Dinner in Paris With Gérard and Marcelle Mourou
Top row, left to right: Jahaira Santoyo, Phoebe Marcus-Porter, Hana Warner, Ty Naquin, Ahmad Abed
Lower row: Arianna Giguere, Christopher Valdes, Gérard Mourou, Marcelle Mourou, Tayari Coleman
Last night of the program
2019 Projects
Tayari Coleman: Visualizing and modeling droplet nucleation and the role of laser tweezing
Whiteboard Video:
This summer the majority of my research consisted of Comsol simulation and I spent an uncountable number of hours at the computer running simulations. My mentor long ago had finished a simulation showing the diffusion of 1,4-dioxane in water, and the concentration of DBDCS within the solution. I was tasked with creating a simulation that successfully implemented and laser and its constituent optical forces as to observe the effects. This was an effort to try and recreate experimental results which currently go unexplained. Zhengyu and Robert had observed that when in the presence of a laser the DBDCS is further focused and begins droplet formation and nucleation much sooner. They had theories about the what the laser was doing but no concrete physics to back them up. Upon viewing the data and the physics I too proposed my own theory. I proposed that the optical gradient forces were causing a focusing of the dioxane thus focusing the DBDCS. I also believed that since this focusing occurred in all directions that there was a pressure differential as well and that the drop in pressure following the laser could account for the earlier nucleation. By effectively simulating this laser and its accompanying forces in Comsol we could pick apart the physics and more easily back track through it for a much more concrete explanation. It wasn’t until the very end of my research, but I was able to create a simulation which showed potential and closely resembled the experimental results. Although my simulation needs further refinement it was a great step and hopefully very useful for Zhengyu and Robert.
The other part of my research was helping to achieve a form of Schlieren microscopy. Instead of using a knife edge for phase gradient imaging we will use a special mask made up of multiple sections of varying optical densities and sharp cutoffs to achieve the knife edge. I created and 3D printed parts for the aperture and condenser to hold the mask, the slit and some Polarizers.
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Christopher Valdes: Optimizing a Set-Up Coupling Electrochemical and Fluorescence Microscopies
Whiteboard Video:
This summer, I worked at the Photophysique et Photochimie Supramoléculaires et Macromoléculaires Laboratory (PPSM) at ENS Paris-Saclay, and I worked on Optimizing a Set-Up Coupling Electrochemical and Fluorescence Microscopies. My lab has a set-up that combines an electrochemical microscope with a fluorescence one so researchers can study a molecule’s fluorescent characteristics while simultaneously applying an electric field. However, recording data from this set-up was very inefficient, so my project focused on developing a more efficient data-acquisition system. The system we decided to use was Master-Slave Synchronization, which basically designates one machine as the “master” in order to control and monitor other devices known as the “slaves". In the set-up, we used a Potentiostat called VersaSTAT 4 as our master and an S2000 spectrometer as our slave. I started by figuring out how to externally trigger our spectrometer, and I found that by using specific pins on an auxiliary interface, we can connect V4 to the spectrometer and cause it to start acquiring data. We also placed the spectrometer in synchronization mode and adjusted acquisition parameters, such as integration time, capture period, and average number of scans, to see if any changes resulted in a timing delay. We found that the delay timing remained consistent regardless of these parameters; the only time delay would change is if we adjusted the frequency of the signal.
Despite doing this, we wanted to use LabVIEW to trigger the spectrometer instead, and there is a reason behind this. In Master-Slave Synchronization, we have three types of signals: clock signals, asynchronous signals, and synchronous signals. Clocks are periodic square signals that provide a sampling frequency, so they remain constant during acquisition time and provide a timing reference source for slave devices to follow. Asynchronous signals are not correlated in phase to these clock signals while synchronous signals are, so by sending a synchronous signal and make it followed by the clocks’ slaves, we can ensure that time intervals between acquired data and master will be consistent, and we can synchronize other data acquisition chains to the same signal as well.
Overall, we want a V4 trigger to trigger a LabVIEW program. This program will generate a clock frequency that can trigger and monitor spectrometer acquisition in an asynchronous fashion; that way, we can acquire spectra in a controlled manner, fire signals, and synchronize recordings of acquisition. With this being said however, there are numerous ways to do this, and I started to use LabVIEW to reconfigure different codes and test each code out to see which is the best set-up, but unfortunately, my time in the lab came to an end. In the future, my lab will be able to synchronize a CCD detector to the same signal, and even potentially the microscopes themselves. Either way, thank you to Jeff Audibert for helping and working alongside me throughout the summer!
Works Cited:
Guerret-Legras, L., Audibert, J., Ojeda, I.G., Dubacheva, G., Miomandre, F., 2019. Combined SECM-fluorescence microscopy using a water-soluble electrofluorochromic dye as the redox mediator. Electrochimica Acta.
Guerret, L., Audibert, J.-F., Dubacheva, G.V., MIOMANDRE, F., 2018. Combined Scanning Electrochemical and Fluorescence Microscopies using a tetrazine as a single redox and luminescent (electrofluorochromic) probe. Chemical Science.
National Instruments. “Synchronization Explained.” Synchronization Explained – National Instruments, 6 June 2017, www.ni.com/product-documentation/11369/en/.
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Phoebe Marcus-Porter: Fabrication of two dimensional waveguide structures containing a single quantum dot
Whiteboard Video:
This summer I worked at Le Laboratoire de photonique quantique et moléculaire under the guidance of professor Lai and master’s student Long on a project that focused on realizing two dimensional waveguide structures containing a single quantum dot.
Waveguide fabrication for single-photon sources has important applications in quantum computing which can expand the capabilities of computers for more complex problem-solving and advancements in scientific research, medicine, modeling quantum systems, data storage, and much more.
We created a structure which mimics a Mach-Zehnder interferometer, well known in classical optics. Mach-Zehnder interferometers classically work with multiple photons at once. In order to use this structure for quantum optics we used a single photon source, coupled at one end of the structure, which can be detected at the other end after being guided through one at a time.
In order to do this, we used low one photon absorption direct laser writing, a process by which we focus a 532 nm laser beam at low power on a specific location on a photosensitive material to polymerize it.
We created samples using glass substrates coated with an SU-8 photoresist layer and a layer of quantum dot solution. We placed the sample onto a piezoelectric translation stage which allowed us to move different parts of the sample into the focus of the beam according to a program we created on Labview which instructed the PZT to move in the shape of a straight line followed by a series of quarter ellipses and a final straight line. We used an objective lens to focus the beam onto the sample which can both excite the material and collect the emission signal from the sample in order to image it and view where the quantum dots are located. Once a quantum dot is located, we then determined whether or it was a single photon source, by passing its signal through a Hanbury Brown Twiss system, a beam splitter with detectors on either side, if it was a single photon source, a signal would only be detected from one side of a beam splitter at a time, and we were able to see this on an anti-bunching curve that was zero at the center. We then ran our Labview program to fabricate the 2-D structure.
After developing the structures, we scanned the field again to determine whether or not the quantum dot was coupled into the structure.
This summer, we successfully fabricated the Mach-Zehnder structure coupled with the single photon source. During the last week of this program, we have started working on adding a structure to the 2-D waveguide which directs the light toward the objective lens and allows us to elevate the structure so that it is no longer against the glass where the difference in refractive indices is not sufficient to detect a waveguide effect and so that the light will be bent down into the objective lens where it can successfully be detected.
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Ty Naquin: Optimizing production of reactive oxygen species with localized surface plasmon resonance
Whiteboard Video:
Light incident on a metal nanoparticle causes the conduction electrons to oscillate. Excitation of the nanoparticles at a resonant frequency results in the creation of a strong electromagnetic field. This phenomenon is known as localized surface plasmon resonance, or LSPR. Thanks to the properties of LSPR one can quickly and efficiently inject energy into the nanoparticle system using ultrashort pulses of light. The properties exhibited by the LSPR vary widely with the size, shape, and composition of the nanoparticle as well as the features of the surrounding medium. This phenomenon can be employed to attain many chemical and biological functions. It has been shown that that through the excitation of gold nanoparticles it is possible to generate reactive oxygen species such as the singlet oxygen. The first step of my project was to use pump-probe spectroscopy to detect the solvated (free) electrons in water. In order to increase our chances of detecting these electrons, I assembled an apparatus to sparge the dissolved oxygen inside a sample of water by flushing argon through the system. After detection of solvated electrons, our lab will excite gold nanorods of varying aspect ratios to optimize the production of reactive oxygen species. Our lab seeks to establish a link between the generation of reactive oxygen species and the presence of electrons in solution.
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Arianna Giguere: Chirp-Scan Measurement of Ultrashort Laser Pulses
Whiteboard Video:
This summer I worked on a project titled Chirp-Scan Measurement of Ultrashort Laser Pulses. The goal of my project was to connect a frequency-resolved optical gating device (or FROG) to one of Ecole Polytechnique’s high-intensity femtosecond lasers for characterization of its pulses. High intensity lasers are important for current applications within medicine and industry, for example, and these powerful lasers also have the potential to be a solution for eliminating harmful nuclear waste in the future. The ability to fully characterize pulses of such lasers is essential for determining their capabilities, and the FROG device conveniently allows for measurements of both the spectral frequency and phase of the pulses.
This summer I began by helping another intern to optimize and couple the beam into a hollow-core fiber which is meant for spectral broadening. Through a process called ‘self-phase modulation’, the beam going through the fiber is essentially “cleaned up”, which is useful for practical experiments. We wanted to use the FROG to look at the pulses coming out of the fiber. My lab already had a FROG device available from years ago, and using a small part of the high-intensity beam, I worked to resurrect the FROG and learn about its sensitivities and characteristics. In addition, when Louis Daniault connected his FROG retrieval software to the physical device, I worked to bridge the gap between the device and computer program. By the end of the program, this involved exporting data from a successful FROG trace retrieval, making plots, and applying Fourier transforms to manipulate the information in relevant ways.
Overall, much of my work this summer involved debugging errors. Without taking into consideration the issues we came across with the laser itself or problems we faced setting up the interface between the FROG retrieval code and the device itself, the FROG device had quite a few of its own issues experimentally. First of all, the device is so incredibly sensitive to any changes in alignment that sometimes I would leave work, come back the next day, and find the alignment was off. Besides this, some of the largest issues I came across included the following:
1. Understanding why I saw fringes at the focus spot of the SHG crystal (and why they were slanted in orientation)
2. The existence of uneven pulse replicas (for example, one bigger than the other) which was clearly detrimental to the FROG trace
3. Difficulty producing a measured trace as seen by the camera good enough to be processed
4. Calibrating the time axis
5. Performing the retrieval and finding that the algorithm could not identify the FROG trace below 1% (even though we knew the retrieval code was capable of doing this, so something was off experimentally).
My solutions to these problems were the following:
1. The fringes were due to the interference of the pulse replicas as expected - it meant the pulses were directly on top of one another. The slanting pulses had no real significance besides being an effect of the delay. As long as the fringes were equal in size across the majority of the pattern and they were clear, the pulses were overlapping like we wanted them to.
2. A consistent beam height all the way through the device was crucial. Minor adjustments were acceptable, but a steady beam height made a difference. In addition, the separation between the bi-mirrors needed nearly perfect symmetry about the center of the beam, otherwise the pulses would not be equal in size.
3. Aligning the FROG just right to produce a good trace before trying the retrieval takes the utmost care. I developed a short guide of tips that I used when aligning the FROG in order to help save future researchers some time, and I sent this to Rodrigo.
4. I discovered we needed a more precise tool, as in the Feinpruf Millitron to measure the mirror displacement in order to calibrate successfully.
5. I found that a time coefficient value of ~0.45 femtoseconds per pixel is necessary for the retrieval to identify the trace within 10E-3 error. However, this discovery was made accidentally since that particular calibration was performed with a poorly-focused trace. It works for well-focused traces and we are still unsure why. Additionally, it is important to note that when I refer to a “well-focused” trace, I mean an un-chirped elliptical trace focused the long way along the spectral domain (for a broad spectral domain - the temporal domain will subsequently be narrow).
Moving forward, the FROG will be used to characterize the pulses coming out of the hollow-core fiber, and later it may be used to look at the pulses after further compression.
I would like to thank my mentor, Rodrigo Lopez-Martens for all his guidance this summer, and Louis Daniault, without whom I could not have gotten started using the GPA. To Konstantin, Zhao, and Jean-Baptiste - thank you for answering my many questions. And to all those at the University of Michigan who gave me this opportunity - Dr. Steven Yalisove, Dr. John Nees, Bett Weston, etc., as well as the NSF, thank you for making this experience possible.
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Ahmad Abed: Confocal microscopy for the characterization of spontaneous emission by quantum light sources
Whiteboard Video:
The study of atomic system in solids is of great importance due to their potential applications in quantum technology. The main focus of the setup that is developed is to study quantum light from defects in solids. Point defects in solids can behave as single-photon sources. During this internship, we assembled a small platform based on confocal microscopy to characterize light from emitters such as point defects in hexagonal boron nitride (hBN). We used a helium-neon (He-Ne) laser as our excitation source and a spectrometer (Andor Shamrock SR-750) to study the spectrum of the emitted light. The platform is based on three stages. The first stage involves the collimation of the He-Ne laser that will excite the emitter. The collimation of white light from a LED provides illumination for the inspection of the sample surface by a CMOS (uEye) camera. The second stage is dedicated to the excitation of the sample. The third stage collects the fluorescence light and directs it into the spectrometer. Furthermore, some intensity correlation measurements will take place by using a beamsplitter. I would like to thank Dr. Benjamin Vest and the Plasmonics and Quantum Nanophotonics team for their help throughout the summer, the Institut d’Optique Graduate School for welcoming and hosting me, the University of Michigan for organizing this amazing program, and the National Science Foundation for all the support.
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Hana Warner: Characterizing and manipulating cholosteric liquid crystal droplet microlasers
Whiteboard Video:
My research this summer was conducted at the Laboratoire de Photonique Quantique et Moléculaire (LPQM) at ENS Paris-Saclay under the advisement of Dr. Mélanie Lebental. My work involved characterizing and manipulating cholosteric liquid crystal droplet microlasers. Liquid crystals have both highly ordered properties, like crystalline solids, and unordered properties, like isotropic liquids. They are often in a nematic phase, where the axis of the molecules are parallel, but a chiral dopant can be used to induce a pitch in the crystals, which is called the cholesteric phase. Liquid crystals can be formed into droplets when they are suspended in a viscous fluid.
These droplets are interesting because they can act as a spherical optical cavity, allowing us to make 3D microlasers. Light becomes trapped in the cavity and is reemitted as either Bragg Modes or whispering gallery modes. Bragg modes result from periodic changes in refractive index within the droplet due to the cholesteric pitch, allowing for repeated reflections back into the cavity. These modes are emitted from the droplet center. Whispering gallery modes result in light being trapped at the droplet edge and are emitted when it can complete a closed cycle. They are emitted at the radius. Lasing action can be accessed by doping the liquid crystals with a laser dye, like we do, or by coupling an optical waveguide to the cavity. The emitted wavelengths are related to the pitch of the cholesteric dopant in the droplet and the type of dye used in the sample, and the microlasers have applications in photonic circuits, sensing systems, and imaging.
My liquid crystal samples were provided by Dr. Brigitte Pansu at Université Paris-Sud. I began my project by determining the process to create textures within liquid crystal droplets and how these defects change the lasing spectrum. Liquid crystal textures describe topological defects within the cavity. They can be created during the manufacturing process and via stress, but we were interested in creating textures via laser excitation to give greater control in their formation. Changes to the droplet result in changes to Bragg and whispering gallery modes, and I was successful in creating textures at low energy in droplets both with and without laser dye. I determined that the presence of dye tended to lower the threshold to create defects in a droplet and that using different solvents did not change the threshold to create defects in most cases.
After characterizing texture formation in the droplets, I moved on to manipulating the droplets with a magnetic field. Our liquid crystals contain a benzene ring, so their magnetic moment rotates to align with the direction of the field. We saw a change in the center structure but no fluctuation of the radius, which corresponded with a clear shift in the Bragg mode while whispering gallery modes remained unchanged. This means that, by placing the droplets in a magnetic field, we can tune their lasing spectrum. This is both an exciting alternative to electric field tuning of liquid crystal droplet microlasers and has potential applications in photonic circuits and magnetic field sensing systems.
The last part of my project involved characterizing the polarization of our laser spectrum. This was accomplished by suspending a fiber, polarizer, and quarter wave plate above a capillary containing the droplets. I measured linear polarization in both the Bragg modes and whispering gallery modes emitted by the droplet; however, this is not exactly expected behavior, and I am curious if some changes in polarization could be induced by the edge of the capillary. In the future, I am interested to see if we can measure polarization while the droplets are suspended in a large cylindrical capillary, since we ran into some limitations with our capillary size that prevented us from studying polarization from cylindrical capillaries.
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Jahaira Santoyo: Characterization of a Phase Plate Overview
Whiteboard Video:
The goal for this summer was to characterize a home-made phase plate. There are a couple of main parameters our research group was most interested in: thickness of the deposition that introduces the pi-dephasing, exact dephasing introduced, and the homogeneity of the phase phate. It is important to note that all of my work was experimental with a side of coding to make data presentable.
I spent the first week reading articles and books to familiarize myself with optics and the math behind it. Once I was familiar with the mathematical concepts and the behavior of light when passing through different optical devices, I was ready to start testing these concepts myself.
For example, one of the very first experiments I did was to propagate light through 2 quarter wave plates (QWP). Maha, the grad student I was working with, encouraged me to prove mathematically with sines and cosines what would happen to the light wave. We moved forwards to explaining what would happen to the light wave using Jones matrices.
Mathematically I had realized that incident vertically polarized light onto 2 QWP would result in horizontally polarized light. I then tested my hypothesis in the lab and saw that my prediction was correct. I did a lot of little experiments to familiarize myself with some of the optical devices.
Originally, the experiment I was suppose to work on consisted of 2 QWP and the phase plate. Light would propagate through one QWP, half the beam would hit the pi-dephasing section and half would hit the 0-dephasing section of the phase plate, then the light would go through another QWP. This process would result in the beam containing both horizontally and vertically polarized light, which combined create higher order modes in an optical fiber. This experiment would also allow me to test for the parameters we were interested in the phase plate.
My task was to prove the above process with Sines and Cosines and with Jones matrices. I couldn’t figure out why my math wasn’t working out, so after banging my head on the desk for a couple of days I decided to ask for help. Turns out I helped discover a mistake in our experiment. The pi-dephasing needed to be introduced between the x and y components the beam propagating on the pi-dephasing section in order to change the direction of polarization. Our experiment failed to do so, therefore we could no longer rely on this specific set up.
While we brainstormed how to move on, Maha showed me how to draw fibers into nanofibers. She taught me how to strip them and how to inject a laser light into the fiber in the most efficient way. This a REALLY fun part ! I got to see the coding on labView that allowed the machinery to draw the nanofibers.
To figure out the homogeneity of the phase plate I simply incident a laser beam into the two sections independently and made power measurements. I also used a polarizer to characterize the polarization of the input and output light. I took extensive notes and coded up my data to present to my advisor.
The other parameters would be determined by the Mach-Zehnder interferometer, but I quickly moved away from this because calibrating was VERY difficult and would take up too much of my time. I moved onto the Michelson interferometer. I spent a couple of days trying to calibrate and was successful. Some strange but very insightful interferences were discovered.
I have sadly run out of time, so I could not continue to work on this project. I learned so much more than expected and I feel really confident with working in optical labs back at my university. Overall, this summer I definitely increased my experimental skills.
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Whiteboard Video:
This summer the majority of my research consisted of Comsol simulation and I spent an uncountable number of hours at the computer running simulations. My mentor long ago had finished a simulation showing the diffusion of 1,4-dioxane in water, and the concentration of DBDCS within the solution. I was tasked with creating a simulation that successfully implemented and laser and its constituent optical forces as to observe the effects. This was an effort to try and recreate experimental results which currently go unexplained. Zhengyu and Robert had observed that when in the presence of a laser the DBDCS is further focused and begins droplet formation and nucleation much sooner. They had theories about the what the laser was doing but no concrete physics to back them up. Upon viewing the data and the physics I too proposed my own theory. I proposed that the optical gradient forces were causing a focusing of the dioxane thus focusing the DBDCS. I also believed that since this focusing occurred in all directions that there was a pressure differential as well and that the drop in pressure following the laser could account for the earlier nucleation. By effectively simulating this laser and its accompanying forces in Comsol we could pick apart the physics and more easily back track through it for a much more concrete explanation. It wasn’t until the very end of my research, but I was able to create a simulation which showed potential and closely resembled the experimental results. Although my simulation needs further refinement it was a great step and hopefully very useful for Zhengyu and Robert.
The other part of my research was helping to achieve a form of Schlieren microscopy. Instead of using a knife edge for phase gradient imaging we will use a special mask made up of multiple sections of varying optical densities and sharp cutoffs to achieve the knife edge. I created and 3D printed parts for the aperture and condenser to hold the mask, the slit and some Polarizers.
Top
Christopher Valdes: Optimizing a Set-Up Coupling Electrochemical and Fluorescence Microscopies
Whiteboard Video:
This summer, I worked at the Photophysique et Photochimie Supramoléculaires et Macromoléculaires Laboratory (PPSM) at ENS Paris-Saclay, and I worked on Optimizing a Set-Up Coupling Electrochemical and Fluorescence Microscopies. My lab has a set-up that combines an electrochemical microscope with a fluorescence one so researchers can study a molecule’s fluorescent characteristics while simultaneously applying an electric field. However, recording data from this set-up was very inefficient, so my project focused on developing a more efficient data-acquisition system. The system we decided to use was Master-Slave Synchronization, which basically designates one machine as the “master” in order to control and monitor other devices known as the “slaves". In the set-up, we used a Potentiostat called VersaSTAT 4 as our master and an S2000 spectrometer as our slave. I started by figuring out how to externally trigger our spectrometer, and I found that by using specific pins on an auxiliary interface, we can connect V4 to the spectrometer and cause it to start acquiring data. We also placed the spectrometer in synchronization mode and adjusted acquisition parameters, such as integration time, capture period, and average number of scans, to see if any changes resulted in a timing delay. We found that the delay timing remained consistent regardless of these parameters; the only time delay would change is if we adjusted the frequency of the signal.
Despite doing this, we wanted to use LabVIEW to trigger the spectrometer instead, and there is a reason behind this. In Master-Slave Synchronization, we have three types of signals: clock signals, asynchronous signals, and synchronous signals. Clocks are periodic square signals that provide a sampling frequency, so they remain constant during acquisition time and provide a timing reference source for slave devices to follow. Asynchronous signals are not correlated in phase to these clock signals while synchronous signals are, so by sending a synchronous signal and make it followed by the clocks’ slaves, we can ensure that time intervals between acquired data and master will be consistent, and we can synchronize other data acquisition chains to the same signal as well.
Overall, we want a V4 trigger to trigger a LabVIEW program. This program will generate a clock frequency that can trigger and monitor spectrometer acquisition in an asynchronous fashion; that way, we can acquire spectra in a controlled manner, fire signals, and synchronize recordings of acquisition. With this being said however, there are numerous ways to do this, and I started to use LabVIEW to reconfigure different codes and test each code out to see which is the best set-up, but unfortunately, my time in the lab came to an end. In the future, my lab will be able to synchronize a CCD detector to the same signal, and even potentially the microscopes themselves. Either way, thank you to Jeff Audibert for helping and working alongside me throughout the summer!
Works Cited:
Guerret-Legras, L., Audibert, J., Ojeda, I.G., Dubacheva, G., Miomandre, F., 2019. Combined SECM-fluorescence microscopy using a water-soluble electrofluorochromic dye as the redox mediator. Electrochimica Acta.
Guerret, L., Audibert, J.-F., Dubacheva, G.V., MIOMANDRE, F., 2018. Combined Scanning Electrochemical and Fluorescence Microscopies using a tetrazine as a single redox and luminescent (electrofluorochromic) probe. Chemical Science.
National Instruments. “Synchronization Explained.” Synchronization Explained – National Instruments, 6 June 2017, www.ni.com/product-documentation/11369/en/.
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Phoebe Marcus-Porter: Fabrication of two dimensional waveguide structures containing a single quantum dot
Whiteboard Video:
This summer I worked at Le Laboratoire de photonique quantique et moléculaire under the guidance of professor Lai and master’s student Long on a project that focused on realizing two dimensional waveguide structures containing a single quantum dot.
Waveguide fabrication for single-photon sources has important applications in quantum computing which can expand the capabilities of computers for more complex problem-solving and advancements in scientific research, medicine, modeling quantum systems, data storage, and much more.
We created a structure which mimics a Mach-Zehnder interferometer, well known in classical optics. Mach-Zehnder interferometers classically work with multiple photons at once. In order to use this structure for quantum optics we used a single photon source, coupled at one end of the structure, which can be detected at the other end after being guided through one at a time.
In order to do this, we used low one photon absorption direct laser writing, a process by which we focus a 532 nm laser beam at low power on a specific location on a photosensitive material to polymerize it.
We created samples using glass substrates coated with an SU-8 photoresist layer and a layer of quantum dot solution. We placed the sample onto a piezoelectric translation stage which allowed us to move different parts of the sample into the focus of the beam according to a program we created on Labview which instructed the PZT to move in the shape of a straight line followed by a series of quarter ellipses and a final straight line. We used an objective lens to focus the beam onto the sample which can both excite the material and collect the emission signal from the sample in order to image it and view where the quantum dots are located. Once a quantum dot is located, we then determined whether or it was a single photon source, by passing its signal through a Hanbury Brown Twiss system, a beam splitter with detectors on either side, if it was a single photon source, a signal would only be detected from one side of a beam splitter at a time, and we were able to see this on an anti-bunching curve that was zero at the center. We then ran our Labview program to fabricate the 2-D structure.
After developing the structures, we scanned the field again to determine whether or not the quantum dot was coupled into the structure.
This summer, we successfully fabricated the Mach-Zehnder structure coupled with the single photon source. During the last week of this program, we have started working on adding a structure to the 2-D waveguide which directs the light toward the objective lens and allows us to elevate the structure so that it is no longer against the glass where the difference in refractive indices is not sufficient to detect a waveguide effect and so that the light will be bent down into the objective lens where it can successfully be detected.
Top
Ty Naquin: Optimizing production of reactive oxygen species with localized surface plasmon resonance
Whiteboard Video:
Light incident on a metal nanoparticle causes the conduction electrons to oscillate. Excitation of the nanoparticles at a resonant frequency results in the creation of a strong electromagnetic field. This phenomenon is known as localized surface plasmon resonance, or LSPR. Thanks to the properties of LSPR one can quickly and efficiently inject energy into the nanoparticle system using ultrashort pulses of light. The properties exhibited by the LSPR vary widely with the size, shape, and composition of the nanoparticle as well as the features of the surrounding medium. This phenomenon can be employed to attain many chemical and biological functions. It has been shown that that through the excitation of gold nanoparticles it is possible to generate reactive oxygen species such as the singlet oxygen. The first step of my project was to use pump-probe spectroscopy to detect the solvated (free) electrons in water. In order to increase our chances of detecting these electrons, I assembled an apparatus to sparge the dissolved oxygen inside a sample of water by flushing argon through the system. After detection of solvated electrons, our lab will excite gold nanorods of varying aspect ratios to optimize the production of reactive oxygen species. Our lab seeks to establish a link between the generation of reactive oxygen species and the presence of electrons in solution.
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Arianna Giguere: Chirp-Scan Measurement of Ultrashort Laser Pulses
Whiteboard Video:
This summer I worked on a project titled Chirp-Scan Measurement of Ultrashort Laser Pulses. The goal of my project was to connect a frequency-resolved optical gating device (or FROG) to one of Ecole Polytechnique’s high-intensity femtosecond lasers for characterization of its pulses. High intensity lasers are important for current applications within medicine and industry, for example, and these powerful lasers also have the potential to be a solution for eliminating harmful nuclear waste in the future. The ability to fully characterize pulses of such lasers is essential for determining their capabilities, and the FROG device conveniently allows for measurements of both the spectral frequency and phase of the pulses.
This summer I began by helping another intern to optimize and couple the beam into a hollow-core fiber which is meant for spectral broadening. Through a process called ‘self-phase modulation’, the beam going through the fiber is essentially “cleaned up”, which is useful for practical experiments. We wanted to use the FROG to look at the pulses coming out of the fiber. My lab already had a FROG device available from years ago, and using a small part of the high-intensity beam, I worked to resurrect the FROG and learn about its sensitivities and characteristics. In addition, when Louis Daniault connected his FROG retrieval software to the physical device, I worked to bridge the gap between the device and computer program. By the end of the program, this involved exporting data from a successful FROG trace retrieval, making plots, and applying Fourier transforms to manipulate the information in relevant ways.
Overall, much of my work this summer involved debugging errors. Without taking into consideration the issues we came across with the laser itself or problems we faced setting up the interface between the FROG retrieval code and the device itself, the FROG device had quite a few of its own issues experimentally. First of all, the device is so incredibly sensitive to any changes in alignment that sometimes I would leave work, come back the next day, and find the alignment was off. Besides this, some of the largest issues I came across included the following:
1. Understanding why I saw fringes at the focus spot of the SHG crystal (and why they were slanted in orientation)
2. The existence of uneven pulse replicas (for example, one bigger than the other) which was clearly detrimental to the FROG trace
3. Difficulty producing a measured trace as seen by the camera good enough to be processed
4. Calibrating the time axis
5. Performing the retrieval and finding that the algorithm could not identify the FROG trace below 1% (even though we knew the retrieval code was capable of doing this, so something was off experimentally).
My solutions to these problems were the following:
1. The fringes were due to the interference of the pulse replicas as expected - it meant the pulses were directly on top of one another. The slanting pulses had no real significance besides being an effect of the delay. As long as the fringes were equal in size across the majority of the pattern and they were clear, the pulses were overlapping like we wanted them to.
2. A consistent beam height all the way through the device was crucial. Minor adjustments were acceptable, but a steady beam height made a difference. In addition, the separation between the bi-mirrors needed nearly perfect symmetry about the center of the beam, otherwise the pulses would not be equal in size.
3. Aligning the FROG just right to produce a good trace before trying the retrieval takes the utmost care. I developed a short guide of tips that I used when aligning the FROG in order to help save future researchers some time, and I sent this to Rodrigo.
4. I discovered we needed a more precise tool, as in the Feinpruf Millitron to measure the mirror displacement in order to calibrate successfully.
5. I found that a time coefficient value of ~0.45 femtoseconds per pixel is necessary for the retrieval to identify the trace within 10E-3 error. However, this discovery was made accidentally since that particular calibration was performed with a poorly-focused trace. It works for well-focused traces and we are still unsure why. Additionally, it is important to note that when I refer to a “well-focused” trace, I mean an un-chirped elliptical trace focused the long way along the spectral domain (for a broad spectral domain - the temporal domain will subsequently be narrow).
Moving forward, the FROG will be used to characterize the pulses coming out of the hollow-core fiber, and later it may be used to look at the pulses after further compression.
I would like to thank my mentor, Rodrigo Lopez-Martens for all his guidance this summer, and Louis Daniault, without whom I could not have gotten started using the GPA. To Konstantin, Zhao, and Jean-Baptiste - thank you for answering my many questions. And to all those at the University of Michigan who gave me this opportunity - Dr. Steven Yalisove, Dr. John Nees, Bett Weston, etc., as well as the NSF, thank you for making this experience possible.
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Ahmad Abed: Confocal microscopy for the characterization of spontaneous emission by quantum light sources
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The study of atomic system in solids is of great importance due to their potential applications in quantum technology. The main focus of the setup that is developed is to study quantum light from defects in solids. Point defects in solids can behave as single-photon sources. During this internship, we assembled a small platform based on confocal microscopy to characterize light from emitters such as point defects in hexagonal boron nitride (hBN). We used a helium-neon (He-Ne) laser as our excitation source and a spectrometer (Andor Shamrock SR-750) to study the spectrum of the emitted light. The platform is based on three stages. The first stage involves the collimation of the He-Ne laser that will excite the emitter. The collimation of white light from a LED provides illumination for the inspection of the sample surface by a CMOS (uEye) camera. The second stage is dedicated to the excitation of the sample. The third stage collects the fluorescence light and directs it into the spectrometer. Furthermore, some intensity correlation measurements will take place by using a beamsplitter. I would like to thank Dr. Benjamin Vest and the Plasmonics and Quantum Nanophotonics team for their help throughout the summer, the Institut d’Optique Graduate School for welcoming and hosting me, the University of Michigan for organizing this amazing program, and the National Science Foundation for all the support.
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Hana Warner: Characterizing and manipulating cholosteric liquid crystal droplet microlasers
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My research this summer was conducted at the Laboratoire de Photonique Quantique et Moléculaire (LPQM) at ENS Paris-Saclay under the advisement of Dr. Mélanie Lebental. My work involved characterizing and manipulating cholosteric liquid crystal droplet microlasers. Liquid crystals have both highly ordered properties, like crystalline solids, and unordered properties, like isotropic liquids. They are often in a nematic phase, where the axis of the molecules are parallel, but a chiral dopant can be used to induce a pitch in the crystals, which is called the cholesteric phase. Liquid crystals can be formed into droplets when they are suspended in a viscous fluid.
These droplets are interesting because they can act as a spherical optical cavity, allowing us to make 3D microlasers. Light becomes trapped in the cavity and is reemitted as either Bragg Modes or whispering gallery modes. Bragg modes result from periodic changes in refractive index within the droplet due to the cholesteric pitch, allowing for repeated reflections back into the cavity. These modes are emitted from the droplet center. Whispering gallery modes result in light being trapped at the droplet edge and are emitted when it can complete a closed cycle. They are emitted at the radius. Lasing action can be accessed by doping the liquid crystals with a laser dye, like we do, or by coupling an optical waveguide to the cavity. The emitted wavelengths are related to the pitch of the cholesteric dopant in the droplet and the type of dye used in the sample, and the microlasers have applications in photonic circuits, sensing systems, and imaging.
My liquid crystal samples were provided by Dr. Brigitte Pansu at Université Paris-Sud. I began my project by determining the process to create textures within liquid crystal droplets and how these defects change the lasing spectrum. Liquid crystal textures describe topological defects within the cavity. They can be created during the manufacturing process and via stress, but we were interested in creating textures via laser excitation to give greater control in their formation. Changes to the droplet result in changes to Bragg and whispering gallery modes, and I was successful in creating textures at low energy in droplets both with and without laser dye. I determined that the presence of dye tended to lower the threshold to create defects in a droplet and that using different solvents did not change the threshold to create defects in most cases.
After characterizing texture formation in the droplets, I moved on to manipulating the droplets with a magnetic field. Our liquid crystals contain a benzene ring, so their magnetic moment rotates to align with the direction of the field. We saw a change in the center structure but no fluctuation of the radius, which corresponded with a clear shift in the Bragg mode while whispering gallery modes remained unchanged. This means that, by placing the droplets in a magnetic field, we can tune their lasing spectrum. This is both an exciting alternative to electric field tuning of liquid crystal droplet microlasers and has potential applications in photonic circuits and magnetic field sensing systems.
The last part of my project involved characterizing the polarization of our laser spectrum. This was accomplished by suspending a fiber, polarizer, and quarter wave plate above a capillary containing the droplets. I measured linear polarization in both the Bragg modes and whispering gallery modes emitted by the droplet; however, this is not exactly expected behavior, and I am curious if some changes in polarization could be induced by the edge of the capillary. In the future, I am interested to see if we can measure polarization while the droplets are suspended in a large cylindrical capillary, since we ran into some limitations with our capillary size that prevented us from studying polarization from cylindrical capillaries.
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Jahaira Santoyo: Characterization of a Phase Plate Overview
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The goal for this summer was to characterize a home-made phase plate. There are a couple of main parameters our research group was most interested in: thickness of the deposition that introduces the pi-dephasing, exact dephasing introduced, and the homogeneity of the phase phate. It is important to note that all of my work was experimental with a side of coding to make data presentable.
I spent the first week reading articles and books to familiarize myself with optics and the math behind it. Once I was familiar with the mathematical concepts and the behavior of light when passing through different optical devices, I was ready to start testing these concepts myself.
For example, one of the very first experiments I did was to propagate light through 2 quarter wave plates (QWP). Maha, the grad student I was working with, encouraged me to prove mathematically with sines and cosines what would happen to the light wave. We moved forwards to explaining what would happen to the light wave using Jones matrices.
Mathematically I had realized that incident vertically polarized light onto 2 QWP would result in horizontally polarized light. I then tested my hypothesis in the lab and saw that my prediction was correct. I did a lot of little experiments to familiarize myself with some of the optical devices.
Originally, the experiment I was suppose to work on consisted of 2 QWP and the phase plate. Light would propagate through one QWP, half the beam would hit the pi-dephasing section and half would hit the 0-dephasing section of the phase plate, then the light would go through another QWP. This process would result in the beam containing both horizontally and vertically polarized light, which combined create higher order modes in an optical fiber. This experiment would also allow me to test for the parameters we were interested in the phase plate.
My task was to prove the above process with Sines and Cosines and with Jones matrices. I couldn’t figure out why my math wasn’t working out, so after banging my head on the desk for a couple of days I decided to ask for help. Turns out I helped discover a mistake in our experiment. The pi-dephasing needed to be introduced between the x and y components the beam propagating on the pi-dephasing section in order to change the direction of polarization. Our experiment failed to do so, therefore we could no longer rely on this specific set up.
While we brainstormed how to move on, Maha showed me how to draw fibers into nanofibers. She taught me how to strip them and how to inject a laser light into the fiber in the most efficient way. This a REALLY fun part ! I got to see the coding on labView that allowed the machinery to draw the nanofibers.
To figure out the homogeneity of the phase plate I simply incident a laser beam into the two sections independently and made power measurements. I also used a polarizer to characterize the polarization of the input and output light. I took extensive notes and coded up my data to present to my advisor.
The other parameters would be determined by the Mach-Zehnder interferometer, but I quickly moved away from this because calibrating was VERY difficult and would take up too much of my time. I moved onto the Michelson interferometer. I spent a couple of days trying to calibrate and was successful. Some strange but very insightful interferences were discovered.
I have sadly run out of time, so I could not continue to work on this project. I learned so much more than expected and I feel really confident with working in optical labs back at my university. Overall, this summer I definitely increased my experimental skills.
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