First Day of the Program
From Left to Right: Joshua Mann, Mir Henglin, Margaret Lutz, Laura Maguire, Jay Angel, Linda Lee, Lauren McLeod, Caleb Holt, Chrissy Porter, Andrew Kerr, Kiersten Daviau, Zak Burkley, Ngoc Chau Vy
Last Night of the Program
The first week was spent being introduced to the instrumentation. The set up uses a Picometrix pulse laser that supplies a signal to the transmission antenna through an optical umbilical. The optical signal hits a semiconductor material that is made out Gallium Arsenide. On the outside of the width of the optical beam sit two electrodes. The pulse of this signal creates electron holes in the semiconductor and this produces an electrical potential. This changing potential creates an electromagnetic wave that goes through the lens in the antenna. This electromagnetic wave is of terahertz in nature. In order to image an object this terahertz signal will reflect off the image and go into the other antenna, which takes this electromagnetic signal and produces and optical signal. Then the signal is analyzed using multiple computer programs created through LabView.
For practice the instruments were set up to image an old coin. This was done using normal reflection that requires a collinear adapter. This adapter attaches the two antennas together at ninety degrees and uses a beam splitter to send the signal to the receiving antenna. This process introduced the practice of maximizing the signal using translation stages, adjustable mounts, and position on the optical table.
During the first week the set up was brought to an art graduate school (INP) to image a Fresco Painting. This meeting was arranged because this Fresco was believed to have a much older painting underneath the visible painting and in theory terahertz should be able to see this. The setup used x and y mechanical translation stages. These are controlled using the LabView programs as the antenna scans. The max signal was found using a third manual translation stage on the axis perpendicular to the paintings surface and the angle at which the signal hits the surface. After a long day a small portion of the painting was imaged and ready to be analyzed. The graduate student had prepared several small samples of different kind of fresco’s to be used a references.
The following week was spent imaging these samples using a similar setup as at INP. This time data processing was up to the students. This was done using an image calculator program made in LabView. The program lets a specific part of the signal to be looked at in many different ways. Some of these are the time domain, the frequency domain, the minimum peak, and the max peak. If any of these produce a satisfactory image it is saved. This file is then open using an image-editing program (ImageJ) to enlarge the picture. This was done for all of the samples and only a very small amount of the scans produced images where the hidden fresco was hidden.
When this was completed scans of twenty-three different pigments were imaged to be used a references for the advisors colleagues. These scans were using transmission and for each sample a front and back image was used
After this the instrument was brought to a company called Imagine Optics. This company makes a deformable mirror that can correct wave fronts. This technique has been proven to work using visible light and the company wanted to see if it did the same for terahertz. In order to see if the terahertz wave front changed a knife-edge scan was taken. Bringing a knife down perpendicular to the beam at its focal point to see if the mirror changes the spot size tested this. After a few weeks the company analyzed the data and showed that the spot size did in fact change.
The next day the analysis of the fresco painting data was presented at LMRH, which is a center for restoration and conservation of monuments and buildings. The results of the fresco samples where shown first and then images of the actual painting were shown. In these images it was agreed upon that a face can be clearly seen which is not part of the surface painting.
The following week three days were spent imaging cave etchings in Derbyshire England. The etchings were located in caves at Cragswell Crags. The collinear adapter was used to images these as well as a large tripod as the etchings were about six feet above the ground. This work was done in partnership with archeologists that were at the site excavating an area in front of the opening of the cave as well as a group of this project’s advisor’s colleagues from England. With such a large amount of people on a single scan the students were left to mostly grunt work which included; heavy lifting, maintaining the generator, setting up, taking down the set up, and manufacturing a platform out of spare material that could support the tripod and antennas with minimal vibrations.
The next two weeks were spent imaging a resolution target. This target was imaged using normal refection, off-axis refection and transmission. While imaging this target several problems arose. First it was observed that when imaged using the smallest step size the data was not being saved properly. In order to correct this the data was selected to be saved by pixel number instead of position. Then when this switched the data appeared all right but the images themselves appeared to be mirrored and useless. Later on it was found that the original data saved by position could still be used and produced excellent images.
This summer I worked with a Picometrix Terahertz laser system on Cultural Heritage and Artifact Preservation under the supervision of Bianca Jackson and in conjunction with the Louvre (Centre De Recherche et De Restauration De Musees De France (C2RMF)). The purpose of this ongoing research project is to use terahertz radiation to image cultural artifacts in order to learn more about their sub-surface layers in a non-destructive fashion. I worked closely with another iREU student, Jay Angel, on a project in which we learned the theory behind the terahertz laser’s production, used the system to take scans of a variety of samples, analyzed the data we collected and joined Bianca to observe and help out on several on-sight research expeditions.
Our terahertz laser was a pulsed beam emitted through a transmitter lens and then picked up by a receiver lens attached to a computer which displayed the signal as a waveform. By placing a sample between the transmitter and receiver we were able to detect the signal after it had either been transmitted through the sample or reflected off the sample’s surface. When scanning a sample, our raw data consisted of one waveform for each step (pixel) of the scan. The finest step size our motorized translation stages allowed was 100 microns. We used a software program written in labview which organized the waveforms to correspond to their position on the sample and created an image based on the intensity of different aspects of the waveform.
The first set of measurements we took was on a series of plaster samples provided from the Institute National Patrimoine, an art restoration school funded by France’s Ministry of Culture. Our samples consisted of patches of pigment painted onto four different bases and then covered with nine different combinations of materials (a total of 36 samples). We scanned each sample using a normal reflection set up in order to see how the different surrounding material’s affected our ability to pick out the pigments. After creating a variety of images, we found only three samples in which the pigment was visible. Our poor results may have been due to the construction of the samples themselves, as they had irregular surfaces and compositions, characteristics which our images showed clearly rather than the pigments.
We then took measurements on a series of different colored mixtures of dried pigment and casein (a protein used as a binder in paint) provided from the University of Reading in the UK. Rather than scanning the entire surface of each pigment we instead focused on one pixel at a time, averaging together a large number of received waveforms to find one, accurate waveform for the pigment. We ran several trials per pigment sample and measured the width of the samples at the places we measured. Using this data we were able to find the indexes of refraction of the samples and compared how this changed with the color. This data will be sent back to the University of Reading for a master student’s research.
Our final set of measurements was done on a resolution target sent from Teraview (dimensions of 25 mm x 25 mm). The target was covered with differently spaced, reflective dashes which we scanned with normal reflection, transmission, and off-axis reflection lens set-ups in order to determine how fine of a line the terahertz could pick out. We ran into difficulties with the way our software saved files, as we were running scans with the smallest step size our translation stages allowed, resulting in large amounts of closely spaced data. After some trial and error, Bianca helped fix the problem, allowing us to analyze the data we had taken. The best images we created had a resolution of about 350 microns.
In addition to collecting our own data, Jay and I accompanied Bianca on several on sight research trips. The first week of the program we traveled to INP (Institute National Patrimoine) to image a fresco in the hopes of seeing whether another, older painting lay beneath the surface. Our results look promising, as we can see a face with the terahertz which isn’t visible on the surface, and it has been proposed that the top layer of the fresco be removed to see if our images are accurate. We then traveled to Imagine Optics, a small institution in Orsay, Paris which produces a deformable mirror which has been proved to improve the wavefront and decrease the spot size of visible light. We provided the terahertz laser to see if such improvements could also apply to light of a longer wavelength. Our results were preliminary but hopeful, meaning that further testing will be done in the future. The final trip we took was to Creswell Crags in Derbyshire, England to image the oldest cave paintings in the UK (estimated at around 13,000 years old). We imaged an etching of either a woman or a bird (it is still disputed what the etching depicts) in order to see if our terahertz could pick up the lines both visible to the naked eye and those covered up by mineral deposited flow rock. The data is still being analyzed, though our raw data images clearly display the uncovered lines and possibly some of the covered as well.
This has been a unique research experience as we were able to not only develop a better understanding of optics but were also able to discover some of the many different (and unexpected) fields which physics is applicable to. On our onsite visits we had the chance to talk to archeologists and art restorers, seeing how scientific collaboration plays out on an international level. This interplay and overlap between careers and countries was the most interesting and thought provoking aspect of my research here and something which will play a large part in my decision for graduate programs.
I spent the summer working under Rodrigo Lopez-Martens, PhD student Aurelion, and post-doc Antonin. I was joined by Matthieu Fechant, a student at École Normale Supériere, one of the top schools in France (or the top one depending on who you ask) for theoretical physics, and two high school students, Mathieu and Michael. During my first two weeks there, I helped clear the lab out and set up the new laser blocks. I also learned how to start the 50 Watt green pump laser.
After that, I found out about Mach-Zehnder Interferometers, and how to set one up with Michael, the first high school student. We used the interferometer to measure how stable the target was. Our target needs to be stabilized down to ~10 nanometers, in order to accurately control the attosecond pulses, which is the overall goal of our lab. They are created by focusing a very high powered laser down to the order of a micron onto a glass target. This creates a solid-density plasma, which acts as a relativistic oscillating mirror, basically reflecting the light and imparting energy to the light. However, light cannot increase its speed because it’s light, so instead the pulse compresses and becomes much shorter, down to attosecond pulses (~10^-18). In more detail, what happens is the electric field of the light oscillates the electrons of the plasma, which then create the attosecond pulses. The target needs to stay within the Rayleigh length of the focused spot, and also needs to rotate smoothly so all pulses are reflected in the same direction. We also need to ensure that the laser strikes a fresh part of the target that is optically smooth (the glass target itself changes by 1/20 of a wavelength of light over the entire surface), or else the light will not be reflected correctly.
My overall project for the next couple of weeks would be to test how stable we could successfully make the target. The Matthieu’s arrived the following week, and we went over the program we would use to minimize the noise. We also ran the motors of the target for the first time and saw the target rotate and how that rotation affects the fringes of the Mach-Zehnder interferometer we had built the week before. In order to minimize the noise, we tested the motor of the original, which was a homemade gadget, against a manufactured motor. The manufactured motor was terrible. It would shake the target so much our software for observing the fringes could not function properly. We did run a couple of other tests using a millitron and a coarse alignment method to see if it even rotated properly. However, the shaking was still so violent that the screws holding the mirror in place in the mount and the screws for aligning the mount would jiggle out. After replacing the original target system, we worked on minimizing the noise by covering up the paths of the lasers, and turning on the vacuum chamber. What we ended up concluding was that the system was most stable when there was minimal movement going on in the room itself. Covering the paths helped a little, but the biggest improvement we observed was when no one moved.
This IREU experience gave me the opportunity to participate in two projects that were taking place at Laboratory for the Utilization of Intense Lasers (LULI), a plasma physics laboratory at Ecole Polytechnic, during the summer of 2012. Specifically, I was involved in a project regarding the collimation of plasma plumes with magnetic fields as well as an experiment designed to study the collision of two plasma jets.
Working alongside Julien Fuch's Ph D student, Bruno Albertazzi, I helped analyze the data from Bruno's recent experiment regarding a poloidal magnetic field collimating a plasma jet. Using a pre-existing Matlab code designed to obtain plasma density graphs from interferograms, I individually analyzed the interferograms obtained from numerous different laser-driven plasma shots. Since only a certain spatial portion (approx 4 mm) of the plasma jet can be analyzed via interferometry with each shot, my individual goal was to combine plasma shots that shared all the same variables, except location in space, into a continuous plasma jet of approximately 10 - 20 mm in length. Using these combined density plots of the plasma, the group I worked with hoped to confirm the accuracy of existing computer code that simulates a plasma jet interacting with a poloidal magnetic field. As well, since our group hopes to publish these results, I was required to create an aesthetically focused Matlab code in order to produce graphs that match the quality found in peer-reviewed journals.
The other project in which I was involved dealt with an experiment run by Lorenzo Romagnani designed to study the collision of two plasma jets. This consisted of 1.5 weeks of set-up and 1.5 weeks of running the experiment in the LULI 2000 experimental room. During set-up I assisted in mounting optical equipment, creating the beam paths, aligning the lasers, and helping Lorenzo whenever needed. While the experiment was running I aligned the targets for each shot. This consisted of using a microscope setup to align the two aluminum wires that were then ionized to generate the colliding plasma; it also involved aligning the gold target used to create the laser-driven accelerated proton beam. As well, I assisted in creating dosimetry packets from radiochromic film sheets that were used to detect the proton beam after interacting with the plasma in order to map the plasma's electric field.
As seen in the description of my experience above, my time at LULI allowed me to not only gain insight regarding the computer work involved in the analysis of data obtained from plasma physics experiments, it also gave me the chance to personally set up a plasma physics experiment and observe how the data is actually collected. The result was a tremendous increase in my knowledge in plasma physics, high-energy laser experimental work, and graduate physics as a whole. As well, the chance to interact with and develop relationships with such a diverse spectrum of scientists taught me how imperative collaboration is for successful science. This was truly a phenomenal experience that will be benefit me not only in my future scientific aspirations but personal as well.
The research project that I worked on this summer was titled “Modeling the Pressure, Temperature, and Velocity Profiles of a Laminar Flow Gas Lens”. The primary goal of the project was to develop the theory that would be needed to construct a gas lens for an ultra-intense laser. Although outside the scope of my research, the potential applications of this technique would be to focus the laser pulse over an extended distance. My specific research objectives were to investigate the phenomena of laminar fluid flow and heat transport of an ideal gas, use this knowledge to determine the velocity, temperature, and density profiles of the gas lens, and model these profiles for helium and xenon gas.
The investigation of the laminar fluid flow theory allowed me to clarify the conditions and configuration of the gas lens model. One necessary condition for this model is that the speed of the gas be significantly lower than the speed of sound. This ensures that the flow remains laminar and the velocity profile remains constant with respect to time at every point in the flow. Another necessary condition is that the flow behaves as a continuum. Consequently, the pressure density of force and the viscous density of force can both be determined by taking a small fluid element (representing the average of a large number of particles) and integrating it to determine the pressure, temperature, and viscosity of the fluid as a whole.
The result of applying laminar flow theory to the gas lens problem was the equation for the velocity profile of the gas as a function of its position between the flat plates.