Thesis/Capstone Archive

Year:
Advisor:

2017

Miriah Peterson (Senior Thesis, August 2017, Advisor: Justin Peatross )

Abstract

We are conducting a search for incoherently scattered 2nd harmonic light, in support of the Bohmian interpretation of quantum mechanics. I calibrate a high-throughput spectrometer to mea- sure the hyper Rayleigh scattered 2nd harmonic and 3rd harmonic. Hyper Rayleigh scattering refers to incoherent harmonic light scattered by atoms in a direction perpendicular to the laser propagation. To benchmark spectrometer performance, I generate fundamental beams of 266nm and 400nm that replicate the 2nd and 3rd harmonics of our 800 nm, 35 fs laser. Relying on linear Rayleigh scattering, I measure the efficiencies of the optics used in the spectrometer. I conclude that the spectrometer measures 266 nm and 400 nm light adequately so that it can be used to de- tect hyper Rayleigh scattered 2nd harmonic of our 800 nm laser, if existent. Unfortunately, our attempts to measure hyper Rayleigh scattering of third harmonic as a benchmark is far weaker than expected, making it difficult to rule out the possible existence of incoherent second harmonic light.

David Squires (Senior Thesis, April 2017, Advisor: Justin Peatross )

Abstract

Existing wavefront sensing technologies, like those used in various adaptive optics systems, often have problems dealing with high turbulence conditions. Holographic interferometry could function as a more robust wavefront sensor in high turbulence conditions if the procedure can successfully be taken from theory to practice. To make that step, I built a Mach-Zehnder-style holographic interferometer that measured turbulent phase shifts accumulated in a signal beam over 80 m. To convert holographic interferograms into wavefront-sensing phase profiles, I converted the interferogram into the Fourier plane and isolated features of interest. The experimental results were inconclusive due to design problems and equipment issues.

2015

Chris Olsen (Senior Thesis, April 2015, Advisor: Justin Peatross )

Abstract

Within the semi-classical framework for strong-field laser harmonic generation, using Bohmian trajectories as radiation sources suggests emission of both even and odd harmonics. In contrast, using expectation values as the sources for Maxwell's equations predicts only odd-order harmonics, which are well known to emit coherently. We perform a photon-counting experiment to search for an incoherent second-harmonic signal, which is expected to be very weak. We constructed a spectrometer to detect hyper-Rayleigh scattering across second and third harmonic from helium. The third harmonic is found to have a superradiant enhancement. An upper bound on the strength of possible second harmonic is established assuming no subradiant suppression.

Grayson Joel Tarbox (Masters Thesis, April 2015, Advisor: Justin Peatross )

Abstract

An experiment currently underway at BYU is designed to test whether the size of a free electron wave packet affects the character of scattered radiation. Using a semi-classical argument wherein the wave packet is treated as a diffuse charge distribution, one would expect strong suppression of radiation in the direction perpendicular to the propagating field as the wave packet grows in size to be comparable to the wavelength of the driving field. If one disallows the interaction of the wave packet with itself, as is the case when calculating the rate of emission using QED, then regardless of size, the electron wave packet radiates with the strength of a point-like emitter. In support of this experiment, we explore a variety of physical parameters that impact the rate of scattered photons. We employ a classical model to characterize the exposure of electrons to high-intensity laser light in a situation where the electrons are driven by strong ponderomotive gradients. Free electrons are modeled as being donated by low-density helium, which undergoes strong-field ionization early on in the pulse or during a pre-pulse. When exposed to relativistic intensities (i.e. intensities sufficient to cause a Lorentz drift at a significant fraction of c), free electrons experience a Lorentz drift that causes redshifting of the scattered 800 nm laser light. This redshift can be used as a key signature to discern light scattered from the more intense regions of the focus. We characterize the focal volume of initial positions leading to significant redshifting, given a peak intensity of 2 x 10^18 W/cm 2 , which is sufficient to cause a redshift in scattered light of approximately 100 nm. Under this scenario, the beam waist needs to be larger than several wavelengths for a pulse duration of 35 fs in order to ensure free electrons remain in the focus sufficiently long to experience intensities near the peak pulse intensity despite strong ponderomotive gradients. We compute the rate of redshifted scattered photons from an ensemble of electrons distributed throughout the focus and relate the result to the scattered-photon rate of a single electron. We also estimate to what extent the ionization process may produce unwanted light in the redshifted spectral region that may confound the measurement of light scattered from electrons experiencing intensities greater than 1.5 x 10^18 W/cm^2.

2014

Eddie Hansen (Capstone, December 2014, Advisor: Justin Peatross )

Abstract

In 2009, E. Fischbach and J. Jenkins reported what looked like small variations in the rate of beta decay of several different isotopes. They postulated that the observed variations stem from a solar influence (via the Earth-Sun Distance) on the observed decay rate. This is a controversial topic because the rate of decay is widely believed to be constant. It has been suggested that annual variation seen in their data might arise from seasonal changes such as humidity and temperature as opposed to Earth-Sun distance. At BYU we have constructed an apparatus to carefully control for environmental factors while measuring the rate of decay from a variety of samples with a precision (of one part in 10^4 per day). Multiple samples will be measured on the same detector, and the same sample will be measured by different detectors to remove environmental factors. The apparatus will enable us to unambiguously either confirm or dismiss the earlier claims.

2013

Nathan Gundlach (Senior Thesis, April 2013, Advisor: Justin Peatross )

Abstract

We use the Dirac equation to model electron behavior in a strong laser field. We also present simulations of the density of spin expectation as the Gaussian wave packet undergoes complex relativistic motion. This work extends an analytic technique for computing a Gaussian wave packet in an intense laser field to the case of a Dirac electron. This technique previously had only been applied to the Klein-Gordon equation.

Josh Olson (Senior Thesis, July 2013, Advisor: Justin Peatross )

Abstract

We employ an interferometric technique to diagnose the plasma density in an intense laser focus that ionizes neon gas. A probe beam, arriving after the ionizing pulse, experiences a phase shift as it travels through the refractive index of the plasma. The phase shift is apparent when the probe beam interferes with a reference beam that avoids the plasma. The plasma density is characterized as a function of delay over several nanoseconds after the 35- femtosecond ionizing laser pulse. Our data shows a 50% drop in density after 4.5 ns.

Lukas Pritchett (Senior Thesis, April 2013, Advisor: Justin Peatross )

Abstract

We investigate time-resolved quantum field theory (QFT), which seeks the time evolution of QFT states. Time-resolution in the Schrödinger picture is possible with commonly available computational resources. In demonstration of this, we have developed a time-resolution method and applied it to problems in phi-4. We detail the method and provide some example computation times. The study of time-resolved QFT can provide insight into the structure of QFT and also an alternative method for solving familiar problems.

Ryan Sandberg (Senior Thesis, August 2013, Advisor: Justin Peatross )

Abstract

We present the 3D visualization of an electron wave packet in an intense laser field. Our visualization is based on a virtual camera model. The camera can be rotated around the wave packet, allowing the wave packet to be seen from any angle. The camera program integrates probability density along the line of sight and displays the calculated wave packet as a cloud. The probability density relies on a formula for calculating the wave packet in an intense laser field that was developed previously. To improve the 3D perspective of the images, we display coordinate lines in the wave packet visualization. We apply our camera model to an electron in an intense laser field and show frames from the movies we made. Features of note include the helical path of the wave packet in circularly polarized light and the multi-peaked structure that develops when the wave packet reaches the size of a laser wavelength.

2012

Michael Johnson (Senior Thesis, August 2012, Advisor: Justin Peatross )

Abstract

This thesis describes methods for successfully characterizing and controlling the output of an optical parametric oscillator (OPO) producing 30-ps pulses between 400 and 700 nm. The output pulses are strongly influenced by variations in the 30-ps, 355 nm pump pulses delivered by an amplified neodymium vanadate oscillator that has undergone second- and third-harmonic generation. This work took place at Arkansas State University in the group of Dr. J. Bruce Johnson. To be suitable for use with remote laser spectroscopy, the pulses from the OPO had to approach the Fourier bandwidth limit. Also described is a method for stabilizing the output of a regenerative train amplifier.

2011

John Purvis Corson (Masters Thesis, June 2011, Advisor: Justin Peatross )

Abstract

We use quantum electrodynamics (QED) to investigate the possibility of radiative interference from a single laser-driven electron wave packet. Intuition gleaned from classical electrodynamics suggests that radiation from a large electron wave packet might interfere destructively when different regions of the packet oscillate out of phase with each other. We show that when the incident light is represented with a multi-mode coherent state, the relative phases of the electron’s constituent momenta have no influence of the amount of scattered light. Hence, the radiation does not depend on the amount of free-particle spreading experienced by the electron before the interaction. This result is shown to hold to all orders of perturbation theory. We extend our conclusions using the Furry picture of QED, where the (now-classical) incident light pulse is treated non-perturbatively with Volkov functions. We connect our results to a first-quantized picture by comparing transition probabilities between QED and semiclassical models. We are able to match these probabilities by choosing the classical scattered light field to be a single mode with energy h¯ω 0 .

Nicholas Herrick (Senior Thesis, September 2011, Advisor: Justin Peatross )

Abstract

We constructed an extreme ultraviolet (EUV) polarimeter which uses laser-induced high harmonics as the light source. This device is capable of making absolute reflectance measurements as low as 0.2% over a wavelength range of 8-89 nm. The polarimetry positioning system allows incident reflectance angles of 0-40º. In order to increase the dynamic range of the EUV detector we introduce low-pressure gas downstream of the harmonic-generation region to predictably attenuate EUV flux during incident measurements. We also use the gas cell to measure the photoabsorption cross sections of hydrogen and oxygen compared to that of water vapor, revealing the influence of chemical bonds on EUV absorption.

Kamal Pangeni (Senior Thesis, July 2011, Advisor: Justin Peatross )

Abstract

We constructed a time-of-flight ion detector to measure the intensity at the focus of an ultra-short-pulse high- intensity laser. The intensity can be inferred indirectly by measuring the ionization state of a gas introduced into the focus.This work supports the overall e fforts of our group to perform laser experiment using intensities up to 1019 W=cm2. The laser is focused to a spot a few microns across and has a duration of only 35 fs. An absolute calibration of the laser intensity is critical to proper interpretation of experiments. We report on test of time-of-flight measurement using argon and provide an overview of the design of the instrument.

Phillip Smith (Senior Thesis, April 2011, Advisor: Justin Peatross )

Abstract

Small particles such as dust or graphite, a few microns in size, can become caught and suspended in the focus of a continuous laser beam. The opaque particles heat up and interact with surrounding air molecules in such a way that they become stably trapped in the laser focus. We have found that crossed laser beams increase the stability of trapped particles. The natural oscillatory excursions change from 100 microns for a single beam down to a few microns when a particle is trapped in the intersection of two beams. Crossed beams also increase the chances of trapping particles at the same location. Moreover, the levitated particles stay in the intersection of the two beams as one beam is moved with respect to the other. We characterized the strength of the trap by oscillating the point of intersection and determined that a particle can undergo 0.33 g of acceleration while remaining trapped.

Grayson Tarbox (Senior Thesis, December 2011, Advisor: Justin Peatross )

Abstract

We examine both near-field work and far-field radiated power from a classical distribution of oscillating dipoles. It is shown that interference in the far field implies near-field work between different members of the distribution. When considering radiation from a quantum mechanical electron wave packet, interference in the emitted light would imply near-field work performed by one spatial portion of the wave-packet on another part. However, quantum electrodynamics does not countenance the possibility of interference in the far-field radiation emitted from a single electron. Thus, the absurd notion of an electron wave packet interacting with itself is avoided.

2010

David Black (Senior Thesis, August 2010, Advisor: Justin Peatross )

Abstract

Our group has made a variety of observations regarding how small opaque particles can be levitated in a focused CW laser beam. The phenomenon relies on a radiometric effect, where the particle is heated by the laser and interacts with surrounding air molecules. Intuitively, one would expect opaque particles to heat up more on the sunny side and thus be propelled out of the beam. Thus, the mechanism by which some particles become trapped in the beam remains a mystery. The distribution of energy absorption within tiny, absorbing spheres (~10 wavelengths across) is investigated to try to gain insight into the levitation of small particles. This is computed using standard Mie scattering analysis as outlined in the textbook Principles of Light and Optics by Born and Wolf. In our analysis, we concentrate on the internal fields rather than the scattered fields that are usually the subject of such analyses. We investigate whether there can be "shady-side heating," where more energy is absorbed on the side of the particle opposite the direction of the beam propagation, which could potentially explain the trapping mechanism. It is found that there is no such effect.

Jacob Johansen (Senior Thesis, August 2010, Advisor: Justin Peatross )

Abstract

In the Bohmian interpretation of quantum mechanics, a point particle follows one of many possible trajectories within its quantum wave packet, which influences the particle's motion. The Bohmian interpretation is applied to the hydrogen atom interacting with an external field. Magnetic force on the electron is analyzed as a possible mechanism for the impulse measured during a stimulated transition. The impulse predicted in the Bohmian formalism is found to be of an appropriate size. An electric dipole moment which arises in the Bohmian interpretation is analyzed, and is found to be extremely dicult to measure, which explains why it has not been observed. The Bohmian interpretation of high harmonic generation is considered. The Bohmian formalism is consistent with other interpretations for ensembles of atoms, but individual trajectories are found to exhibit even as well as odd harmonics. This, and the electric dipole moment, might lead to experimental tests which, while technically diffcult, might enable the Bohmian interpretation to be distinguished from other interpretations of quantum mechanics.

2009

Nicole Brimhall (PhD Dissertation, July 2009, Advisor: Justin Peatross )

Abstract

We developed an extreme ultraviolet (EUV) polarimeter, which employs lasergenerated high-order harmonics as the light source. This relatively high-flux, directional EUV source has available wavelengths between 10 nm and 47 nm with easily rotatable linear polarization. The polarimeter has allowed us to characterize the optical constants of materials that may be useful for EUV optics. The instrument has a versatile positioning system and a spectral resolution of about 180, and we have demonstrated that reflectance as low as 0.1% can be measured repeatably at EUV wavelengths. We investigate the high harmonic source used for polarimetry measurements by documenting the spatial evolution of the generating laser in a semiinfinite helium-filled gas cell under conditions suitable for harmonic generation. The laser is observed to focus, diverge, and refocus, accompanied by a flattop beam profile and extended harmonic phase matching. We numerically simu- late the propagation to investigate these experimental results. We find that harmonic energy comes from the forward portion of the laser pulse, whereas the latter portion gives rise to the incidental double laser focusing. Good phase matching for the harmonics arises in large measure from a balance between the linear phase delay of the neutral atoms and the Gouy shift, which is elongated and nearly linearized when an aperture is partially closed on the beam. We compare reflectance data taken with the polarimeter instrument with expected results from well-characterized samples and find that they agree. To improve repeatability and reduce the effects of systematic measurement errors we have measured the ratio of p- to s-polarized reflectance and shown that optical constants can be extracted from this data as efficiently as from absolute reflectance measurements. These ratio measurements allow more accurate recovery of optical constants than our absolute reflectance measurements for our well-characterized samples. We use the polarimeter instrument and the ratio reflectance technique to determine the optical constants of copper, uranium, and their natural oxides from 10-47 nm. For copper, this measurement resolves previously conflicting data sets, where disagreement on optical-constant values arises from failure to keep samples from oxidizing before measurement. Uranium has been proposed as a high-reflectance material in the EUV for several years, however difficulties with oxidation have prevented its careful characterization previous to this work. We find that measured optical constants for uranium do not agree well with previously accepted theoretical calculations.

Jordan Cox (Honors Thesis, August 2009, Advisor: Justin Peatross )

Abstract

We recover the trajectory of a fateful field goal attempt in the bowl game between Brigham Young University (BYU) and the University of California-Los Angeles (UCLA) in December 2007. The primary task that enables the analysis is the recovery of the locations of the TV cameras from the information contained within the frames. In this project, we develop and implement a method for determining the camera position in several frames. Using screen shots from two cameras showing the same event, we deduce the path of the football before it was tipped by a BYU player. Our analysis shows clearly that the ball would have resulted in a field goal if the BYU player had not removed a significant amount of momentum from the football. We calibrate the camera using points on the football field, which are coplanar, a difficult condition notorious for resulting in a singular matrix in traditional Direct Linear Transformations (DLT). We develop an alternative method for retrieving the camera location. We find good agreement between our method and a recently developed 2D analogue of DLT that does not suffer from the singular matrix problem.

Eric Cunningham (Senior Thesis, December 2009, Advisor: Justin Peatross )

Abstract

We examine characteristics of Bohmian trajectories associated with the Klein- Gordon equation for a charged particle in an electromagnetic potential. We describe both Hamilton-Jacobi theory and Bohmian mechanics in the relativistic regime, showing how to write the Klein-Gordon equation in the form of a relativistic Hamilton-Jacobi equation. Problematic negative probability density regions associated with the Klein-Gordon equation can occur for a self-interfering wave packet. This is mitigated by the fact that the regions are always spatially localized to isolated regions less than about 6% of a Compton wavelength. This means that the Bohmian formulism for the Klein-Gordon equation, long deemed to be unsatisfactory, is dismissible only on spatial scales that necessitate quantum field theory.

Jordan Maucotel (Capstone, December 2009, Advisor: Justin Peatross )

Abstract

We designed and implemented a technique for creating a closely spaced sequence of high-intensity femtosecond IR laser pulses with adjustable spacing. This was implemented for an experiment where counter-propagating laser pulses collided in region of high-harmonic generation. The laser beam was split before amplification with and one beam entering a Mach-Zender-style interferometer to create a pair of closely spaced pulses. After amplification, the beam containing the pair of pulses was temporally chirped so that the two pulses overlapped in time, creating a rapid beat pattern in time. The beat pattern became the sequence of counter-propagating pulses for the experiment, and they were characterized using a cross-correlator. The two beams were focused and made to collide in an argon jet inside a vacuum system. With this setup, we were able to show that quasi-phase matching is insufficient to make ionization gating a feasible method for generating bright attosecond high-harmonic pulses.

2008

Robert Petersen (Senior Thesis, August 2008, Advisor: Justin Peatross )

Abstract

We observe the stability of ne graphite particles (diameter 1-10 microns) sus- pended radiometrically in a laser beam, as the ambient gas pressure increases from 1 atm to 10 atm. Particles are self-selectively captured near the focus of a 2.5 W CW 532 nm beam, by sprinkling graphite powder above the beam. After a particle becomes trapped in the beam, the ambient pressure of nitro- gen gas is gradually increased until the particle is observed to `fall out' of the laser. Results show that 90% of the graphite particles do not remain trapped in the beam past an ambient pressure of 4 atm. The mean `fall-out' pressure is about 2 atm. Only 2% of the particles remained trapped in the laser above 8 atm. No particle in our 40-particle sample group remained trapped at 10 atm. Qualitatively, we observed that smaller particles (i.e., particles that scatter less light) tend to survive to higher pressures.

2007

Nicole Brimhall (Masters Thesis, July 2007, Advisor: Justin Peatross )

Abstract

We developed an extreme ultraviolet (EUV) polarimeter, which employs lasergenerated high-order harmonics as the light source. This relatively high-flux directional EUV source has available wavelengths between 8 nm and 62 nm and easily rotatable linear polarization. The polarimeter will aid researchers at BYU in characterizing EUV thin films and improving their understanding of materials for use in EUV optics. This first-time workhorse application of laser high harmonics enables polarization-sensitive reflection measurements not previously available in the EUV. We have constructed a versatile positioning system that places harmonics on the microchannel plate detector with an accuracy of 0.3 mm, which allows a spectral resolution of about 180. We have demonstrated that reflectance as low as 0.2% can be measured at EUV wavelengths and that this data is repeatable to within the error of our source stability (∼7% fluctuation). We have compared reflectance data with that taken from the same sample at Beamline 6.3.2 at the Advanced Light Source. This data agrees well from 5 degrees to 30 degrees and the angular locations of the interference fringes also agree.

Gavin Giraud (Senior Thesis, August 2007, Advisor: Justin Peatross )

Abstract

High-order harmonics are generated by intense ultrashort laser pulses focused into a helium- lled cell. The semi-in nite gas cell extends all the way from the focusing mirror to a thin foil that partitions the helium- lled region from a subsequent region of vacuum. The laser pulse interacts strongly with the gas prior to and during the harmonic-generation process, which takes place near the exit foil. The laser beam pro le undergoes distortion, which has previously been associated with improved phase matching and increased high-harmonic signal. An imaging system was prepared for viewing the laser beam pro le near the focus, during operation at full power under conditions ideal for high harmonic generation. A mirror can be quickly inserted and removed for quasi- simultaneous imaging of the the residual laser pulse or the harmonic signal. The measured laser-beam pro les show evidence of self-guiding of a laser pulse, in the same region that produces the strongest harmonic signal.

Mirinda Martin (Capstone, August 2007, Advisor: Justin Peatross )

Abstract

Matthew Turner (Senior Thesis, August 2007, Advisor: Justin Peatross )

Abstract

We numerically simulate the propagation of a high-intensity laser pulse to investigate the role of nonlinear effects in our high-harmonics generation experiments. The model uses an approximation of the nonlinear wave equation, which includes the Kerr effect and plasma generation, to model the evolution of an electric field envelope as it propagates through the region of interest. The initial condition for the field is calculated from Fresnel integrals to decrease computation time and include the effects of a aperture located before the focusing lens. Numerical results for the radial fluence profile, fluence full-width at halfmaximum, spectral shift, and ionization levels agree very well with previous experimental results. Removal of the nonlinear effects from the propagation code shows that the Kerr effect has a negligible influence in our setup. The simulations demonstrate that plasma generation is responsible for an observed v double focus in the fluence.

2006

John Charles Painter (Masters Thesis, May 2006, Advisor: Justin Peatross )

Abstract

We investigate the spatial evolution of an intense laser pulse as it generates high-order harmonics in a long gas cell, filled with 80 torr of helium. A thin foil separates the gas-filled region of the cell from a subsequent evacuated region. The exit plane of the gas cell can be scanned along the laser axis so that the evolution of the laser throughout the focus can be observed (full scanning range of 9 cm). We constructed an apparatus that images the laser radial energy profile as it exits the cell. The high harmonics, odd orders ranging from 45 to 91, are observed at the same time that the laser spot is characterized. Re-absorption of the harmonics within the gas cell restricts the region of harmonic emission to the final centimeter (or less) of the cell. We present the first direct evidence (to our knowledge) of laser filamentation under conditions ideal for high-order harmonic generation. The 30 fs, 4 mJ, laser pulses were observed to undergo double focusing within the gas cell, with about 4 cm separating the two foci. The region with best harmonic emission occurs midway between the two foci. The radial profile of the laser focus, 150-200 microns in diameter, evolves from a Gaussian-like profile to a more square-top profile as it propagates over several centimeters. The filamentation phenomenon as well as the brightness of the harmonics improves when an aperture is partially closed on the laser beam before reaching the focusing mirror. A spectral sampling of the imaged laser focus revealed a 4 nm blue-shift associated with the generation of plasma in the gas cell. The blue-shifting occurs primarily in the center of the laser beam and less at the wider radii. The initial laser pulse had a spectrum centered at 800 nm with a 35 nm bandwidth. The energy associated with each of the observed 26 harmonic beams was found to be approximately 1 nJ, yielding a conversion efficiency of approximately 2×10−7 .

2005

Mark Adams (Senior Thesis, August 2005, Advisor: Justin Peatross )

Abstract

n/a

Eric Christensen (Senior Thesis, April 2005, Advisor: Justin Peatross )

Abstract

n/a

Adam Hendrickson (Senior Thesis, June 2005, Advisor: Justin Peatross )

Abstract

We investigate the trapping of opaque microscopic particles in mid air by a laser beam. Radiometric interactions with ambient gas molecules suspend and hold the particles in place. Tungsten, charcoal, and black liquor particles are observed. A microscope and CCD camera are used to image the particles and to capture video footage of particle jitter motion while trapped. A shutter controller is used to determine particle jitter speeds, which typically range from 1.5 x10 -3 m/s to 4.8 x10 -3 m/s. Particle sizes up to 14 microns are observed with various shapes and jitter. The on-axis jitter motion has a typical range of excursion of 30 to 60 microns. Many particles can simultaneously be trapped near the focus. Particles separated by less than 15 microns often exhibit synchronized coupled motion. 2

Rhett Lindsey (Senior Thesis, August 2005, Advisor: Justin Peatross )

Abstract

n/a

Julia Robin Miller Sutherland (Masters Thesis, July 2005, Advisor: Justin Peatross )

Abstract

Ten-millijoule, thirty-five femtosecond, 800 nm (~40 nm bandwidth) laser pulses are used to study high-order harmonic generation in helium- and neon-filled gas cells of various lengths. Harmonic orders in the range of 50 to 100 are investigated. A semiinfinite cell geometry produces brighter harmonics than cells of sub-centimeter length. In the semi-infinite geometry, the gas occupies the region from the focusing lens to a thin exit foil near the laser focus. Counter-propagating light is used to directly probe where the high harmonics are generated within the laser focus and to investigate phase matching. The phase matching under optimized harmonic generation conditions was found to be unexpectedly good with phase zones many millimeters long. Restricting the laser beam with an 8 mm aperture in front of the focusing lens increases the emission of most harmonic orders observed by as much as an order of magnitude. Optimal harmonic generation pressures were found to be about 55 torr in neon and 110 torr in helium. The optimal position of the laser focus was found to be a few millimeters inside the exit foil of the gas cell. Probing with counter-propagating light reveals that in the case of neon the harmonics are generated in the last few millimeters before the exit foil. In helium, the harmonics are produced over a longer distance. Direct measurement shows the re-absorption limit for mid-range harmonics in neon has been reached.

2004

Brigham Russell Frandsen (Honors Thesis, July 2004, Advisor: Justin Peatross )

Abstract

David Niemi (Senior Thesis, August 2004, Advisor: Justin Peatross )

Abstract

n/a

John Painter (Senior Thesis, August 2004, Advisor: Justin Peatross )

Abstract

n/a

Benjamin Pratt (Senior Thesis, January 2004, Advisor: Justin Peatross )

Abstract

Kelly Smith (Honors Thesis, February 2004, Advisor: Justin Peatross )

Abstract

2002

Andrew Landen (Senior Thesis, May 2002, Advisor: Justin Peatross )

Abstract

Sergei Leonidovich Voronov (PhD Dissertation, December 2002, Advisor: Justin Peatross )

Abstract

Laser high-order harmonic generation in the presence of relatively weak interfering light is investigated. The interfering pulses intersect the primary harmonicgenerating laser pulse at the laser focus. The interfering light creates a standing intensity and phase modulation on the field, which disrupts microscopic phase matching and shuts down local high harmonic production. Suppression of the 23rd harmonic (by two orders of magnitude) is observed when a counter-propagating interfering pulse of light is introduced. A sequence of counter-propagating pulses can be used to shut down harmonic production in out-of-phase zones of the generating volume to achieve quasi phase matching. Harmonic emission is enhanced in this case. A new high-power laser system with higher pulse energy has been constructed to further investigate quasi phase matching of high-order harmonics generated in difficultto-ionize atomic gases (e.g., neon as opposed to argon). The new system can also be used to study harmonic generation in ions. A new counter-propagating beam produces a train of 5 pulses with regulated timing. In preliminary tests, the new system has produced high harmonics up to the 65th order in neon. This should increase with additional adjustments to the laser system. The high-order harmonics have also demonstrated to be useful for polarized reflectometry measurements of optical surfaces in the extreme ultraviolet (EUV) wavelength range.

2001

John Benson Madsen (Masters Thesis, December 2001, Advisor: Justin Peatross )

Abstract

We have conducted two experiments in laser high harmonic generation with multiple beams. In the first experiment, high harmonic light in the direction of a weak counter propagating beam is looked for, but not observed. We determined that the reverse signals of harmonics with orders in the twenties produced in an argon jet are at least 1000 times less intense than the forward traveling harmonics we measured. The emission of harmonics in the direction of a weak counter propagating beam has been predicted to appear only when intensity-dependent phases are present. Phase matching in the direction opposite to the generating beam is otherwise so poor that it eliminates the possibility of emission in that direction. In the second experiment, weak light entering from near counter propagating and perpendicular angels creates a standing modulation in both intensity and phase. The standing modulations disrupt normal phase matching and shut down local high harmonic production. Disruption of one order of magnitude is observed when perpendicular propagating light in introduced. Perpendicular propagating light is used to shut down a small region of harmonic generation in large generating medium to achieve quasi phase matching. Harmonic emission is enhanced by one order of magnitude.

Michael (Vern) Pack (Senior Thesis, December 2001, Advisor: Justin Peatross )

Abstract

Jed Simmons (Senior Thesis, March 2001, Advisor: Justin Peatross )

Abstract

Nathan Burrett Terry (Masters Thesis, December 2001, Advisor: Justin Peatross )

Abstract

Two hundred milliwatts of 457.9 nm laser light incident on a BBO crystal was used to produce 915.8 nm photon pairs through type II parametric down conversion. We measured the cut of the BBO (Beta Barium Borate) crystal by comparing the visible down converted rings with those rings numerically predicted to obey the phasematching conditions associated with type II down conversion. 916 nm down converted photons were detected using a silicon avalanche photodiode. The construction and design of the ice cooled housing for the avalanche photodiode is discussed. With the crystal aligned, down converted 916 nm light was found to account for 25% of the photons present; the other 75% came chiefly from fluorescence. The number of down converted photons observed is show to be in agreement with quantum theory. Future experiments utilizing the correlated photon pairs are discussed. These will be incorporated into the curricula of the undergraduate optics and quantum mechanics classes.

Jamie Titensor (Senior Thesis, December 2001, Advisor: Justin Peatross )

Abstract

Michael J Ware (PhD Dissertation, December 2001, Advisor: Justin Peatross )

Abstract

A new context for the group delay function is presented describing electromagnetic pulse propagation in a uniform linear dielectric medium. In contrast to the traditional formulation, this new context retrains validity for pulses of any bandwidth, propagating in media with arbitrarily complicated resonance structures. The new context defines the arrival time of a light pulse at point in space (using a time expectation integral over the Poynting vector) and considers the delay between pulse arrival times at two distinct points. This delay consists of two parts: a spectral superposition 9f group delays and a delay due to spectral reshaping vie absorption or amplification. The traditional formulation of group velocity is recovered by taking a narrowband limit of this generalized context. The use of the new context is illustrated for pulses propagating both superluminally and subluminally in amplifying and absorbing media. The inevitable transition to subliminal behavior for any initially superluminal pulse is also demonstrated. The energy exchanged between an electromagnetic pulse and linear dielectric medium in which it propagates is also considered. It is this exchange of energy which allows for the superluminal behavior of the centroid of field energy. While group velocity indicates the presence of field energy (the locus of which can move with arbitrary speed), the velocity of energy transport maintains strict liminality. This indicates that the medium exchanges energy differently with the leading and trailing portions of the pulse. The reason for this asymmetric treatment is clearly demonstrated by rewriting the exchange energy in terms of the instantaneous spectrum (i.e., the spectrum of the pulse truncated at each new instant as a given locale in the medium experiences the pulse). This description for the exchange energy directly emphasizes the role of the principle of causality and gives insight into the phenomenon of superluminal pulse propagation.

1999

Kenneth S. Anderson (Honors Thesis, April 1999, Advisor: Justin Peatross )

Abstract

Sterling W. Cornaby (Senior Thesis, April 1999, Advisor: Justin Peatross )

Abstract

Ian T Kohl (Masters Thesis, December 1999, Advisor: Justin Peatross )

Abstract

We have constructed a Ti:Sapphire regenerative amplifier, operating at 1 kHz, that amplifies nanojoule femtosecond pulses to the millijoule level. The system utilizes the technique of chirped-pulse amplification. A Pockels cell introduces the seed pulse into the cavity and later ejects it after 12 round trips. The output of the amplifier is 2 mJ per pulse which after compression to 40 femtoseconds becomes 1 mJ. The laser system is being used for an experiment (in progress) in high harmonic generation which is described in this thesis. For the experiment, we are exploring the extent to which intrinsic atomic phases of individual harmonics vary with laser intensity. These variations play an important role for phase-matching in the harmonic generation process. Our experiment uses weak counter propagating laser light to induce a standing intensity modulation on the main laser pulse. The counter-propagating light is too weak to generate harmonics by itself. However, the intensity dependence of the intrinsic phase in the presence of the standing intensity modulation can overcome the extremely poor phase-matching in the backwards direction. Observation of backwards-traveling high harmonics therefore promises to be a sensitive probe of this behavior.

Paul Sorensen (Senior Thesis, April 1999, Advisor: Justin Peatross )

Abstract

Michael Ware (Honors Thesis, March 1999, Advisor: Justin Peatross )

Abstract

Angularly dispersive systems introduce frequency dependent phase shifts to the frequency components of laser pulses. With appropriate understanding, these systems may be employed to build laser systems with ultra-short high intensity pulses. The traditional method of describing the phase delay introduced by these systems can be cumbersome at times. This thesis introduces a revised approach to calculating the phase delay in angularly dispersive systems. We also consider the energy flow in these systems. The group delay is usually derived by approximating the phase delay with the first few terms of a Taylor series expansion. We show that under a specific definition for the temporal position of a pulse, we can give a precise meaning to the group delay without approximation.

John Batierun Yelifari (Masters Thesis, December 1999, Advisor: Justin Peatross )

Abstract

In interferometric measurements, one beam probes the surface under test and then interferes with a second reference beam. A distortion in the wave front of the probe beam relative to that of the reference beam reveals errors in the test surface as seen in the variations of the interference fringe pattern. In conventional interferometry (Michelson, Twyman-Green, etc. interferometers) large reference surfaces are used to generate reference beams. The accuracy of measurements of the surface errors due to the test surfaces is therefore limited by the imperfection of the reference surface. However, the accuracy of these results can be improved as the size of the source and the reference surface is decreased to a point, so that t represents a truly unaberrated surface/ with such unaberrated surfaces, it appears to be possible to use visible light to measure the precision of optical surfaces to a quality sufficient for use in the deep ultraviolet (~λ/100). In this thesis we present two approaches in developing such a high precision interferometer. The first method virtually eliminates the reference surface and all auxiliary optics (including the beam splitter) through the technique of wavefront division. The exit of a narrow fiber optic serves as the point source both to create the probe beam and the test beam. The second approach utilizes a conventional reference surface but the beam splitter is a pellicle illuminated over a very small area (about 80 microns). The influence of the aberrations of this minute surface on the overall results is therefore negligible. In both of the two approaches a fiber with a core diameter of 5 microns is used as our source of the initial beam. The performances of our interferometers were compared to that of a good precision commercial interferometer (Talandic Research interferometer model 302) and the results were comparable or better. The measurements of a 2-inch concave mirror suggested an accuracy ofλ/20, which matched the specifications of the mirrors. This work may impact the emerging field of EUV lithography where unprecedented determination of optical surfaces will be essential.

1998

Troy Rockwood (Honors Thesis, January 1998, Advisor: Justin Peatross )

Abstract