Optics in the City of Light

2013 Projects

Amanda Alexander: Observing the confinement of bacterial pore-forming toxin receptors in HeLa cells using Europium-doped oxide nanoparticles

This summer I worked at the Laboratoire d’Optique et Biosciences at l’École Polytechnique under the supervision of Antigoni Alexandrou and Max Richly. My research project involved observing the confinement of bacterial pore-forming toxin receptors in HeLa cells using Europium-doped oxide nanoparticles. The observation of toxin receptors in HeLa cells can provide insight into the complex architecture of the cellular membrane. HeLa cells are derived from a human cervical cancer cell line and are very commonly used in research. A similar project had previously been conducted in this lab involving Madin-Darby canine kidney (MDCK) cells. Therefore, another goal of my project was to compare my observations in HeLa cells to those previously found in MDCK cells.

The cellular membrane plays a very important role as the between the cell and its environment. Thus, experimental investigation to determine the membrane’s complex structure is of high scientific importance. We used fluorescent single-particle tracking (SPT) of rare earth-doped nanoparticles coupled to the pore forming α-toxin of Clostridium septicum (CSαT) to obtain the trajectories of the toxin’s receptor. Since the nanoparticles used do not exhibit blinking or photobleaching, this setup allows us to obtain exceptionally long, uninterrupted trajectories.

The trajectories obtained were then analyzed with an algorithm based on Bayesian inference. Currently employed techniques for extracting data from recorded trajectories are only able to extort a small fraction of the available information. However, the Bayesian Inference method allows for the extraction of more information stored in a single molecule trajectory, while making fewer assumptions about the forces acting on the biomolecules. This way we can extract the diffusion coefficient of the receptor, the force map within the domain, and the confining potential from the receptor trajectories.

After Bayesian Inference analysis of the collected trajectories, the toxin receptors in HeLa cells was found to be similarly confined to domains as seen in the MDCK cells. We determined a diffusion coefficient (Dinf) of 0.06 ±0.01 μm2/s, and a confining domain area (A) of 0.29±0.05 μm2 in HeLa cells. These values are much smaller compared to those found in MDCK cells (Dinf = 0.18 ±0.01μm2/s and A = 0.40±0.05 μm2). Additionally, the inferred second-order spring-like potential that confines the receptor gave a radial spring constant (kr) of 0.52±0.06 pN/μm. This kr was much larger than the value found in MDCK cells (0.30±0.03 pN/μm).

Future work on this project would involve conducting further experiments using sphyingomyelinase, cholesterol oxidase, and latrunculin B in order to determine if the receptor confinement is dependent on sphyingomyelin and cholesterol (two lipids prominent within lipid rafts) or whether confinement is affected by cytoskeletal actin. These experiments previously conducted with MDCK cells indicated that the observed receptors are confined in lipid raft platforms.



Hadalia Bergeron: developing a rare-earth doped nanoparticle system used to detect hydrogen peroxide concentrations

Through the University of Michigan ‘’Optics in the City of Light’’ iREU program, I worked under Dr. Antigoni Alexandrou at the Laboratory of Optics and Biosciences (LOB) at the École Polytechnique. I was supervised by Mouna Adbesselem, a Ph.D. student of the LOB. The research project I worked on for the duration of the program consisted in developing a rare-earth doped nanoparticle system used to detect hydrogen peroxide concentrations. The work I did expanded upon their previous work published in 2009 in Nature Nano [Ref[1]] using Europium doped nanoparticles. The detection of hydrogen peroxide concentrations is relevant for biological systems such as cells and tissues, where reactive oxygen species mediate cellular signaling pathways and are involved in inflammatory responses.

The research project began with familiarization with the theory behind the nanoparticle system used to detect hydrogen peroxide concentrations. This consisted in reading the publications of the research group and shadowing the experiments that I would later be doing. By the end of the first week, I began the first experiments on my own. These experiments were calibration experiments for the improved nanoparticle sensor. The nanoparticle hydrogen peroxide sensor system is based upon the luminescence of the rare-earth ions over time. The calibration is thus performed by measuring the intensity of the luminescence of individually detected nanoparticles in response to known concentrations of hydrogen peroxide in vitro. Using these calibration curves, the hydrogen peroxide concentration can be obtained after measuring the luminescence dynamics of the nanoparticles. The experiments were performed in a lab using argon lasers and laser diodes and the nanoparticles are observed under a microscope. The image from the microscope is gathered by an EM-CCD camera and displayed on a computer screen. The analysis of the images was done using a MATLAB program.
To complement the calibration experiments, I also worked on spectrofluorometry of the nanoparticles. Through these experiments, I was able to confirm the phenomenon observed from a tabletop optical system using a more chemically oriented technique. Approaching the same phenomenon from multiple perspectives further outlined the multidisciplinary nature of not only research in general, but especially the work done at the LOB.
In addition to the calibration experiments I was performing, I was also able to accompany Mouna to an MRI lab in a hospital in Paris. Her work with the Georges-Pompidou European Hospital Cardiovascular Research Center concerns the use of rare-earth doped nanoparticles as MRI contrast enhancers. Though this work did not directly involve my own, it was a great opportunity to witness the collaboration of the LOB with other labs and the versatility of the nanoparticle system they are developing. I also participated in biweekly meetings with Dr. Alexandrou, Dr. Bouzigues, and Mouna. Through these meetings I was able to get a bigger picture of the work that goes into developing innovative technology.
The opportunity to be a part of the LOB and to participate in this iREU program allowed me to engage in incredibly enjoyable and stimulating research. Above all, the two months I spent at the École Polytechnique was a remarkable learning experience and submersion into the academic research environment. I owe my gratitude to Dr. Alexandrou, Dr. Bouzigues, and my mentor Mouna Abdesselem.

[1] D. Casanova, C. Bouzigues, T.-L. Nguyên, R. O. Ramodiharilafy, L. Bouzhir-Sima, T. Gacoin, J.-P. Boilot, P.-L. Tharaux, and A. Alexandrou, “Single europium-doped nanoparticles measure temporal pattern of reactive oxygen species production inside cells.,” Nature nanotechnology, vol. 4, no. 9, pp. 581–5, Sep. 2009.



Brandon Fain: laser induced breakdown spectroscopy (LIBS) for Cultural Heritage Studies

I worked at the Laboratoire de Recherche des Monuments Historiques (LRMH). Unlike many of the other labs my peers worked in, the LRMH is not associated with any university in Paris, it is a government research lab located just outside of Paris on the east side in the Champs-sur-Marne suburb. The lab studies historical monuments and pieces of art using scientific methods. The LRMH houses biology, chemistry, and physics labs, and has a mix of researchers in these sciences and professionals in art conservation.
My project focused on an analytical spectroscopy technique called laser induced breakdown spectroscopy (LIBS). In short, the technique entails irradiating a sample (presumably one with an unknown composition) with a pulse laser focused to a very small point. This creates a large fluence at the target area on the sample. Under appropriate absorption conditions, this heats the material rapidly, fully vaporizing it into plasma in our case. This plasma then expands rapidly in the direction perpendicular to the surface, fully dislodging from the target material by laser ablation. This plasma cools quickly and radiates as it does so. During the initial 5-1000 ns the plasma radiation is dominated by blackbody effects of the kinetic movement of free particles within the plasma. Thereafter, radiation is characterized by photon emission from electron transitions inside of the atoms.
The second and later emission can be used for spectroscopy, thus the full name of LIBS. By studying the spectral lines characteristic of electron transitions in a particular atomic structure, one can identify the elements present in the given sample. Thus, LIBS is an analytical spectroscopy technique allowing one to determine the elements present in a given unknown sample. With regard to art conservation as studied at LRMH, LIBS is used as a method for quickly determining chemical components of samples on site. The pertinent advantages for this application are the relative ease of transportation and set up, the ability to yield results in nearly real time, and the elimination for sample cutting from precious artworks for chemical laboratory analysis. The method is-of course-destructive, but on a much smaller scale than such sample cutting.
I performed two types of studies with LIBS: stratigraphic and molecular. The stratigraphic studies were attempting to examine samples with known prepared layers of pigments or metals (gold sheet on lead or azurite and minium pigments on plaster, for example) and identify the transitions between the materials as the LIBS system ablated through the strata. The molecular studies focused on determining appropriate delay and gate parameters to “fine tune” the spectrometer to be able to view radiation from molecular species, a much more difficult task given that the elemental and blackbody radiation tends to dominate in terms of intensity given off during most of the radiation time.
I was lucky enough to be allowed free reign to perform experimental work using the LIBS apparatus at the LRMH independently, so most of my summer involved preparing samples, running the LIBS system, acquiring and analyzing data, and preparing reports from that. Ultimately, I was able to establish a method for examining strata transitions in multilayered sample and qualify for what strata this would be difficult. The molecular studies proved more challenging, but my work hopefully prepares for future work in using the techniques to identify the binding materials present in a paintings (binding materials are those used to adhere the pigment to the medium). These methods, along with the other analytical techniques at the LRMH, are then applied toward actual conservation, restoration, and historical projects undertaken in the field study of historical monuments in Europe.



Matthew Feldman: how vibration of cell cytoskeletons is translated into information at the nucleus of a cell

I worked in the Institut d’Optique with Arnaud Dubois in the Biophotonics group. My research mentor was Antoine Federici. The overall goal of the research project I was involved in was to determine how vibration of cell cytoskeletons is translated into information at the nucleus of a cell. In order to study this, the Institut d’Optique group is building an optical tweezer system to tug on beads that are fixed to cell cytoskeletons. My role in the project was to build the imaging system so that we can observe the cells in 3D. I built a full-field optical coherence microscopy (FF-OCM) system.

Because the OCM and optical tweezers are to be set up on the same table and use the same objective, we are unable to move the sample in order to take deeper image slices, as traditional OCM requires. Therefore, the experiment I worked on attempted to characterize the spherical aberration and image degradation using the camera position to control the depth of the focal plane. I spent the first few weeks in the REU building up the system on a table, configuring the camera, motor, and piezo software to work with the computer, and tracking down the correct objectives and lenses.

Over the course of the summer, I was responsible for two major programs. One was a LabVIEW program to control the motors, camera, and piezoelectric as well as probe the image and capture data. The other program was a Matlab script to analyze and process the data by fitting Gaussian and logistic curves via the optimization toolbox.

My project was a series of three experiments. The first was to characterize the depth of field of the system. For this experiment, there was no reference arm. I put the camera and sample into their optimal focused positions, then translated the camera over the range of the stage (100mm) and tracked the PSF and edge sharpness. We found that the image just becomes unfocused after 50mm (PSF doubles).

The next experiment was to trace image quality as the sample is re-conjugated with the camera at different points along the numerical aperture curve. The sample was fixed to a 100um piezoelectric stage. This experiment showed no noticeable image degradation over the course of 100mm. Furthermore, this showed that the system has a penetration depth of up to about 15um, given a 100mm camera stage.

The final experiment was to observe the fringe contrast as the camera moves. To do this, I started with the camera and sample perfectly focused. Then, I moved the reference mirror to give maximum fringe contrast. I then repeated the previous experiment, keeping the fringes in the same place with each slice, relative to the sample. This experiment showed that the fringe contrast decreased by about 40% over the course of 100mm, or 15um for the sample. This means that the OCM system I built will be acceptable for the cytoskeleton experiments.



Chelsea Hendrus: Terahertz Studies at Laboritoire de la Recherche des Monuments Historiques

During my stay with the Optics in the City of Lights iREU, I worked as an intern with the Laboritoire de la Recherche des Monuments Historiques, a lab funded by the French Ministry of Culture, that works to design and improve techniques used on the field to study, care for, and restore historical monuments. Initially, I was assigned to work with their Wall Paintings division on Terahertz water content sensing, however it was a very busy season for my adviser, and he ended up on conferences for a majority of my first month. To keep me occupied, the LRMH Wall Painting division transitioned me into their work with the Nanomatch project, a multinational endeavor to find a better stone consolidant specifically for use with national heritage sites, as most commercial construction consolidants tend to have adverse affects over time when used on painted stone. I worked with an Italian intern, who's formal training is in Conservation Sciences, and we used colorimetry and high resolution photography, along with a few surface-stress tests to see how the proposed consolidants affected the adherence strength and color of some of the more common pigments of antiquity, applied with several different types of traditional techniques. At the beginning of July, I also received the opportunity to go on the field, to the Chapel Sainte Marie, in Fontaine Chaalis, to observe the Terahertz machine being used on the field, as a part of another multinational project for comparing methods of non-invasive testing for the condition, and sources of deterioration in wall paintings. The final report I wrote for the lab ended up as a review on the function and applications of Terahertz sensing. Sadly, due to serious issues with the electrical motors (they posed a serious shock hazard) I was unable to work hands-on or independently with the THz system. This technology, and the numerous other applications it possesses is fascinating, and I would look forward to a chance to study it in the future. Over all, this was an incredibly unique opportunity to see science, art, and history come together in a way that one doesn't often find.



Cassandra Hastings: transmission band shifts for hollow core, liquid-filled, photonic crystal fibers

This summer, my research involved both experimental and theoretical work which would help validate a theory that predicted transmission band shifts for hollow core, liquid-filled, photonic crystal fibers. The experimental work involved locating the transitions of the transmission spectra passing through the fibers when laser light was shined through them. The theoretical work involved plotting the transmission center data points, taken to be the half-way point between the right-hand side and left-hand side position of the spectra, and mapping them against the predicted theory to find any abnormalities associated with the prediction.

In order to identify the transition bands for the transmission spectra, an optical spectrum analyzer was used. First the laser light, after passing through the fiber, was focused by using a camera and monitor, to make sure that the only light being measured was the light that was passing through the core of the fiber. Then, a scan was taken of the transmission band and the transitions were identified by means of wavelength optimization and specific wavelength filters. This was done for two different types of photonic crystal fibers filled with varying indices of refraction, as the theory predicted shifts in the transmission spectra based on index variation between different liquid mixings. The liquid mixings were created by using water and dimethyl sulfoxide (DMSO).

The data for the two fibers filled with varied liquids of different indices was then plotted onto a graph. Using the nominal data provided by the information sheets that came with the fiber, it was found that the data points followed the same curve of the overall prediction, but the center of the transmission bands were slightly higher in wavelength. An experimental model was then written and produced that matched the trend seen in the data points obtained.

In conclusion, the theory was validated with the data points obtained, minus small abnormalities when approaching ultra-violet light. Further research to be done includes testing the theory with other photonic crystal fibers and obtaining more data points for indices of refraction below that of the range of water.



Victoria Treviño Kala: Simulation and Optimization of Multilayer Mirrors

Multilayer mirrors are structures which consist of alternating layers of high- and low- Z materials with thicknesses in nanometers. These structures act as reflective optics in extreme ultraviolet light (EUV) and soft x-ray ranges. Because of this feature, multilayer mirrors have a variety of applications from high resolution imaging in astronomy to attosecond light pulses.

My work at the Laboratoire Charles Fabry at Institut d’Optique focused primarily on the simulation and optimization of multilayer mirrors using commercial software such as Matlab and IMD. I also assisted with the creation of some of these mirrors using a magnetron sputtering deposition machine. After we created these mirrors, we measured their performance by using an x-ray reflectometer to obtain an reflectivity spectrum of the mirror.

The simulation codes I created in Matlab allows the user to define certain parameters (i.e. energy, polarization, grazing incidence angles) as well as the structure of the mirror (i.e. materials, thicknesses, periodicity, roughness). The user also has the option to com- pare the Matlab simulation with an IMD simulation as well as experimental data obtained from the x-ray reflectometer. The optimization code is very similar, as it allows the user to identify the parameters and structure, but the code then makes use of the experimental data to create a new mirror with optimized thicknesses with a better reflectivity spectrum.



Aaron Laursen: fluid-filled Photonic Crystal Fibers

Due largely to their novel non-linear properties, fluid-filled Photonic Crystal Fibers (PCFs) have attracted the interest of both academic and industrial optics researchers through- out the world. A small subset of these researchers (and researchers of assorted other non- linear optics), form the Manolia Research Group at Institut d’Optique Graduate School in Palaiseau, France, and, thanks to the University of Michigan and a generous grant from the National Science Foundation, I was given the opportunity to join this group for a two month internship during the summer of 2013, under the direction of Dr. Philippe Delaye.

During my time with the Manolia group, I worked with another intern (there through the same iREU) to investigate the optical transmission properties of various PCFs. More specifically, we sought to understand the connection between the fluid used as a filler and the location of the resultant fiber’s transmission-band and to experimentally validate a theoretical model for such a relation. To this end, I began by researching current theoretical models and calculating their predicted refractive index – band center relation. I then expanded these to account for such complicated effects as the wavelength and temperature dependences of refractive indexes, generating an even more complete and (hopefully) accurate model.

After gaining a thorough understanding of the theory, I moved on to the experimental side of this project. Having been given a simple proof-of-concept setup the lab had already developed, my lab partner and I refined a stream-lined and optimized laser system for measuring a given fiber’s transition-band shift. In addition to preparing the measuring system, we also prepared a number of fiber samples using custom filling fluids to achieve target refractive indexes. All of this work culminated in a large dataset which I was given the task of analyzing, and, after writing several programs to do so, I managed to extract some excellent results.

These two months gave me wonderful opportunities to not only gain valuable research experience and contribute to a larger scientific endeavor, but also to experience engineering research on an international scale. While some language (I do not know French) barriers and cultural differences may have presented themselves occasionally, thanks to the patience and enthusiasm of my supervisors and collaborators, and indeed the larger community, I had a excellent experience, and would strongly recommend this or a similar program to anyone interested in physics and engineering research with a global scope.



Michael McLoughlin: physics of laser-plasma interactions

This summer, I had the formidable opportunity to engage in scientific research at LOA (Laboratory of Applied Optics). This opportunity was extended to me via the University of Michigan with funding from the National Science Foundation; therein, for the efforts and generosity on behalf of all parties, I am extremely grateful.

The lab that I worked at and the group that I worked with specialize in the physics of laser-plasma interactions. The overarching goal of the my group, as I understand it, is the study of the interaction between high intensity short pulse lasers with plasma in order to better understand and control the phenomenon which results from this phenomenon dubbed ‘wake-field acceleration.’ Crudely, the interaction between the laser and the plasma creates an environment with high electric fields (the ‘wake-field’) allowing, thus, for the rapid acceleration of particles (here, electrons) over short distances. It is this effect that one aims to understand.

My research this summer was related to this overarching goal and consisted of two small projects. At my request, one of these projects more experimental while the other was more theoretical. In the first, which lasted about two-weeks, I worked with Doctoral student Emilien Guillaume. We put the finishing touches on an experimental apparatus that he had been building (before my arrival) which was designed to allow one to measure the density profile of a gas injected through a nozzle into a vacuum chamber. The intention was to learn to control the density profile in such a way as to create a ‘shock’, or rapid density decrease, in the profile. Once mastered, a similar method would be used for injecting the gas that was to become the plasma in the wake-field accelerator in order to, following the hypothesized effects of plasma density on the wake-field, produce a plasma environment more conducive to accelerating electrons.

The second project I worked on was supervised by Remi Lehe, a second Doctoral student, and was more extensive. We worked on improving a laser-wake-field simulation program called CALDER. CALDER was built using a discretized scheme of Maxwell’s equations known as the Yee-Scheme. The Yee-scheme, however, suffers from numerical instabilities (identified as numerical Cherenkov radiation) when modeling particles with velocities approaching that of light (as we had need to do). Remi had a new scheme in mind and it was my job to do the calculation that would adapt it to Maxwell’s equations, implement it into the source code of CALDER, and then work with the simulation to see whether, as was expected according to a previous calculation, it produced the desired effect of removing the this numerical instability. The desired effect was achieved.

Once the new scheme was successfully completed I was set to implementing a new technique that promised to significantly decrease the computation time of the laser-plasma simulations. This technique, first proposed by J. Vay at Berkeley, involves taking data from the simulation from the perspective of a Lorentz Boosted frame. It was shown that such a boost would ease the computational requirements and decrease computation time by a factor on the order of (where is the typical relativistic factor). While the boost has been implemented its promise is delayed by both certain unexpected numerical instabilities which arise in the boosted frame (potentially ‘numerical heating’) and the successful implementation of a module into CALDER which can transform the computational results from the boost frame to the lab frame so that one may quickly interpret the results.



Francisco Nunez: Hydrodynamic Simulations of Thin Foils Ablated by Long and Short Laser Pulses

This summer I conducted research under the supervision of Sophia Chen in conjunction with PI Julien Fuchs at the LULI Laboratory of Ecole Polytechnique, a science institution located in Palaiseau, France. My research project consisted of running hydrodynamic simulations of thin foils being ablated by both a long and short laser pulse. These simulations were based on a physical experiment conducted on the ELFIE 100 TW LULI laser facility dedicated to the study of longitudinal ion acceleration using a high intensity picosecond laser pulse irradiating a 500 nm thick plastic foil exploded by a nanosecond pulse, though in the simulations the thickness of the target was varied, among other things. This experiment was conducted with the belief in mind that it is more effective to accelerate ions by using under-dense targets instead of solids, i.e. thin foils (any target less than 100 microns). Accelerating ions is useful in several applications because it produces high energy and current beams. Accomplishing this with a laser allows us to create compact energy that does not require electrostatic techniques or an enormous amount of space. In the context of plasma physics, producing ions can be used to probe plasmas and/or heat them to conditions similar to stars, among other things.

The first week was dedicated to getting familiarized with the research and gaining perspective on what exactly I was going to be working on from a physics context. I read a number of research papers on topics such as plasma physics, ion acceleration, laser ablation and so forth. I also interacted with my advisors concerning any questions I had. I could not do much more than that because both of my advisors were at conferences for the first week.

The following weeks were spent learning how to use the simulation software I was tasked to work with, which was a software called ESTHER, which is a code that produces a 1D radiation-matter interaction that allows us to study the evolution of a stack of varied backgrounds from the solid phase to the plasma phase by a pulse of intense radiation, in our case a laser pulse. I systematically translated the manual, which was in French and acquired all of the possible knowledge I could extract from it. I then learned how to access the remote cluster, in which the ESTHER software code is housed in order to run the simulations. I ran several simulations of various targets (aluminum, hydrogen, etc.) in order to get acclimated with how to use the program.

For the next couple of weeks, I ran simulations and did some trouble-shooting on different issues associated with particular problematic simulations. Hydrogen initially was one of the main simulations I worked on that was having problems running. After convening with the programmers of the code and troubleshooting the issue we were able to successfully run hydrogen. The issue was that certain files associated with the element’s execution were not present on the cluster. We were focusing on observing the laser profile of these simulations.

We ran into some momentarily debilitating issues with other desired simulations so briefly transferred over to working with software called FLASH. FLASH code is a modular, parallel multi-physics simulation code capable of handling general compressible flow problems found in many astrophysical environments. It is a set of independent code units put together with a Python language setup tool to form various applications. The code is written in FORTRAN90 and C. It is also useful for hydrodynamic simulations after recent update modifications. The main difference with this type of simulating was that it was in 2D, which made it significantly more complex. Because of the computational complexity, each simulation took about 4 hours to run instead of the 2-5 minutes ESTHER simulations took to run. We modeled hydrogen on this software for a short period.

We then went back to working with the ESTHER code and focused on a new main target of interest, which was CH2 (methylene), which was a type of plastic. I ran simulations for this target at varying laser intensities. In order to obtain these intensities, I had to calculate them by hand into units of WATTS/cm^2 and then convert them to WATTS/m^2 by adding 4 orders of magnitude. I found this out later when we realized that our simulations kept crashing and we didn’t know why. The problem was that ESTHER took intensities values in the latter format as opposed to the former.

After about a week of trial and error and establishing all parameters for the given laser shot cases, I began to simulate the plastic foil and record the results. I did this all the while taking into account only one single pulse because aside from intensities with reduced values, the second pulse in the ablation was considered negligible. Later we ended up simulating all cases as dual pulses with the addition of second pulse parameters and some extra associated input files because it was discovered that the second pulse was not negligible after all. After simulating each varied foil thickness, I saved the simulation image as a PNG, converted the output to a text file at a particular time of interest (i.e. when the plasma began to expand at the front and rear), imported the text file to excel and created a scatter plot for the output. On this scatter plot, I created an exponential trend, line of best fit for the front and rear of the plasma, added error bars and calculated the R squared value for each plot. A trend line made the line of best fit by adding an extra column of arbitrary data. This arbitrary data set column was manipulated in order to achieve the desired fit. The rear of the plasma was a very small curve, so I had to zoom in tremendously to get an accurate fit.

In addition to the dual pulses, I also ran a series of simulations with the second pulse only at target thicknesses varying from .1 microns to 4 microns in target thickness. This was done in order to isolate the activity taking placing within the lifespan of the second pulse, which remains constant throughout the simulation. After completing all simulations and plotting them with appropriate fits, I created a comprehensive document for the future reference of other researchers working on this experiment. It included the various shot case intensities, target thicknesses, simulation result figures, adjusted delays between the first and second pulse, among other things. The delay between pulses affected how much the plasma expanded. The thickness of the target affected the distribution of heat transfer.

During the course of this research experience I also got the opportunity to tour the laser labs, which included lasers like ELFIE within the LULI laboratory facility. This was a unique and enriching research experience in a plethora of ways. It exposed me to a foreign culture, allowing me not only to acquire an ability to communicate with French people, but also adopt their customs and gain greater perspective from a culture different than my own. I also gained a great amount of knowledge on the highly interesting and relatively new field of HEDP (high energy density physics). I learned about different lasers, how they affect targets when performing ablation, how the composition of material can change when affected by a laser (i.e. turn into a plasma or something else), how the study of particle interaction has relevance in scientific application and that is just to name a few things. The fact that I learned about all of these incredibly complex concepts and worked on a project based on them, all the while combatting a language barrier in several cases is incredibly satisfying to me. It has shown me what I am capable of doing if I invest the necessary time and dedication.



Timothy Pillsbury: Nonlinear optical phenomena in metallic and semiconductor nanostructures

My research focused on the fundamental study of second harmonic generation (SHG) on gold triangular nano-prisms and of third harmonic generation (THG) on lead sulfide, PbS, quantum dots. It becomes possible to obtain SHG on gold nano-structures due to their small size compared with wavelength that permits observation of the phase retardation effect and because centro-symmetry breaks on their surfaces as a result of their non-centro-symmetric geometry.. The former permits the SHG from centro-symmetric materials. In the case of gold nano-prisms we studied the dependence of SHG on nanostructure geometry and distances between structures. We pursue the fundamental interest of THG in the case of PbS. The potential to generate coherent signal by quantum dots opens a new possibility in molecular and cellular imaging. In both cases, I used polarization-resolved nonlinear optical microscopy setup, which detects both the s and p components of emitted light and, as consequence, establishes the state of polarization of SHG and THG. In addition this setup can perform image acquisition, spectrum measurements, and temporal measurements. My first task was to identify the excitation wavelength where we obtain the strong enhancement of SHG due to localized surface plasmon resonances. Then, using this information, I studied the impact of nano-prisms’ designs on SHG, on amplitude values, and on the resonance position. The results of my measurements were in agreement with results of simulation performed by collaborators. Thus for horizontally polarized light, we found strong SHG enhancement for the spectral region between 700 nm and 850 nm, which corresponds to local surface plasmon resonances. As expected, we observed the spectral shift of resonance band in function of the nano-prisms design. This SHG tunability makes the nanostructures applicable to a variety of possible uses.

Next, I concentrated on measurements of THG of PbS QD’s. I improved the protocol of sample preparation by varying the solvents used and their concentrations, modifying sonication time, and performing AFM measurements to examine sample quality. Using the prepared samples, I identified the signal, validated the THG through power dependence measurements and emission spectroscopy, and proceeded with series of polar plot measurements that should elucidate the questions on THG origin. This research was done in frame of developments of new bio-markers for cellular and tissular imaging [1] taking in account the absence of blinking in PbS THG and the presence of coherent emission, which permits of use very sensitive interferometric techniques.

[1] Yelin, D., Oron, D., Korkotian, E., Segal, M., and Silberberg, Y. (2002) Third-harmonic microscopy with a titanium-sapphire laser. Applied Physics Letters B.



Michael Reynolds: Second and Third Harmonic Generation for Calculating Hyperpolarizability

I worked with Dr. Isabelle Ledoux-Rak in Laboratoire de photonique quantique et moléculaire at École Normale Supérieure de Cachan. Over the course of the summer, we received four sets of molecules: two sets of ferrocenyl acetylenyl linked heterocyclic compounds from IISER Bhopal in India, one set of porphyrin molecules from INSA and Rennes University in France and one set of cavitands from University of Pécs in Hungary. My work was to use electric field induced second harmonic generation (EFISH) and third harmonic generation (THG) to measure the nonlinear response of these molecules. This research will help each of the labs to identify more effective nonlinear molecules and to customize them for applications in the field of photonics. Specifically, the porphyrins and molecules from Bhopal are part of continued research into all-optical signal processing, while the cavitands are being studied for applications in electro-optics.

Harmonic generation is a nonlinear optical phenomenon in which light of sufficient intensity incident on a material excites a higher order response in the polarization of the material. This higher order response results in a higher frequency electromagnetic wave. Effectively, in these materials, several photons are allowed to create one higher energy photon. EFISH and THG experiments measure a series of interference fringes resulting from changing the path length of the laser in the solution, called Maker fringes, to calculate the second- and third-order susceptibility per molecule, respectively. These quantities are also known as the first- and second-hyperpolarizability.

When I arrived in early June, I joined in the work already being performed by another undergraduate in Mme. Isabelle’s lab, Imane Abdouh. I learned from her how to operate the Q-switched Nd3+: YAG nanosecond laser and to prepare and measure samples for third harmonic generation. Together we measured the third harmonic Maker fringes for the first set of molecules from IISER Bhopal. I performed analysis and compiled a report outlining our findings, which was sent to the IISER Bhopal lab.

After Imane departed, I continued the work independently, measuring, analyzing, and reporting the results for the porphyrins from Rennes and the second set of heterocyclic compounds from Bhopal. The porphyrins particularly had a strong third order nonlinear response, some attaining negative values of hyperpolarizability on the order of 10-32 (esu). In the final week of the program, Mme. Isabelle changed the setup to perform EFISH to measure second order nonlinearity. I then performed measurements on the cavitands and wrote a report outlining the findings for these molecules as well.

Immediate future work on this project will look at the first-hyperpolarizability of individual conjugated branches from the cavitands to better understand the interplay between different components of the original molecules and at the second-hyperpolarizability of an even more advanced set of dendritic porphyrin molecules. More generally, it will entail continued correspondence with each of the labs that contributed molecules for our measurements as they look to engineer nonlinear materials for the previously mentioned applications.



Sarah Sofia: Proton Beam Analysis at LULI, Ecole Polytechnique

This summer, I analyzed proton beam spectra from a neutron generation experiment aiming to get a neutron beam with a narrow energy spectrum that was performed by Julien Fuchs lab on the laser Elfie at LULI at Ecole Polytechnique. The experiment used the duel target generation that has been used in previous experiments, shooting the short pulse laser at a target, producing and accelerating a proton beam through TSNA acceleration. However, in order to create a narrower neutron beam spectrum, a more mono energetic proton beam is needed. My work over the summer focused on the analysis of the protons beam spectrum, to analyze the effectiveness of the spectrum narrowing techniques and find the proton beam spectra that can be used in neutron generation simulations.

In order to analyze the proton beams, the beam is shot at a stack of radiochromic film (RCF), where the incident protons deposit energy on each layer until they are stopped according to their response function. Higher energy protons deposit most of their energy further into the pack, while lower energy protons deposit most of their energy in the first few films until they are stopped. By analyzing the signal on the film from the protons, converting optical density to dose and then to the total deposited energy on the film, we can find the energy spectrum that best fits the energy deposition on the RCF.

So, in order to filter the energies of the proton beam, a focusing cylinder was placed in the path of the proton beam. When a second laser pulse is shot at the cylinder, it creates a plasma and resulting electric field on the inside of the cylinder which accelerates the proton beam radially inward, focusing the beam. However, the focal lengths of the protons’ focuses depend on the energy of proton. So this can be used to filter proton energies by placing the target far away, or inserting a pin hole. I spent a large portion of the summer analyzing data from a shot with a focusing cylinder and a distant target. I found that the proton beam that reached the target does seem to have a narrower energy spectrum of roughly 1.5-2 MeV in width. This means that a more mono-energetic neutron beam can likely be achieved; however the method is not very efficient since many protons are lost in the process of passing through the cylinder and reaching the distant target, so more work will likely need to be done to balance the narrowing of the beam with efficiency.