images can then be fused together via an ImageJ plugin, which finds the areas in which the pattern is in-focus on the structured image, and then retrieves the image information from this area on the standard image, to produce a clear, optically sectioned image that has no out-of-focus light pollution from other layers in the cell. This imaging technique gives a resolution comparable to confocal microscopy, but offers a much faster process and a wider field of view [2].

The first part of my project dealt with optimizing the existing setup. For the structured imaging, using extensive MatLab coding, my partner Norquata Allen and I were able to design new patterns that were thought to optimize the sectioning abilities of the microscope. Once the patterns were made, we also had to program them into the LabView acquisition software.

We then used plain fluorescence slides, made with a fluorescent solution embedded between a glass slide and a cover slip, to make comparisons in terms of sectioning performances between the different patterns we designed. We did this by scanning through the entire axial depth range of which the patterns were visible. We analyzed these stack of images using a Fourier transform, which provides a quantitative way to evaluate the visibility of the pattern. Eventually, we found out that one of the pattern we had designed was indeed capable of improving the axial sectioning ability of the set-up.

The second piece of my research was focused on Fluorescence Lifetime Imaging Microscopy (FLIM). We combined the FLIM and HiLo techniques to create time-gated stacks of optically-sectioned fluorescence decays using two different methods: a standard exponential fit technique, which uses 10 (ten) 1-ns long gates covering an 8-ns long interval and a rapid fitting-free technique (RapidFLIM, [3]), which uses only 5 (five) 2-ns long adjacent gates. While the traditional fit method is considered to be the more accurate approach, we wanted to see if the RapidFLIM is an acceptable approach, as it allows for more signal and can be implemented in real-time. We also imaged two different cell types: living HEK cells, transfected with GFP-tagged beta-secretase 1 (BACE1), and living HEK cells, transfected with a GFP-tagged BACE1-APP FRET tandem. The latter present a shorter fluorescence lifetime than the first, because of the non-radiative energy transfer mechanism called FRET (Förster resonance energy transfer). The BACE1-APP complex also produces the Aβ peptide, which is crucially involved in Alzheimer’s disease as the main component of the amyloid plaques found in the brains of Alzheimer patients. The ultimate function of the microscope will be for biomedical research, specifically for Alzheimer’s disease. Cholesterol is thought to increase the BACE1-APP linkage, and our microscope can monitor this increase through fluorescence lifetime imaging.

My work this summer was mostly concerned with optimizing the current microscope setup. The future goals include further optimization of the system and, ultimately, bringing in biomedical researchers from other institutions to perform their research using this microscope setup. I had an amazing summer in this program, and owe the utmost gratitude to my advisor Guillaume Dupuis, the faculty members at Université Paris-Sud, and Steve Yalisove, for making this experience possible.

References
[1] Lim D., Chu K.K., and Mertz J., Opt. Lett. 33 (2008)
[2] Dupuis G., Benabdallah N., Chopinaud A., Mayet C. and Lévêque-Fort S., Proc. of SPIE 8589 (2013)
[3] Padilla-Parra S., Audugé N., Coppey-Moisan M., and Tramier M., Biophys. J. 95(6) (2008)


Dylan Sena: Coupling Phenomenon in Organic Microlasers

This summer, under the supervision of Mélanie Lebental, I worked at the Laboratoire de photonique quantique et moléculaire (LPQM) at Ecole Normale Supérieure de Cachan (ENS-Cachan). I also worked with post-doctoral student Stefan Bittner and ph.D. student Nina Sobeshchuk. The focus of my research was analyzing two and three dimensional, single and double Fabry-Perot cavities and my work can be divided into three main categories: Fabrication, Measurement, and Analysis. These organic lasing microcavities are used as micron-sized coherent light sources and have a wide range of applications in optical telecommunications and chemical and biological sensors.

For about a week of my internship, I worked with Nina in the clean room in order to fabricate single and double 3D Fabry-Perot cavities with various cavity widths, lengths, and gap widths. The samples were then analyzed under a scanning electron microscope (SEM).

The second part of my work was measuring the lasing behavior of our samples in the optics lab. I observed the microlasers that Nina and I created and older ones. In our lab setup, a pump laser is shone onto a cavity on our sample. The pump energy and polarization can be controlled by means of polarizers and half-waveplates. Then the spectrum is measured.

The third and most important part of my work was analyzing the data that I collected. In particular, we evidenced a relationship between spectral features and the gap width. Using the transfer matrix as a model, I analyzed the resonant frequencies and compared the predictions with the experiments. The agreement was not as good as expected and improvements of the model were proposed.


Kevin Liang: LED-droop

This summer, I worked with the Condensed Matter Physics lab group at l’Ecole Polytechnique. Throughout the summer, I worked with my fellow intern Susannah Betts and a Ph.D student named Marco Piccardo. His adviser and other help, Jacques Peretti, Claude Weisbuch, and Lucio Martinelli also occasionally helped us in our tasks.

The summer began with identifying the problem of LED-droop. Our first sets of experiments were all based off of electroluminescence. By running a current through an LED (as it would be commercially used), we get it to emit light. This light is then guided by a series of lenses and ultimately collected by a spectrometer. The spectra from these experiments are able to give us valuable information, such as the external quantum efficiency of the LED, and help us determine any geometric effects of using the LED. For example, it is possible that the orientation of the LED (whether the back or front is facing the spectrometer) can affect the spectra taken. Within this electroluminescence regime, we studied samples with and without electron-blocking layers, spreading layers, aperture grids, and more.

Then next part of the summer can be roughly classified under experiments involving photoluminescence. In this method, the LED is activated not with an injected current, but rather with a laser. A laser creates carriers within the quantum wells of the LED, and these carriers go on to recombine to re-emit photons. However, some of these carriers exit the quantum wells and participate in other processes. These processes (including Auger Recombination, tunneling, overflow, and higher-valley transitions) are highly interesting and can be examined with photoluminescence. An example of an example was to attempt to measure the Auger Recombination process for holes, which is a more elusive process to observe than that of the electrons, which are actual particles. Within the method of photoluminescence, experiments can be further subdivided into transmission and reflection experiments. We got our hands into both and managed to get difficult, but rewarding results.

After being in the lab for the majority of the program, I was assigned to independently tackle a theoretical project involving localization of carriers in the quantum wells. After two weeks, I managed to get some interesting results. We were able to calculate the typical localization area for electrons of a typical energy as they are being injected into the LED. This typical area then gives way to how the effective carrier concentration of electrons is larger than expected. This then tells us that Auger Recombination effects are enhanced, as that process is high carrier concentration-dominated. Our results are now being discussed, and I am hopeful that it can be a gateway to more experiments involving localization.

To wrap up the summer, we worked with LEDs and particularly in how the carriers in them “move”. We looked at how and why the LEDs are as (in)efficient as they are by studying Auger Recombination. It was highly rewarding to be able to use different methods and to analyze the data in different ways. More importantly, I found my experience in the final two weeks (theory project with localization) to be a great learning process as it took a grand amount of time to start, but results started to come in quickly at the end. This summer REU was definitely an experience I would want to do over and over again.


Anna Snellgrove: Second harmonic generation (SHG) of nanoparticles in solution

This summer I had the opportunity of working with Dr. Isabelle Ledoux-Rak at the Laboratoire de Photonique Quantique et Moléculaire at ENS Cachan. I worked primarily with Ngo Hoang Minh, a graduate student studying the second harmonic generation (SHG) of nanoparticles in solution. I worked with him to make measurements on a wide variety of nanoparticles, including gold nanospheres, gold nanorods, gold nanostars, and gold and platinum composite nanoparticles. At the beginning of the summer, I also was able to observe the measurement of second harmonic generation on organic crystals fabricated by a visiting graduate student.

Our goal this summer was to measure the average hyperpolarizability of our samples in order to gain insight into the source of SHG in metallic nanoparticles and understand how to maximize SHG in nanoparticle solutions. My first few weeks were spent learning how to operate the laser and collect data. We excited the sample with a Nd:Yag laser with a wavelength of 1064 nm and measured the SHG signal at a wavelength of 532 nm. We then found the slope of the plot of the 532 nm SHG intensity produced by the solution vs the square of the 1064 nm input intensity. By comparing the value of this slope with the slope of the plot generated by pure water, which has a known hyperpolarizability, we were able to calculate the hyperpolarizability of the nanoparticles.

Later in the summer I was able to observe as Minh fabricated gold and platinum composite nanoparticles. I learned about the chemical process required to make nanoparticles and how it is possible to manipulate size, shape and composition of the partials by adding each chemical at the right time, concentration, and temperature. This helped to expand my knowledge of chemistry and its applications. We then measured the absorption spectrum and hyperpolarizability, so I was able to see every stage of the experiment with these particles.

When I was not working in the lab, I read about the theory associated with SHG in metallic nanoparticles and organic crystals. In order for SHG to occur, it is necessary for the material not to be centosymmetric. For this reason, SHG in bulk gold is impossible. Gold nanoparticles, on the other hand, have hyperpolarizabilites on the order of 10-26 esu. It is theorized that this is possible because of interactions at the surface of the nanoparticles and field gradients within the particles. By examining how altering the size and shape of the nanoparticle changes the hyperpolarizability of the solution, it may be possible to identify more how much each effect contributes to SHG.

Additional work in this area could look at finding the hyperpolarizabilites of an even wider variety of nanoparticle shapes, in different solvents, and with different metals. This could be used to refine the theory, allowing scientist to fabricate nanoparticle with well-known hyperpolarizabilites for applications such as optical communications and biological sensing.

Working at ENS Cachan this summer allowed me to experience an international, interdisciplinary lab as well as learning a lot about my topic. It expanded my understanding of multiple disciplines and the global scientific community.


Julian Girard: Electric field induced second harmonic response (EFISH) from liquid-crystal micro droplets

This summer I was working alongside Professor Abdel El Abed at the Ecole Normale Superior de Cachan. His current experiment is to analyze the electric field induced second harmonic response (EFISH) from liquid-crystal microdroplets. This research experience was highly valuable to me, as it exposed me to a very new style of experimentation and granted me the opportunity to practice many experimental techniques. In addition to the great deal of research experience I gained this summer, I was able to explore the Parisian, and European, lifestyle in a way that would be impossible as a common tourist. Living within Paris, with the freedom to explore the city and all it has to offer, as well as the many surrounding cities worth visiting, was an exceptionally great way to enjoy my summer and expand my cultural horizons.

The project I worked on this summer was motivated by it's various applications to microfluidics and biological sciences, however the experiment itself is more based on optics. I was lucky enough to be paired with a professor that gave me much freedom to explore his experimental setup and work closely with his primary advisee (Son Nyugen). Son and I were primarily working in the optics lab, where our experiment was setup; there was very little theoretical, analytical, or computer-based work as the project was relatively new. This was a nice change of pace for me, as my previous research experiences have been based primarily on analytical, computer-based research.

As the motivation for the experiment is quite novel, we had very little previous work to base our experimental setup on, consequently, much of our time was spent maximizing the sensitivity and accuracy of our optical setup. More specifically, this setup was composed of a femtosecond pulse laser, a small optical track for focus and polarization adjustments, a microscope to focus the beam on our samples, and various detectors to monitor the reflected secondary signal. This stage of the research gave me the chance to learn much about different sorts of optical equipment, and grow accustomed to the frustration associated with maximizing the focus of an optical system. If anything, I learned that patience is just as important as ambition in conducting optics research. Other interesting aspects of my summer research experience included PDMS chip fabrication, which involved work in a clean-room designed for micro-scale fabrication, understanding and working with LabView as a primary interface, and discussing the nonlinear optical theory behind some of the strange results we found with prestigious professors.

The project is still in the beginning stages, so no publication-worthy results have yet been found, but this project will certainly yield some very impressive findings in the near future. I'm happy to have been part of an important experiment. Research aside, I really enjoyed the atmosphere and community at ENS Cachan; everyone was very friendly, accommodating, and impressively intelligent. I enjoyed working at ENS so much that I now plan to apply to their graduate school in the Fall of 2014.

Outside of my work at Cachan, I was able to do a lot with my time in Paris. I explored much of the city, tasting different sorts of cuisine, visiting different corners of the city, going to various museums, historical attractions, concerts, and countless other events and venues. In addition to my time in Paris I had the opportunity to explore both the North and South of France (Normandy and Port-Barcares, respectively), and I even spent a weekend in Amsterdam!

I can't say enough about how important this program has been for me, giving me the chance to expand my research experience, build important professional connections, reinforce my desire to continue into a career in research, and have the cultural adventure of a lifetime. This is a well-organized, highly valuable research opportunity and I would recommend it to any motivated physics student looking to go a little out of his/her comfort zone for a summer research project.