First Week of the Program
Left to right: Eskil Irgens, Jebril Thaxton, Kaley Lollis, Sage Thomas,
Alexander Green, Sonia Mulgund, Zoe Tremitiere, Luis Martin
Dinner in Paris
Left to right: Luis Martin, Alexander Green, Jebril Thaxton, Kaley Lollis, Eskil Irgens, Sage Thomas, Sonia Mulgund, Zoe Tremitiere
2024 Projects
Luis G Martin: Developing a LIDAR Bench Test
Whiteboard Video:
This summer, I worked on creating a LiDAR bench test from scratch. LiDAR stands for Light Detection and Ranging. It is a system very similar to radar but typically employs infrared light rather than radio waves. A LiDAR system sends a pulse of light and detects its reflection to calculate the distance to a point. By repeating this process many times, we can map out a surface.
I began my experiment by trying to calculate the time of flight of one of these pulses. To do this, I connected a laser diode to a wave generator and a photodetector to an oscilloscope. I then attempted to measure the delay of the received signal. What initially seemed like an easy task quickly became challenging due to the precision required for this measurement. A slight error in calculation could result in a significant distance error; for instance, a 3-nanosecond miscalculation corresponds to about a meter of uncertainty.
The first limiting factor in my analysis was the equipment I was using. I had to upgrade my oscilloscope to a faster and more precise model. The next step was to find the most efficient waveform that would provide the best delay measurement. After experimenting with different parameters, I identified the optimal frequency and amplitude for this measurement. However, this was not enough, as the slow rising time of my signals remained an issue.
With the help of my PI, I designed a PCB that improved the rising time, transforming the outputs from both the wave generator and the photodetector into a sharper shape. After a few revisions of the circuit, we printed and assembled the PCB. Further testing showed an improvement in the rise time by about one order of magnitude going from 500-ns to 40-ns. The next step was to include a concave lens in front of the photodetector to increase the amount of light captured and to focus the light more consistently.
Despite these modifications, my best measurements were still off by up to a little more than 3 ns, translating to an error of about 20%. As we can see in this plot, some of the measurements were quite accurate, but the further away the signal traveled, the harder it became to have a reliable calculation.
Finally, I tried analyzing these signals using MATLAB. I connected the computer to the oscilloscope and captured the signal in MATLAB for further analysis. However, my time in the lab has come to an end. I hope all my documentation and notes are useful to the next person who works on this project. It was a pleasure working with everyone at LISV this summer. I would like to give a special thanks to my PI, Professor Luc Chassagne, for teaching me so much during my time here, and also to Bastien Bechadergue and Kevin Acuna for their guidance and mentorship.
Top
Eskil Ergens: Beam Shaping by a Polymeric Waveguide Resonant Grating
Whiteboard Video:
For my research project this summer, I worked with Professor Ngoc Diep LAI in the LUMIN laboratory at École normal supérieure Paris-Saclay. My goal for this summer was to develop a narrow collimated beam of light from a nano-emitter. For my project, I embedded a nanodiamond into a polymer-based circular waveguide resonant grating. The project is a continuation of the work of a previous PhD student of Professor LAI, Gia Long NGO, who has done this with quantum dots instead of nanodiamonds. The difficulty with quantum dots is that it is a single photon emitter, and so it is difficult to accurately measure how well the beam is collimated. Using nanodiamonds instead, we are able to better measure how well the light is collimated.
The first part of my project was focused on simulating grating patterns to understand which patterns give the most focused beam. For this, I used Ansys Lumerical FDTD to simulate models. This software uses numerical approximations to partial differential equations to build an accurate model for how light is affected by the waveguide. Using the simulation, we were able to test which grating patterns gives the most focused beam. While this work had been done for quantum dots, the light emitted from the nanodiamond has a somewhat different wavelength, and so it was necessary to repeat this part of the analysis.
The second part was learning to develop waveguides. Our lab uses a process called low-one photon absorption direct laser writing (LOPA-DLW) where we use a material that polymerizes when exposed to light. We use a 532nm laser, and because our material has a low absorption at this wavelength, the light is allowed to travel far into the material, and will only activate the polymerization process where the laser is focused. We can therefore carefully choose which areas we want to print allowing us to print details. I started by printing 2D structures with a singular layer of grating, before moving on to the more complicated 3D structures.
The next part of my project was focused on characterizing the emittance of nanodiamonds. We want to measure the exact amount of light emitted by any singular nanodiamond before embedding it in a waveguide, as this allows us to calibrate our measurements of how much light our waveguide focuses.
Finally, I worked on embedding the nanodiamonds into the waveguide. For this process, it was important to carefully place the waveguide above a particular nanodiamond far enough away from any other nanodiamond that these would not interfere and embed themselves in the waveguide, affecting our measurements. It was also important to make sure that the nanodiamond did not move around, as this would affect the placement in the waveguide and ruin our measurements.
After I leave, my group will finish up the project, measuring how well our beam has been focused using a high-sensitivity camera to compare the experimental results with the results found in the simulation.
My time in Paris has been an illuminating one, and I have learned much about optics as well as French student culture. It has given me a perspective on current optics research, and opened my eyes for work in physics outside the United States. I am grateful to Professor LAI and Duc Nam TRINH for guiding me through this research experience.
Top
Sonia Mulgund: Development of Broadband Nanosecond Fiber Laser Sources for Fusion Lasers
Whiteboard Video:
I spent this summer working in the laser R&D group at the Laboratoire pour L’Utilisation des Lasers Intenses (LULI), which is a plasma physics laboratory with some of the highest-power kilojoule class lasers in the world. My project focused on the simulation and characterization of broadband, incoherent nanosecond-pulsed fiber laser sources for fusion. Broadband light sources are interesting for high-power laser studies since the light couples better, with fewer instabilities, to plasma targets than continuous-wave (cw) sources, and the low coherence allows higher energy to be delivered without overwhelming optical systems. I have been looking specifically at how to temporally profile broadband pulses using a component known as a semiconductor optical amplifier (SOA), which provides a cheaper and easier method of temporal profiling than traditional methods like an electro-optic modulator. These diodes would provide the seeder sources in high-power laser experiments.
In the lab, I worked on setting up and characterizing superluminescent diode sources with the SOA. I characterized three different diodes at different central wavelengths and bandwidths. I set up the diodes and laser diode drivers, optimizing the temperature control loops for each diode, and then took measurements of the output power and spectral features for different driving currents. I also characterized the SOA pulse shaping capability and contrast with each diode, using a power meter, spectrum analyzer, and digital oscilloscope to make measurements. We demonstrated for the first time using an SOA to temporally shape broadband pulses, with contrast close to that for cw systems, opening the door for many further experiments with broadband high power lasers.
The other portion of my project was the numerical simulation in Python of broadband, incoherent pulses to investigate the effect of applying a temporal gate (such as with the SOA). The main goal was to understand the occurrence of ‘rogue waves,’ or very-high-intensity events due to the chance constructive interference of different wavelengths and phases that would be destructive to an optical system. Although these chances are low, for a high-power fusion laser system where there are potentially thousands of shots in a day, it is important to characterize the possibility. To do this, I used the complex exponential description for EM waves, defining a Gaussian spectral envelope with a broad bandwidth and a random distribution of the imaginary phase. Then, using Fourier transforms between the spectral and temporal domains, I analyzed statistics of the phase and intensity before and after gating. First, I investigated the changing probability of a rogue wave as the number of points in the spectrum increased, showing that the probability becomes exceedingly low with increasing number of points. However, the computation is very slow due to the high number of repeat simulations needed to achieve statistical information for wider spectra. Other methods I used include visualizing the distribution of the root-mean-square spectral and temporal phase and Strehl ratio value (a measure of image quality describing how in-phase the waves are), demonstrating that applying a temporal gate widens the spectral phase rms distribution while narrowing temporal phase distribution to a single distribution from a bimodal one. I also showed that the probability density function of intensity for the ungated light follows exponential decay, while gating results in a decreased ‘stretching exponent’ corresponding to a long-tailed distribution in intensity (more likely occurrence of a high-intensity event).
This summer was a great learning experience to see what it looks like to do laser research at a lab that doesn’t have the traditional American academic structure. Thank you to Loic Meignien and Pierre Lebegue for their help and mentorship. I am also very thankful for the opportunity and funding from the University of Michigan and the National Science Foundation.
Top
Alexander Green: Fabrication of Magnetic Nanostructures Using Direct Laser Writing
Whiteboard Video:
This summer, I worked under Professor Ngoc Diep Lai in the LUMIN laboratory at ENS Paris-Saclay to demonstrate the use of the direct laser writing technique for fabricating magnetic nanostructures.
Direct Laser Writing (DLW) is a photolithography technique that excels in fabricating precise structures at the sub microscale. My project used a “one-photon” absorption setup, in which the wavelength of the laser corresponds to the material’s absorption spectrum. A 532 nanometer laser which was focused onto the thin film material. At the focus spot, the laser light reaches a very high intensity, which generates a strong heating effect in the absorbing material. This essentially “burns” away the material at the focus spot. The sample is mounted on a “PZT”, which moves the sample according to the shape of the intended structure. With this setup, the laser is able to carve complex structures into material.
The use of DLW to fabricate on plasmonic, or metallic, has been well-studied by this group, specifically fabrication on gold film. In order to practice the DLW technique, I began my research fabricating various prints on gold film. When fabricating on gold film, heat at the focus spot causes gold material to melt into nano-islands on the substrate. Different sizes of gold nanoparticles can be observed as different “colors”, which gives rise to printmaking applications.
In contrast to plasmonic material, DLW fabrication on magnetic material is not yet well-studied. Magnetic materials have strong magnetic properties, and can be easily magnetized by an external magnetic field. I demonstrated consistent fabrication on a Tantalum/Platinum/Cobalt magnetic thin film at 40 - 60 mW of laser power. I fabricated tracks of varying distances to investigate how magnetic domain wall propagation would be effected between the tracks. Magnetic domains are regions on the magnetic material in which the magnetic moments of the atoms are grouped together and aligned in the same direction. When exposed to a magnetic field, domains can spontaneously form and propagate across the surface of the magnetic thin film.
In order to observe these domains, I used the Magnetic-Optic-Kerr-Effect, or “MOKE” technique, which characterizes the change in polarized light reflected off magnetic material. When subjected to pulses of magnetic field at 30 milliteslas for 6 microseconds, I found that the domains moved between the fabricated tracks at a slower rate, proportional to their width, compared to outside the structure. At ~33 milliteslas, the velocity of the domain wall was approximately 10 meters per second outside the structure. Meanwhile, the velocity between two tracks 10 microns apart was approximately 2 meters per second. Between tracks 5 microns apart the velocity was approximately 1 meter per second. This result is expected due to the interfacial forces between fabricated tracks. Similar fabricated structures could have potential applications for computing and information storage, in which the direction of the magnetic moments in a structure could signify bits 0 or 1.
I have had an engaging and fun summer here in Paris, and I would like to thank Professor Ngoc Diep Lai and Quang Truong for their mentorship, guidance, and support this summer. Thank you to the Optics in the City of Light program for this amazing opportunity!
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Sage Thomas: Characterizing the coupling strength between a gold nanoparticle and an optical cavity
Whiteboard Video:
The phenomenon of localized surface plasmon resonance has many uses, including testing for the presence of a certain molecule in some fluid, perhaps something that signifies some disease. If you attach receptor molecules to a gold nanoparticle and perform an experiment to determine the plasmon resonance frequency, the frequency of the peak will shift depending on whether the molecule attached to the particle or not, thus detecting the presence of the molecule. However, this peak is somewhat broad, making it hard to determine the exact shift.
When a gold nanoparticle with a plasmon resonance frequency corresponding to one of the cavity's resonance peaks is put in the cavity, the single peak splits in a phenomenon known as rabi splitting. How far apart these peaks split is due to the coupling strength between the particles and the cavity. Here, for a Bragg cavity, we see the transmission spectra that look like this. And, when the particle is introduced, it should split.
The strength of the coupling can be characterized by changing the width of the cavity slightly and recording how the positions of the two peaks change as you do so. Plotting these creates an anti-crossing curve, where the horizontal asymptote is the plasmon resonance frequency of the particle, and the diagonal asymptote is the position of where the cavity resonance peak would be if there was no particle. The coupling strength is then determined by the equation omega = 2g where g is the coupling strength and omega is the distance between the two peaks at the point where the lines ''should'' cross.
Using Lumerical Finite Difference Time Domain (FDTD) software, I modeled Fabry Perot cavities with gold, silver, and aluminum mirrors, as well as Bragg mirrors to see if there would be any difference between the coupling strengths. We were expecting to find that the coupling strengths were all different. However, we found that the coupling strengths for mirrors of all the metals were all very similar, and there were only slight differences in the shape of the graphs. We also saw that the anti crossing curve does not follow the diagonal asymptote exactly.
This can possibly be explained by the fact that the presence of the gold particle creates a different effective refractive index for the material inside the cavity, meaning the simulation run to find the original peak of the bare cavity does not give the correct location. This can be seen through effective medium approximation methods such as the Yamaguchi method or the Maxwell-Garnett model. With this, we do see that the line is below the original, so it indicates that this may be partially the cause of it crossing the diagonal asymptote. However, for the Bragg cavity we saw no splitting at all, and the exact cause of that is not certain yet, as it was the opposite of what was predicted.
Finally, I want to thank Professor Palpant for his guidance this summer, as well as Benny, Baptiste, Camille, Elsa, and Robin for making me so welcome here.
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Kaley Lollis: Single Fluorescent Sensors for 3D Physico-Chemical Mappings
Whiteboard Video:
The goal of my project this summer was to create Single Fluorescent sensors for 3D physico-chemical nanoMappings. I worked with Jeff Audiber, Baptiste Maillot, and Vitor Brasiliense in the PPSM lab at École normale supérieure Paris-Saclay. I started by developing the sensor. Making the sensor consisted of three essential parts. The first part was to create a glass pipette with the optimal tip parameters for the sensor. I used the pipette puller Sutter P-2000. This machine used a laser to heat the glass tube (borosilicate tube OD 0.9 mm, ID 0.5 mm ) at a local point and pull the tube apart. By adjusting the parameters of heat, filament, velocity, delay, and pull, I was able to obtain reproducible tips with an average diameter of two to three micrometers. The next step was photo-grafting the end of the tip with diazonium. A solution of diazonium was made with NaNO2 and acidic water with a pH of 2. This created an N2+ molecule in the solution that was then excited by a laser at 470 nm. Once the solution was excited by this laser, the molecule was grafted onto the end of the tip.
In order to check that grafting was successful. I had to ensure the presence of fluorescein. I did this in three significant ways. I recorded the grafting process and looked for light on the tip of the pipette when the laser was shown. After grafting, using wide-field illumination, an image of the fluorescein was captured. After this, spectra was recorded comparing the intensity of fluorescence versus the wavelength in nm. Once the photo-grafting step was completed, I bound the fluorophore with click chemistry.
During click chemistry, a stable solution was created. I started with fluorescein - NH2, NaNO2 and T3OH were added to this. This resulted in fluorescein - N2+ in order to create a stable solution N3- must be obtained so that the triazole bond can be made, thus making the solution stable. To go from N2+ to N3- NaN3 was added to the solution. This reaction releases N2 (gas). This results in fluorescein - N3-. The tips were placed in a bath of this solution with constant O2 for at least 18 hours and a maximum of 72 hours. After sitting in this bath, the tips were removed and rechecked using spectrasuite for the intensity of the fluorescence.
After click chemistry, the tips were fully functionalized. The next step was to create a calibration curve. The calibration curve allows for a reference for later on when the tip is scanning the electrode. The calibration curve plots the intensity of fluorescence versus pH. This curve shows that the more basic a solution is, the less intense the fluorescence. Once complete, the tip can be used to scan within a 10 by 10 µm electrode sitting on ITO insulated with SU8. Within this electrode sits a pH gradient that is more basic than outside the electrode. After scanning this electrode, a curve is produced. I will use the original calibration curve to identify the pH values found within the electrode.
Throughout my internship, I additionally learned about replacing optic fibers and the stability of laser fibers. I continued to further my knowledge in MATLAB by using various programs throughout the experiment. Being in Paris was a transformative summer, and I enjoyed the wealth of knowledge from my advisors at ENS while also learning about French culture and cuisine.
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Zoe Tremitiere: Characterizing Sources of Noise in Naofibers
Whiteboard Video:
This summer I worked in the nonlinear optics group at the Institute of Optics Graduate School characterizing sources of noise in optical fibers.
An optical fiber is a cylindrical waveguide, capable of transmitting light over long distances. The Nonlinear Optics group at IOGS is especially interested in using optical fibers to generate entangled photon pairs. Entangled photons, which have wide ranging applications, including in telecommunication and quantum information, can be generated through a process called spontaneous four wave mixing, where two photons from the pump field traveling in a nonlinear medium, such as an optical fiber, annihilate to create a pair of entangled photons emitted at different wavelengths.
The characteristics of the fiber have an effect on the pairs of photons generated. For instance, the diameter of the fiber impacts where photons are emitted along the wavelength spectrum
Length also plays a role. Increasing the length of the fiber can increase the intensity of the nonlinear effect. By pulling our fibers over a flame to create nanofibers with diameters up to 100 times smaller, we can increase the likelihood of generating entangled pairs, but we also increase the amount of noise in our measurements.
In our experiment, we define “noise” as single photons that make it harder to detect entangled pairs. We identify pairs by checking their arrival time at the detector. Both photons in a pair should reach the detector at the same time, so when two photons are detected at the same instant, this indicates that they belong to a pair. In the ideal case, all photons generated belong to a pair. However, in reality many single “noise” photons are also generated due to other phenomena.
In order to limit noise, we want to understand its source. We do this by using a continuous pump laser at a fixed wavelength to generate unpaired “noise” photons. The pump signal is much higher than the noise it generates, emitting at a rate of about 1017 photons per second compared to 103 photons per second for the noise. Once we filter out the pump signal, we’re able to pay attention to the noise emission spectrum.
Noise photons are not emitted at a constant rate for all wavelengths. We see peaks in our noise spectrum due to two phenomena: Raman scattering and fluorescence. The Raman spectrum is particularly useful because it serves as a structural material for our fiber medium. Raman peaks are generated when photons scatter off of molecules. As a photon hits a molecule, it causes it to vibrate. The photon loses energy in this process, or shifts down, and the Raman spectrum shows us the shifts in energy resulting from molecular vibrations, which helps us recognize molecules present in our fibers and nanofibers.
Fluorescence describes the property of molecules and atoms to absorb photons at a particular wavelength and re-emit them at a higher wavelength. While it helps us understand at what wavelengths light is absorbed for a given emission wavelength, because the fluorescence spectrum shifts for different excitation wavelengths of the laser, it is harder to identify sources of these emissions. Our experiments thus serve a double purpose of discovering the composition of fibers, and pinpointing whether particular peaks in the spectrum are due to Raman scattering or fluorescence.
Over the summer, we conducted measurements of the spectra of fibers using a few different filter systems that allowed us to count photon emissions for a range of wavelengths. We also built a new diffraction grating based filter system to extend the wavelength range of our spectra because the commercial filters we’d been using didn’t allow us to see as far as we wanted.
I want to thank IOGS for hosting me, École Polytechnique and the University of Michigan for organizing this program, NSF, and Philipe Delaye for his mentorship and support on this project.
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Jebril Thaxton: Developing a closed feedback loop to minimize errors in laser temporal profiles
Whiteboard Video:
Hello my name is Jebril Thaxton from the University of Michigan. I have spent time this summer at Ecole Polytechnique in the LULI lab one of the largest kilo joule lasers in the world. The goal this summer was to develop a closed feedback loop to minimize errors in laser temporal profiles.
This loop contains an Arbitrary waveform generator, a Modulation box for taking electrical signals into optical signals. The use of laser diodes to represent the waveform into an oscilloscope to display the data. With all these devices connected to each other we create noise and some errors in the final output. The goal of the feedback loop is to combat these errors by changing the original waveform to compensate for the errors being created. For example, if the input signal is square wave and the output to the signal is a negative ramp, we would input a positive ramp to compensate and receive a square wave.
This consecutive loop is programmed using python and SCPI commands to their respective devices. Python is used for the automation process, this allows for data like waveform points and screenshots from the oscilloscope to be printed while the loop continues without any interaction from the user. The oscilloscope and the AWG are connected through ethernet and serial communication to the PC while the mod box and laser use fiber cables connected to each device.
The loop starts with the python program sending a command for a specific waveform to the AWG. This waveform is modulated to an optical signal and a laser that is connected to the oscilloscope and displays the error signal. This error signal is measured and adjusted and compensated as a new signal to minimize the difference from the target signal.
The Feedback loop allows for a constant output even if there are slight changes in the parameters. After some number of iterations of the loop depending on the amount of error the output waveform becomes closer and eventually becomes an exact representation of the needed output waveform.
Top
Whiteboard Video:
This summer, I worked on creating a LiDAR bench test from scratch. LiDAR stands for Light Detection and Ranging. It is a system very similar to radar but typically employs infrared light rather than radio waves. A LiDAR system sends a pulse of light and detects its reflection to calculate the distance to a point. By repeating this process many times, we can map out a surface.
I began my experiment by trying to calculate the time of flight of one of these pulses. To do this, I connected a laser diode to a wave generator and a photodetector to an oscilloscope. I then attempted to measure the delay of the received signal. What initially seemed like an easy task quickly became challenging due to the precision required for this measurement. A slight error in calculation could result in a significant distance error; for instance, a 3-nanosecond miscalculation corresponds to about a meter of uncertainty.
The first limiting factor in my analysis was the equipment I was using. I had to upgrade my oscilloscope to a faster and more precise model. The next step was to find the most efficient waveform that would provide the best delay measurement. After experimenting with different parameters, I identified the optimal frequency and amplitude for this measurement. However, this was not enough, as the slow rising time of my signals remained an issue.
With the help of my PI, I designed a PCB that improved the rising time, transforming the outputs from both the wave generator and the photodetector into a sharper shape. After a few revisions of the circuit, we printed and assembled the PCB. Further testing showed an improvement in the rise time by about one order of magnitude going from 500-ns to 40-ns. The next step was to include a concave lens in front of the photodetector to increase the amount of light captured and to focus the light more consistently.
Despite these modifications, my best measurements were still off by up to a little more than 3 ns, translating to an error of about 20%. As we can see in this plot, some of the measurements were quite accurate, but the further away the signal traveled, the harder it became to have a reliable calculation.
Finally, I tried analyzing these signals using MATLAB. I connected the computer to the oscilloscope and captured the signal in MATLAB for further analysis. However, my time in the lab has come to an end. I hope all my documentation and notes are useful to the next person who works on this project. It was a pleasure working with everyone at LISV this summer. I would like to give a special thanks to my PI, Professor Luc Chassagne, for teaching me so much during my time here, and also to Bastien Bechadergue and Kevin Acuna for their guidance and mentorship.
Top
Eskil Ergens: Beam Shaping by a Polymeric Waveguide Resonant Grating
Whiteboard Video:
For my research project this summer, I worked with Professor Ngoc Diep LAI in the LUMIN laboratory at École normal supérieure Paris-Saclay. My goal for this summer was to develop a narrow collimated beam of light from a nano-emitter. For my project, I embedded a nanodiamond into a polymer-based circular waveguide resonant grating. The project is a continuation of the work of a previous PhD student of Professor LAI, Gia Long NGO, who has done this with quantum dots instead of nanodiamonds. The difficulty with quantum dots is that it is a single photon emitter, and so it is difficult to accurately measure how well the beam is collimated. Using nanodiamonds instead, we are able to better measure how well the light is collimated.
The first part of my project was focused on simulating grating patterns to understand which patterns give the most focused beam. For this, I used Ansys Lumerical FDTD to simulate models. This software uses numerical approximations to partial differential equations to build an accurate model for how light is affected by the waveguide. Using the simulation, we were able to test which grating patterns gives the most focused beam. While this work had been done for quantum dots, the light emitted from the nanodiamond has a somewhat different wavelength, and so it was necessary to repeat this part of the analysis.
The second part was learning to develop waveguides. Our lab uses a process called low-one photon absorption direct laser writing (LOPA-DLW) where we use a material that polymerizes when exposed to light. We use a 532nm laser, and because our material has a low absorption at this wavelength, the light is allowed to travel far into the material, and will only activate the polymerization process where the laser is focused. We can therefore carefully choose which areas we want to print allowing us to print details. I started by printing 2D structures with a singular layer of grating, before moving on to the more complicated 3D structures.
The next part of my project was focused on characterizing the emittance of nanodiamonds. We want to measure the exact amount of light emitted by any singular nanodiamond before embedding it in a waveguide, as this allows us to calibrate our measurements of how much light our waveguide focuses.
Finally, I worked on embedding the nanodiamonds into the waveguide. For this process, it was important to carefully place the waveguide above a particular nanodiamond far enough away from any other nanodiamond that these would not interfere and embed themselves in the waveguide, affecting our measurements. It was also important to make sure that the nanodiamond did not move around, as this would affect the placement in the waveguide and ruin our measurements.
After I leave, my group will finish up the project, measuring how well our beam has been focused using a high-sensitivity camera to compare the experimental results with the results found in the simulation.
My time in Paris has been an illuminating one, and I have learned much about optics as well as French student culture. It has given me a perspective on current optics research, and opened my eyes for work in physics outside the United States. I am grateful to Professor LAI and Duc Nam TRINH for guiding me through this research experience.
Top
Sonia Mulgund: Development of Broadband Nanosecond Fiber Laser Sources for Fusion Lasers
Whiteboard Video:
I spent this summer working in the laser R&D group at the Laboratoire pour L’Utilisation des Lasers Intenses (LULI), which is a plasma physics laboratory with some of the highest-power kilojoule class lasers in the world. My project focused on the simulation and characterization of broadband, incoherent nanosecond-pulsed fiber laser sources for fusion. Broadband light sources are interesting for high-power laser studies since the light couples better, with fewer instabilities, to plasma targets than continuous-wave (cw) sources, and the low coherence allows higher energy to be delivered without overwhelming optical systems. I have been looking specifically at how to temporally profile broadband pulses using a component known as a semiconductor optical amplifier (SOA), which provides a cheaper and easier method of temporal profiling than traditional methods like an electro-optic modulator. These diodes would provide the seeder sources in high-power laser experiments.
In the lab, I worked on setting up and characterizing superluminescent diode sources with the SOA. I characterized three different diodes at different central wavelengths and bandwidths. I set up the diodes and laser diode drivers, optimizing the temperature control loops for each diode, and then took measurements of the output power and spectral features for different driving currents. I also characterized the SOA pulse shaping capability and contrast with each diode, using a power meter, spectrum analyzer, and digital oscilloscope to make measurements. We demonstrated for the first time using an SOA to temporally shape broadband pulses, with contrast close to that for cw systems, opening the door for many further experiments with broadband high power lasers.
The other portion of my project was the numerical simulation in Python of broadband, incoherent pulses to investigate the effect of applying a temporal gate (such as with the SOA). The main goal was to understand the occurrence of ‘rogue waves,’ or very-high-intensity events due to the chance constructive interference of different wavelengths and phases that would be destructive to an optical system. Although these chances are low, for a high-power fusion laser system where there are potentially thousands of shots in a day, it is important to characterize the possibility. To do this, I used the complex exponential description for EM waves, defining a Gaussian spectral envelope with a broad bandwidth and a random distribution of the imaginary phase. Then, using Fourier transforms between the spectral and temporal domains, I analyzed statistics of the phase and intensity before and after gating. First, I investigated the changing probability of a rogue wave as the number of points in the spectrum increased, showing that the probability becomes exceedingly low with increasing number of points. However, the computation is very slow due to the high number of repeat simulations needed to achieve statistical information for wider spectra. Other methods I used include visualizing the distribution of the root-mean-square spectral and temporal phase and Strehl ratio value (a measure of image quality describing how in-phase the waves are), demonstrating that applying a temporal gate widens the spectral phase rms distribution while narrowing temporal phase distribution to a single distribution from a bimodal one. I also showed that the probability density function of intensity for the ungated light follows exponential decay, while gating results in a decreased ‘stretching exponent’ corresponding to a long-tailed distribution in intensity (more likely occurrence of a high-intensity event).
This summer was a great learning experience to see what it looks like to do laser research at a lab that doesn’t have the traditional American academic structure. Thank you to Loic Meignien and Pierre Lebegue for their help and mentorship. I am also very thankful for the opportunity and funding from the University of Michigan and the National Science Foundation.
Top
Alexander Green: Fabrication of Magnetic Nanostructures Using Direct Laser Writing
Whiteboard Video:
This summer, I worked under Professor Ngoc Diep Lai in the LUMIN laboratory at ENS Paris-Saclay to demonstrate the use of the direct laser writing technique for fabricating magnetic nanostructures.
Direct Laser Writing (DLW) is a photolithography technique that excels in fabricating precise structures at the sub microscale. My project used a “one-photon” absorption setup, in which the wavelength of the laser corresponds to the material’s absorption spectrum. A 532 nanometer laser which was focused onto the thin film material. At the focus spot, the laser light reaches a very high intensity, which generates a strong heating effect in the absorbing material. This essentially “burns” away the material at the focus spot. The sample is mounted on a “PZT”, which moves the sample according to the shape of the intended structure. With this setup, the laser is able to carve complex structures into material.
The use of DLW to fabricate on plasmonic, or metallic, has been well-studied by this group, specifically fabrication on gold film. In order to practice the DLW technique, I began my research fabricating various prints on gold film. When fabricating on gold film, heat at the focus spot causes gold material to melt into nano-islands on the substrate. Different sizes of gold nanoparticles can be observed as different “colors”, which gives rise to printmaking applications.
In contrast to plasmonic material, DLW fabrication on magnetic material is not yet well-studied. Magnetic materials have strong magnetic properties, and can be easily magnetized by an external magnetic field. I demonstrated consistent fabrication on a Tantalum/Platinum/Cobalt magnetic thin film at 40 - 60 mW of laser power. I fabricated tracks of varying distances to investigate how magnetic domain wall propagation would be effected between the tracks. Magnetic domains are regions on the magnetic material in which the magnetic moments of the atoms are grouped together and aligned in the same direction. When exposed to a magnetic field, domains can spontaneously form and propagate across the surface of the magnetic thin film.
In order to observe these domains, I used the Magnetic-Optic-Kerr-Effect, or “MOKE” technique, which characterizes the change in polarized light reflected off magnetic material. When subjected to pulses of magnetic field at 30 milliteslas for 6 microseconds, I found that the domains moved between the fabricated tracks at a slower rate, proportional to their width, compared to outside the structure. At ~33 milliteslas, the velocity of the domain wall was approximately 10 meters per second outside the structure. Meanwhile, the velocity between two tracks 10 microns apart was approximately 2 meters per second. Between tracks 5 microns apart the velocity was approximately 1 meter per second. This result is expected due to the interfacial forces between fabricated tracks. Similar fabricated structures could have potential applications for computing and information storage, in which the direction of the magnetic moments in a structure could signify bits 0 or 1.
I have had an engaging and fun summer here in Paris, and I would like to thank Professor Ngoc Diep Lai and Quang Truong for their mentorship, guidance, and support this summer. Thank you to the Optics in the City of Light program for this amazing opportunity!
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Sage Thomas: Characterizing the coupling strength between a gold nanoparticle and an optical cavity
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The phenomenon of localized surface plasmon resonance has many uses, including testing for the presence of a certain molecule in some fluid, perhaps something that signifies some disease. If you attach receptor molecules to a gold nanoparticle and perform an experiment to determine the plasmon resonance frequency, the frequency of the peak will shift depending on whether the molecule attached to the particle or not, thus detecting the presence of the molecule. However, this peak is somewhat broad, making it hard to determine the exact shift.
When a gold nanoparticle with a plasmon resonance frequency corresponding to one of the cavity's resonance peaks is put in the cavity, the single peak splits in a phenomenon known as rabi splitting. How far apart these peaks split is due to the coupling strength between the particles and the cavity. Here, for a Bragg cavity, we see the transmission spectra that look like this. And, when the particle is introduced, it should split.
The strength of the coupling can be characterized by changing the width of the cavity slightly and recording how the positions of the two peaks change as you do so. Plotting these creates an anti-crossing curve, where the horizontal asymptote is the plasmon resonance frequency of the particle, and the diagonal asymptote is the position of where the cavity resonance peak would be if there was no particle. The coupling strength is then determined by the equation omega = 2g where g is the coupling strength and omega is the distance between the two peaks at the point where the lines ''should'' cross.
Using Lumerical Finite Difference Time Domain (FDTD) software, I modeled Fabry Perot cavities with gold, silver, and aluminum mirrors, as well as Bragg mirrors to see if there would be any difference between the coupling strengths. We were expecting to find that the coupling strengths were all different. However, we found that the coupling strengths for mirrors of all the metals were all very similar, and there were only slight differences in the shape of the graphs. We also saw that the anti crossing curve does not follow the diagonal asymptote exactly.
This can possibly be explained by the fact that the presence of the gold particle creates a different effective refractive index for the material inside the cavity, meaning the simulation run to find the original peak of the bare cavity does not give the correct location. This can be seen through effective medium approximation methods such as the Yamaguchi method or the Maxwell-Garnett model. With this, we do see that the line is below the original, so it indicates that this may be partially the cause of it crossing the diagonal asymptote. However, for the Bragg cavity we saw no splitting at all, and the exact cause of that is not certain yet, as it was the opposite of what was predicted.
Finally, I want to thank Professor Palpant for his guidance this summer, as well as Benny, Baptiste, Camille, Elsa, and Robin for making me so welcome here.
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Kaley Lollis: Single Fluorescent Sensors for 3D Physico-Chemical Mappings
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The goal of my project this summer was to create Single Fluorescent sensors for 3D physico-chemical nanoMappings. I worked with Jeff Audiber, Baptiste Maillot, and Vitor Brasiliense in the PPSM lab at École normale supérieure Paris-Saclay. I started by developing the sensor. Making the sensor consisted of three essential parts. The first part was to create a glass pipette with the optimal tip parameters for the sensor. I used the pipette puller Sutter P-2000. This machine used a laser to heat the glass tube (borosilicate tube OD 0.9 mm, ID 0.5 mm ) at a local point and pull the tube apart. By adjusting the parameters of heat, filament, velocity, delay, and pull, I was able to obtain reproducible tips with an average diameter of two to three micrometers. The next step was photo-grafting the end of the tip with diazonium. A solution of diazonium was made with NaNO2 and acidic water with a pH of 2. This created an N2+ molecule in the solution that was then excited by a laser at 470 nm. Once the solution was excited by this laser, the molecule was grafted onto the end of the tip.
In order to check that grafting was successful. I had to ensure the presence of fluorescein. I did this in three significant ways. I recorded the grafting process and looked for light on the tip of the pipette when the laser was shown. After grafting, using wide-field illumination, an image of the fluorescein was captured. After this, spectra was recorded comparing the intensity of fluorescence versus the wavelength in nm. Once the photo-grafting step was completed, I bound the fluorophore with click chemistry.
During click chemistry, a stable solution was created. I started with fluorescein - NH2, NaNO2 and T3OH were added to this. This resulted in fluorescein - N2+ in order to create a stable solution N3- must be obtained so that the triazole bond can be made, thus making the solution stable. To go from N2+ to N3- NaN3 was added to the solution. This reaction releases N2 (gas). This results in fluorescein - N3-. The tips were placed in a bath of this solution with constant O2 for at least 18 hours and a maximum of 72 hours. After sitting in this bath, the tips were removed and rechecked using spectrasuite for the intensity of the fluorescence.
After click chemistry, the tips were fully functionalized. The next step was to create a calibration curve. The calibration curve allows for a reference for later on when the tip is scanning the electrode. The calibration curve plots the intensity of fluorescence versus pH. This curve shows that the more basic a solution is, the less intense the fluorescence. Once complete, the tip can be used to scan within a 10 by 10 µm electrode sitting on ITO insulated with SU8. Within this electrode sits a pH gradient that is more basic than outside the electrode. After scanning this electrode, a curve is produced. I will use the original calibration curve to identify the pH values found within the electrode.
Throughout my internship, I additionally learned about replacing optic fibers and the stability of laser fibers. I continued to further my knowledge in MATLAB by using various programs throughout the experiment. Being in Paris was a transformative summer, and I enjoyed the wealth of knowledge from my advisors at ENS while also learning about French culture and cuisine.
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Zoe Tremitiere: Characterizing Sources of Noise in Naofibers
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This summer I worked in the nonlinear optics group at the Institute of Optics Graduate School characterizing sources of noise in optical fibers.
An optical fiber is a cylindrical waveguide, capable of transmitting light over long distances. The Nonlinear Optics group at IOGS is especially interested in using optical fibers to generate entangled photon pairs. Entangled photons, which have wide ranging applications, including in telecommunication and quantum information, can be generated through a process called spontaneous four wave mixing, where two photons from the pump field traveling in a nonlinear medium, such as an optical fiber, annihilate to create a pair of entangled photons emitted at different wavelengths.
The characteristics of the fiber have an effect on the pairs of photons generated. For instance, the diameter of the fiber impacts where photons are emitted along the wavelength spectrum
Length also plays a role. Increasing the length of the fiber can increase the intensity of the nonlinear effect. By pulling our fibers over a flame to create nanofibers with diameters up to 100 times smaller, we can increase the likelihood of generating entangled pairs, but we also increase the amount of noise in our measurements.
In our experiment, we define “noise” as single photons that make it harder to detect entangled pairs. We identify pairs by checking their arrival time at the detector. Both photons in a pair should reach the detector at the same time, so when two photons are detected at the same instant, this indicates that they belong to a pair. In the ideal case, all photons generated belong to a pair. However, in reality many single “noise” photons are also generated due to other phenomena.
In order to limit noise, we want to understand its source. We do this by using a continuous pump laser at a fixed wavelength to generate unpaired “noise” photons. The pump signal is much higher than the noise it generates, emitting at a rate of about 1017 photons per second compared to 103 photons per second for the noise. Once we filter out the pump signal, we’re able to pay attention to the noise emission spectrum.
Noise photons are not emitted at a constant rate for all wavelengths. We see peaks in our noise spectrum due to two phenomena: Raman scattering and fluorescence. The Raman spectrum is particularly useful because it serves as a structural material for our fiber medium. Raman peaks are generated when photons scatter off of molecules. As a photon hits a molecule, it causes it to vibrate. The photon loses energy in this process, or shifts down, and the Raman spectrum shows us the shifts in energy resulting from molecular vibrations, which helps us recognize molecules present in our fibers and nanofibers.
Fluorescence describes the property of molecules and atoms to absorb photons at a particular wavelength and re-emit them at a higher wavelength. While it helps us understand at what wavelengths light is absorbed for a given emission wavelength, because the fluorescence spectrum shifts for different excitation wavelengths of the laser, it is harder to identify sources of these emissions. Our experiments thus serve a double purpose of discovering the composition of fibers, and pinpointing whether particular peaks in the spectrum are due to Raman scattering or fluorescence.
Over the summer, we conducted measurements of the spectra of fibers using a few different filter systems that allowed us to count photon emissions for a range of wavelengths. We also built a new diffraction grating based filter system to extend the wavelength range of our spectra because the commercial filters we’d been using didn’t allow us to see as far as we wanted.
I want to thank IOGS for hosting me, École Polytechnique and the University of Michigan for organizing this program, NSF, and Philipe Delaye for his mentorship and support on this project.
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Jebril Thaxton: Developing a closed feedback loop to minimize errors in laser temporal profiles
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Hello my name is Jebril Thaxton from the University of Michigan. I have spent time this summer at Ecole Polytechnique in the LULI lab one of the largest kilo joule lasers in the world. The goal this summer was to develop a closed feedback loop to minimize errors in laser temporal profiles.
This loop contains an Arbitrary waveform generator, a Modulation box for taking electrical signals into optical signals. The use of laser diodes to represent the waveform into an oscilloscope to display the data. With all these devices connected to each other we create noise and some errors in the final output. The goal of the feedback loop is to combat these errors by changing the original waveform to compensate for the errors being created. For example, if the input signal is square wave and the output to the signal is a negative ramp, we would input a positive ramp to compensate and receive a square wave.
This consecutive loop is programmed using python and SCPI commands to their respective devices. Python is used for the automation process, this allows for data like waveform points and screenshots from the oscilloscope to be printed while the loop continues without any interaction from the user. The oscilloscope and the AWG are connected through ethernet and serial communication to the PC while the mod box and laser use fiber cables connected to each device.
The loop starts with the python program sending a command for a specific waveform to the AWG. This waveform is modulated to an optical signal and a laser that is connected to the oscilloscope and displays the error signal. This error signal is measured and adjusted and compensated as a new signal to minimize the difference from the target signal.
The Feedback loop allows for a constant output even if there are slight changes in the parameters. After some number of iterations of the loop depending on the amount of error the output waveform becomes closer and eventually becomes an exact representation of the needed output waveform.
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