PhD Defenses 2017

Dissertation Title: Semitauonic B_c decays and quark flavor identification methods

Date and Time: Wednesday, December 13, 1:00 pm

Location: PSC 3150

Dissertation Committee Chair: Prof. Hassan Jawahery

Committee:

Dr. Nicholas Hadley
Dr. Sarah Eno
Dr. Kaustubh Agashe
Dr. Eric Slud

Abstract:

The LHCb experiment at the Large Hadron collider is a unique laboratory for studying the properties of heavy quarks. The physics program of the experiment includes studies of CP violation, measurements of CKM matrix parameters, searches for rare decays, quarkonia studies, and other flavor physics, forward physics, and new physics topics.

This dissertation presents an analysis measuring the ratio of the semileptonic branching fractions B(B_c -> J/ψ τ ν) and B(B_c -> J/ψ μ ν) of the doubly-heavy B_c meson, denoted R(J/ψ). Such semitauonic branching fraction measurements have played an increasingly prominent part of B physics research at BaBar, Belle, and LHCb. These measurements are powerful probes of the universality of the couplings of leptons (e, μ, and τ) in electroweak interactions. Currently, measurements of the quantities R(D) and R(D*) by all three experiments are in excess of precise Standard Model predictions.

A second topic of the dissertation is the creation of a new algorithm for the tagging the flavor of neutral mesons in CP violation studies, and a powerful method for calibrating these flavor tagging algorithms via binomial regression. This work ties in to the increasingly prominent use of machine learning techniques in particle physics, and the need for solid understanding of the behavior of their output.

Dissertation Title: Understanding Phase Transitions, Symmetry Breaking, and Interaction Enhanced Sensing in Optomechanical and Cold Atomic Systems

Date and Time: Wednesday, December 13, 2:30 pm

Location: 2115 CSS/ATL

Dissertation Committee Chair: Prof. Victor Galitski

Committee:

​​Dr. Jacob Taylor (advisor)
Dr. Gretchen Campbell
Dr. Jay Deep Sau
Dr. Mohammad Hafezi

Abstract: 

We focus on the interaction of light and matter in atomic and optomechanical systems. These highly controllable and engineerable systems present access to new regimes and research opportunities that often do not exist outside the laboratory. As such, they frequently depart from more commonplace systems which are well understood. We extend our understanding of thermodynamic phase transitions, spontaneous symmetry breaking, and quantum-enhanced sensing to new regimes.

Traditionally, phase transitions are defined in thermodynamic equilibrium. However, inspired by the success of the phase transition paradigm in non-equilibrium fields, we derive an effective thermodynamics for the mechanical excitations of an optomechanical system. Noting the common frequency separation between optical and mechanical components, we study the dynamics of the mechanical modes under the influence of the steady state of the optical modes. We identify a sufficient set of constraints which allow us to define an effective equilibrium for the mechanical system. We demonstrate these constraints by studying the buckling transition in an optomechanical membrane-in-the-middle system, which spontaneously breaks a parity symmetry. Having established a thermodynamic limit, we characterize the nature of the phase transition, which can change order based on system parameters. We extend our framework, proposing an photonic systems which realizes an SO(N) symmetry breaking transition of the same nature as the membrane-in-the-middle system. While we have treated these systems in the classical limit, their open nature has pronounced effects when other noise sources are suppressed. We study the canonical optomechanical system to unravel the origin of the semiclassical force and potential on the mechanics. We find that this force, while conservative with respect to the mechanics, deeply depends on the quantum back-action due to photon loss from the cavity.

Additionally, we study the ability of cold atoms to sense rotation. We consider bosonic atoms confined to a one-dimensional ring. Employing Luttinger liquid theory to study the excitations, we find that in the strongly-repulsive regime, atomic currents can be manipulated and superposed by controlling a laser barrier. These superpositions provide a Heisenberg-limited rotation sensing method. When we include noise, the precision is reduced, but the performance still surpasses the standard quantum limit. We comment on the applicability of such a sensor for inertial sensing.

Dissertation Title: Student Sensemaking in Quantum Mechanics: Lessons to Teachers from Studies of Groupwork and Representation Use

Date and Time: Monday, December 11, 11:00 am

Location: PSC 3150

Dissertation Committee Co-Chairs: Profs. Andrew Elby and Ayush Gupta (advisor)

Committee:

Dr. Janet Walkoe
Dr. Kara Hoffman
Dr. Patricia Alexander

Abstract:

This dissertation covers two distinct threads of research; both threads focus on understanding student-thinking in quantum mechanics and then draw implications for future research and instruction. The primary goal of this collection of work is, in any way possible, to improve instruction and find ways to better support students in their learning.

The first thread of research focuses on tension negotiation in collaborative group problem-solving. While group-work has become more commonplace in physics classes, this research provides instructors some means of seeing just how complicated group dynamics can be. In particular, I highlight one interactional pattern through which students resolve tension emerging in group interaction by closing conversations or conversational topics. In doing so, students leave some conceptual line of reasoning unresolved. This work provides important insights into helping instructors understand and respond to group dynamics and conversational closings. The second thread of work focuses on flexible representation use. This thread has two similar lines of research. The first focuses on how particular representations (wavefunction and external potential graphs) associated with the infinite-well and finite-well potentials can be used by students as tools to learn with. Adapting these models to new situations can lead to deeper understandings of both the model being adapted and the new situation. In some cases, the process of adaptation is not impeded by the student lacking a sophisticated understanding of the model being adapted.

The second line of research on representation use focuses on the reflexiveness of student inquiry with representations. In reflexive reasoning, the student’s sensemaking shapes, and is shaped by, the representations they draw and animate. This form of inquiry stands in contrast with traditional notions of proficiency in using representations which tend to highlight reproducing standard representational forms and then reading-out information from those forms. In this work, I highlight how this non-linear, reflexive sense-making is supported by the development of coherent, coupled systems of representations and attention to particular figural features, leading to the generation of new meaning.

Dissertation Title: Innovative Scanning Probe Methods for Energy Storage Science: Elucidating the Physics of Battery Materials at the Nano-to-Microscale

Date and Time: Friday, December 8, 9:30 am

Location: CHM 0112 (Marker Seminar Room)

Dissertation Committee Chair: Prof. Theodore Einstein

Committee: 

Dr. Janice Reutt-Robey
Dr. Ellen Williams
Dr. James Williams
Dr. Sangbok Lee

Abstract: 

Energy storage research is uniquely positioned in modern science and technology. Advancements in the field (or lack thereof), will affect the future of humans, ecosystems, environments, and economies in a positive (or negative) way. While there has been decades-long progress in the energy storage solutions of everyday portable electronic devices, major energy storage hurdles persist such as grid-scale storage, and economically palatable, safe vehicular batteries. In an attempt to tackle these massive issues, the research community is looking beyond typical storage concepts, chemistries, electrolytes, and geometries. Many of these approaches make use of nanoscale and mesoscale technologies. For example, one such promising approach replaces conventional planar electrodes with a collection of nanostructures, arraigned in dense mesoscale architectures, aiming to increase key figures of merit, like power density (via nanostructures) and energy density (via dense mesoscale architectures). Regardless of the novel approach, new techniques are needed for characterization and scientific discovery at the nano-to-microscale. In this talk, I will discuss my PhD research which has precisely targeted these needs within the energy storage community. The work has resulted in the devolvement and application of innovative scanning probe approaches for basic energy storage discovery at the nano-to-microscale. Leveraging a new class of scanning probes, invented here, “battery probes” help to enable scanning nanopipette and probe microscopy, pascalammetry with microbattery probes, inverted scanning tunneling spectroscopy, and nanoscale solid-state electrochemistry with nanobattery probes.

Dissertation Title: Dynamics of Topological Defects in Hybrid Quantum Systems.

Date and Time: Thursday, December 7, 1:00 pm

Location: PSC 2136

Dissertation Committee Chair: Prof. Victor Galitski

Committee: 

Dr. Victor Yakovenko
Dr. Maissam Barkeshli
Dr. Ian Spielman
Dr. John Weeks

Abstract: 

This dissertation focuses on dynamics and transport effects of semiclassical topological defects in systems with important quantum degrees of freedom, which we term "hybrid quantum systems". The topological defects under consideration are skyrmions and magnetic vortices in layered heterostructures of three-dimensional (3D) topological insulators (TI) and magnetic materials, and dark solitons in Bose-Einstein condensates (BEC).

We examine the proximity effect between a 3D TI and two types of insulating magnets: a chiral magnet with a single skyrmion in a ferromagnetic background, and an XY-magnet with strong easy-plane anisotropy which undergoes a vortex unbinding transition. The skyrmion magnetic texture leads to confinement of Dirac states at the skyrmion radius, resulting in a charged skyrmion that can be manipulated by an external electric field. We show that the bound states are robust in the presence of an external magnetic field. Magnetic vortices in the XY-magnet affect electronic transport at the TI surface. Scattering at classical magnetic fluctuations influences surface resistivity of the TI, and near the transition temperature we find that the resistivity has a clear maximum and scales linearly with temperature on either side of the transition. We discuss the limits of mapping the TI XY-magnet model to the classic theoretical problem of free Dirac fermions in a random magnetic field.

Secondly, we study dark solitons in a BEC coupled to thermal non-interacting impurity atoms acting as a dissipative bath. We calculate the friction coefficient due to scattering and find that it can be tuned with accessible experimental probes. We develop a general theory of stochastic dynamics of the negative-mass dark soliton and solve the corresponding Fokker-Planck equation exactly. From the time-dependent phase-space probability distribution function we find the soliton can undergo Brownian motion only in the presence of friction and a confining potential. Finally, we numerically study the ground-state properties of a spin-1 BEC gas in the "synthetic dimensions" experimental set-up. Ground state phases depend on the sign of the spin-dependent interaction parameter and the strength of the spin-orbit field. We find "charge"- and spin-density-wave phases related to helical spin order.

Dissertation Title: An Experimental Realization of a Griffiths Phase in 87Rb in Three Dimensions

Date and Time: Wednesday, December 6, 12:00 pm

Location: PSC 1136

Dissertation Committee Chair: Prof. Steven Rolston

Committee:

Dr. Gretchen Campbell
Dr. Jay Deep Sau
Dr. Ian Spielman
Dr. Mohammad Hafezi

Abstract:

We describe a novel High Bandwidth Arbitrary Lattice Generator (HiBAL) we've created to skirt limits imposed on monochromatic standing waves of light. With its current iteration we can phase and amplitude modulate optical lattices over a broad range of wavevectors simultaneously at MHz frequencies. We characterize its behavior with a multi-Mach-Zehnder interferometer and a 0.5 NA diffraction limited imaging system, both designed and built in-house. We report lattice phase control to within a few parts in a thousand.

Disorder plays an important role in the phase diagrams of many materials. Crystal defects can cause exotic phases to coexist with the mundane in real world systems, and some phase diagrams are even dominated by the effects of disorder. We report the trapping and characterization of a Bose gas in an optical field isotropic in two dimensions and disordered in a third. We evaluate the phase diagram of our system as a function of temperature and disorder depth, and find favorable comparisons with indications of an intermediate Griffiths phase predicted by previous Monte Carlo and Renormalization Group studies separating 2D and 3D superfluid regimes.

Finally, I discuss the possibility of realizing the BKT transition in a non-orientable space. The BKT phase transition an infinite order phase transition in two dimensions from a normal gas to a superfluid mediated by vortices, which are orientable topological phase defects in two dimensions. I discuss the properties of vortices and their intractions on a Mobius strip, and describe how a relay-imaged bichromatic optical potential could be used to form a Mobius strip out of ultracold gases.

Dissertation Title: Studies of Ultracold Strontium Gases

Date and Time: Monday, November 20, 2:00 pm

Location: PSC 2136

Dissertation Committee Chair: Prof. Steven Rolston

Committee: 

Dr. Gretchen Campbell (advisor)
Dr. Trey Porto
Dr. Christopher Monroe
Dr. Amy Mullin

Abstract:

We describe the operation and performance of an ultracold strontium apparatus that is capable of generating quantum degenerate gases. The experiment has produced Bose-Einstein condensates (BECs) of 84Sr and 86Sr as well as degenerate Fermi gases (DFGs) of 87Sr with a reduced temperature of T/TF = 0.2 at a Fermi temperature of TF = 55 nK. Straightforward modifications could be made to allow for isotopic mixtures and BECs of the fourth stable isotope, 88Sr.

We also report on a technique to improve the continuous loading of a magnetic trap by adding a laser tuned to the 3P1 - 3S1 transition. The method increases atom number in the magnetic trap and subsequent cooling stages by up to 65% for the bosonic isotopes and up to 30% for the fermionic isotope of strontium. We optimize this trap loading strategy with respect to laser detuning, intensity, and beam size. To understand the results, we develop a one-dimensional rate equation model of the system, which is in good agreement with the data. We discuss the use of other transitions in strontium for accelerated trap loading and the application of the technique to other alkaline-earth-like atoms.

Finally, we also report on an updated investigation of photoassociation resonances relative to the 1S0 + 3P1 dissassociation limit in bosonic strontium. Multiple new resonances for 84Sr and 86Sr were measured out to binding energies of -5 GHz and several discrepancies in earlier measurements were resolved. These measurements will allow for the development of a more accurate mass-scaled model and a better theoretical understanding of the molecular potentials near the 3P1 state. We also measure the strength of the 84Sr 0u transitions in order to characterize their use as optical Feshbach resonances.

Dissertation Title: MAGNETIZED PLANE WAVE AND STRIPE-ORDERED PHASES IN SPIN-ORBIT-COUPLED BOSE GASES

Date and Time: Friday, November 17, 9:00 am

Location: 2115 CSS/ATL

Dissertation Committee Chair: Prof. Steven Rolston

Committee:

Dr. Ian B. Spielman (advisor)
Dr. Gretchen Campbell
Dr. Frederick Wellstood
Dr. Mohammad Hafezi

Abstract:

Quantum degenerate gases have provided rich systems to simulate engineered Hamiltonians and to explore quantum many-body problems in laboratory-scale experiments. In this work, we focus our study on spin-orbit-coupled (SOC) Bose-Einstein condensates (BECs) of Rubidium-87 atoms realized using two-photon Raman coupling scheme in which various novel phases are predicted to exist due to competing energies from the atomic internal structure, coupling strength, and many-body collisions.

BECs are observed primarily using the interaction between light and matter, where it is common to probe the atoms with near-resonant light and image their shadow on a camera. This absorption imaging technique measures the integrated column density of the atoms where it is crucial to focus the imaging system. I present a systematic method to bring the ultracold atom systems into an optimal focus using power spectral density (PSD) of the atomic density-density correlation function. The spatial frequency at which the defocus-induced artifacts first appear in the PSD is maximized on focus. The focusing process thus identifies the range of spatial frequencies over which the PSD is uncontaminated by finite-thickness effects.

Next, I describe magnetic phases which exist in spin-1 spin-orbit-coupled condensates in a near-zero temperature. We observe ferromagnetic and unmagnetized phases which are stabilized by the locking between the spin and linear momentum of the system. Our measurements of both the first- and second-order transitions are in agreement with theory.

Finally, I discuss the stripe-ordered phase which occurs in SOC Bose gases favoring the miscibility configuration. The stripe phase is theoretically predicted to have excitation spectrum analogous to that of supersolidity and to exhibit spatial density modulation within specific regions of parameter space. We use the optical Bragg scattering to probe any small density modulation present in the atomic spatial distribution. I present for the very first time the observation of the stripe phase in full phase diagrams. Our measurement results of the phase boundaries are consistent with existing theory and all observations to date.

Dissertation Title:​​ ​​CYCLOTRON RESONANCE GAIN IN THE PRESENCE OF COLLISIONS

Date and Time: ​​​​Wednesday, November 15, 10:00 am

Location: ​​​​IPT 1116 (IPST Conference Room)

Dissertation Committee Chair: ​​​​Prof. Thomas Antonsen

Committee:

Dr. Mohammad Hafezi
Dr. Edward Ott
Dr. Thomas E. Murphy
Dr. Wendell T. Hill

Abstract:

The conditions needed for the amplification of radiation by an ensemble of magnetized, relativistic electrons that are collisionally slowing down are investigated. The current study is aimed at extending the work of other researchers in developing solid-state sources of Terahertz radiation. The source type considered here is based on gyrotron-like dynamics of graphene electrons, or it can alternately be viewed as a solid state laser source which uses Landau levels as its band structure and is thus similar to a quantum cascade laser. Such sources are appealing because they offer the potential for a compact, tunable source of Terahertz radiation that could have commercial applications in scanning, communication, or energy transfer. An exploration is undertaken, using linear and nonlinear theories, of the conditions under which such sources might be viable, assuming realistic parameters. Classical physics is used, and the model involves electrons in graphene assumed to be pumped by a laser, follow classical laws of motion with the dissipation represented by a damping force term, and lose energy to the electromagnetic field as well. The graphene is assumed to be in a homogeneous magnetic field, and is sandwiched between two partially-transmissive mirrors so that the device acts as an oscillator.

This thesis incorporates the results of two approaches to the study of the problem. In the first approach, a linear model is derived semi-analytically, which is relevant to the conditions under which there is gain in the device and thus stable operation is possible, versus the regime in which there is no net gain. In the second approach, a numerical simulation is employed to explore the nonlinear regime and saturation behavior of the oscillator. The simulation and the linear model both assume the same original equations of motion for the field and particles that interact self-consistently. The model used here is very simplified, but the aim here is to elucidate the basic principles and scaling behavior of such devices, not necessarily to calculate what the exact dynamics, outputs, and parameters of a fully commercially realized device will be.

Dissertation Title: Energy Deposition in Femtosecond Filamentation: Measurements and Applications

Date and Time: Friday, November 3, 10:00 am

Location: ERF 1207 (IREAP large conference room)

Dissertation Committee Chair: ​​Prof. Howard Milchberg

Committee:

Dr. Phillip Sprangle
Dr. Gregory Nusinovich
Dr. Jared Wahlstrand
Dr. Ki-Yong Kim

Abstract:

Femtosecond filamentation is a nonlinear optical propagation regime of high peak power ultrashort laser pulses characterized by an extended and narrow core region of high intensity whose length greatly exceeds the Rayleigh range corresponding to the core diameter. Providing that a threshold power is exceeded, filamentation can occur in all transparent gaseous, liquid and solid media. In air, filamentation has found a variety of uses, including the triggering of electric discharges, spectral broadening and compression of ultrashort laser pulses, coherent supercontinuum generation, filament-induced breakdown spectroscopy, generation of THz radiation, and the generation of air waveguides. Several of these applications depend on the deposition of energy in the atmosphere by the filament. The main channels for this deposition are the plasma generated in the filament core by the intense laser field and the rotational excitation of nitrogen and oxygen molecules. The ultrafast deposition acts as a delta function-like pressure source to drive a hydrodynamic response in the air. This thesis experimentally demonstrates two applications of the filament-driven hydrodynamic response. One application is the ‘air waveguide’, which is shown to either guide a separately injected laser pulse, or act as a remote collection optic for weak optical signals. The other application is the high voltage breakdown of air, where the effect of filament-induced plasmas and hydrodynamic response on the breakdown dynamics is elucidated in detail. In all of these experiments, it is important to understand quantitatively the laser energy absorption; detailed absorption experiments were performed as a function of laser parameters. Finally, as check on simulations of filament propagation and energy deposition, we measured the axially resolved energy deposition of a filament; in the simulations, this profile is quite sensitive to the choice of the nonlinear index of refraction (​n2). We found that using our measured values of ​n2 in the propagation simulations results in an excellent fit to the measured energy deposition profiles.

Dissertation Title: SEARCH FOR NATURAL OCCURRING SUPERCONDUCTIVITY AND NOVEL PHENOMENA: MAGNETIC TRANSITIONS IN NATURAL TRANSITION METAL COMPOUNDS

Date and Time: Wednesday, October 25, 1:00 pm

Location: PHY 2126

Dissertation Committee Chair: Prof. Johnpierre Paglione

Committee: 

Dr. Richard Greene
Dr. Nicholas Butch
Dr. Efrain Rodriguez
Dr. Ichiro Takeuchi

Abstract:

​​Transition metal chalcogenides and transition metal arsenides are important families of natural mineral compounds widely distributed in the natural world. With similar structural and electronic properties of transition metal oxides, natural transition metal compounds are expected to have similar novel phenomena. With an ongoing project for searching natural superconductors in collaboration with Department of Mineral Science, Smithsonian National Museum of Natural History, we had a chance to investigate several natural minerals from the Smithsonian Museum in order to study previously unexpected naturally occurring mineral compounds for interesting ground states.

We found several interesting magnetic transitions in these natural occurring mineral samples. Some of the magnetic transitions are not reported, some of the transitions are associated with other unreported novel quantum phenomena. In this thesis, I will discuss Bornite (Cu5FeS4), Berthierite (FeSb2S4), Nagyagite (Pb5Au(Te,Sb)4S5 8), Maucherite (Ni11As8) and related experiments in detail.

Bornite (Cu5FeS4) has a semiconductor-insulator transition accompanied with an antiferromagnetic transition. As shown by our ability to tune the transition temperature and low-temperature metallicity by applying external pressure, Bornite may be a good candidate for Mott system and searching new superconductors.

Berthierite (FeSb2S4) is a quasi-1-dimensional antiferromagnet. With strong anisotropic physical properties, berthierite may provide a very good system for understanding the low dimensional magnetic material.

A Ferromagnetic order was found in natural Nagyagite (Pb5Au(Te,Sb)4S5 8) samples. The magnetic order, the weak anti-localization property with strong spin-orbital coupling and the 2-dimensional structure of this compound makes it a very interesting system for realizing topological properties in a natural compound.

The magnetic order and transitions in both natural and synthetic Maucherite (Ni11As8) samples show interesting finite-size scale effect. It gives us a different approach to understand the differences in some physical properties between natural and synthetic compounds.

Also, we will present a summary of other magnetic transitions and magnetic properties of more than 40 distinct minerals for this study and show the relation and similarities between strongly correlated transition metal oxide materials and other quantum materials. We will also make a list of other transition metal minerals that are worthy of investigation based on our research experience.

Dissertation Title: DEMONSTRATION OF A QUANTUM GATE WITH ULTRAFAST LASER PULSES

Date and Time: Monday, October 16, 3:00 pm

Location: PSC 2136

Dissertation Committee Chair: Prof. Christopher Monroe

Committee: 

Dr. Alexey Gorshkov
Dr. Mohammad Hafezi
Dr. Luis A. Orozco
Dr. Howard Milchberg

Abstract:

One of the major problems in building a quantum computer is the development of scalable and robust methods to entangle many qubits. Quantum computers based on trapped atomic ions are one of the most mature and promising platforms for quantum information processing, exhibiting excellent coherence properties, near-perfect qubit detection efficiency and high-fidelity entangling gates. Entangling operations between multiple ions in a chain typically rely on qubit state-dependent forces that modulate their Coulomb-coupled normal modes of motion. However, scaling these operations to large qubit numbers in a single chain must account for the increasing complexity of the normal mode spectrum, and can result in a gate time slowdown or added complexity of the control forces. In this thesis, I present an alternative route to the scalability problem using optical interactions faster than any state evolution. The experiments shown here represent a proof of principle for quantum manipulation of atoms in the strong coupling regime. This work relies on spin dependent forces (SDK) with short laser pulses and use it as our fundamental building block for thermometry and non-trivial motional state preparation. Together with a robust stabilization of the ion trap and high light collection efficiency, we demonstrate two-atom entanglement with ten ultrafast pulses. Due to the nature of the interaction, the demonstrated entangling operation can be made arbitrarily fast only limited by laser engineering.

Dissertation Title: Bridging quantum, classical and stochastic shortcuts to adiabaticity

Date and Time: Friday, September 15, 1:00 pm

Location: IPT 1116 (IPST conference room)

Dissertation Committee Chair: Prof. Christopher Jarzynski

Committee: 

Dr. Alexey Gorshkov
Dr. Rajarshi Roy
Dr. Victor Yakovenko
Dr. Perinkulam Krishnaprasad

Abstract: 

Adiabatic invariants -- quantities that are preserved under the slow driving of a system's external parameters -- are important in classical mechanics, quantum mechanics and thermodynamics. Adiabatic processes allow a system to be guided to evolve to a desired final state. However, the slow driving of a quantum system makes it vulnerable to environmental decoherence, and for both quantum and classical systems, it is often desirable and time-efficient to speed up a process. Shortcuts to adiabaticity are strategies for preserving adiabatic invariants under rapid driving, typically by means of an auxiliary field that suppresses excitations, otherwise generated during rapid driving. Several theoretical approaches have been developed to construct such shortcuts. In this dissertation we focus on two different approaches, namely counterdiabatic driving and fast-forward driving, which were originally developed for quantum systems. The counterdiabatic approach introduced independently by Dermirplak and Rice [J. Phys. Chem. A, 107:9937, 2003], and Berry [J. Phys. A: Math. Theor., 42:365303, 2009] formally provides an exact expression for the auxiliary Hamiltonian, which however is abstract and difficult to translate into an experimentally implementable form. By contrast, the fast-forward approach developed by Masuda and Nakamura [Proc. R. Soc. A, 466(2116):1135, 2010] provides an auxiliary potential that may be experimentally implementable but generally applies only to ground states.

The central theme of this dissertation is that classical shortcuts to adiabaticity can provide useful physical insights and lead to experimentally implementable shortcuts for analogous quantum systems. We start by studying a model system of a tilted piston to provide a proof of principle that quantum shortcuts can successfully be constructed from their classical counterparts. In the remainder of the dissertation, we develop a general approach based on flow-fields which produces simple expressions for auxiliary terms required for both counterdiabatic and fast-forward driving. We demonstrate the applicability of this approach for classical, quanutum as well as stochastic systems. We establish strong connections between counterdiabatic and fast-forward approaches, and also between shortcut protocols required for classical, quantum and stochastic systems. In particular, we show how the fast-forward approach can be extended to highly excited states of quantum systems.

Dissertation Title: THE EFFECTS OF GEOMETRY AND PATCH POTENTIALS ON CASIMIR FORCE MEASUREMENTS

Date and Time: Friday, August 18, 10:00 am

Location: ERF 1207 (IREAP large conference room)

Dissertation Committee Chair: Prof. Jeremy Munday

Committee:

Dr. Ian Applebaum
Dr. Victor Galitski
Dr. Min Ouyang
Dr. Jan Sengers

Abstract: 

Electromagnetic fluctuations of the quantum vacuum cause an attractive force between surfaces, called the Casimir force. In this dissertation, the first Casimir force measurements between two gold-coated spheres are presented. The proximity force approximation (PFA) is typically used to compare experiment to theory, but it is known to deviate from the exact calculation far from the surface. Bounds are put on the size of possible deviations from the PFA by combining several sphere-sphere and sphere-plate measurements.

Electrostatic patch potentials have been postulated as a possible source of error since the first Casimir force measurements sixty years ago. Over the past decade, several theoretical models have been developed to characterize how the patch potentials contribute an additional force to the measurements. In this dissertation, Kelvin probe force microscopy (KPFM) is used to determine the effect of patch potentials on both the sphere and the plate. Patch potentials are indeed present on both surfaces, but the force calculated from the patch potentials is found to be much less than the measured force. In order to better understand how KPFM resolves patch potentials, the artifacts and sensitivities of several different KPFM implementations are tested and characterized. In addition, we introduce a new technique, called tunable spatial resolution (TSR) KPFM, to control resolution by altering the power-law separation dependence of the KPFM signal.

Dissertation Title: Optical properties of a quantum-noise limited phase-sensitive amplifier

Date and Time: Thursday, August 10, 2:00 pm

Location: PSC 2136

Dissertation Committee Co-Chairs: Prof. Steven Rolston and Prof. Paul Lett

Committee:

Dr. William Phillips
Dr. Gretchen Campbell
Dr. Julius Goldhar

Abstract:

This thesis is a summary of investigations on the optical properties of the phase-sensitive and phase-insensitive amplifiers. Both optical amplifiers are implemented using four-wave mixing in Rb 85 atomic vapor based on a double-lambda level scheme.

We first study the effects of input phase and amplitude modulation on the output of a quantum-noise-limited phase-sensitive amplifier (PSA). We investigate the dependence of phase modulation imposed on a signal by an acousto-optical modulator on its alignment, and demonstrate a novel approach to quantifying the phase modulation by using the PSA as a diagnostic tool. We then use this method to measure the alignment-dependent phase modulation produced by an optical chopper, which arises due to diffraction effects as the chopper blade passes through the optical beam.

The phase-insensitive amplifier (PIA) has been proven to be a reliable source of two-mode squeezed light, and of quantum entanglement. In this dissertation, the PIA is utilized to generate a two-mode squeezed state, i.e., a twin-beam state, and is also used as a gain-assisted anomalous dispersion medium. We experimentally demonstrate the ability of a PSA to pre-amplify quantum correlations in twin light beams before degradation due to loss and detector inefficiency. By including a PSA before loss, one is able to preserve the correlations as well as the two-mode squeezing level. We compare the results to simulations employing a simple quantum-mechanical model and find a good agreement.

We have demonstrated that the cross-correlation between the two modes of a bipartite entangled state can be advanced by propagation through a fast-light medium. The extra noise added by a PIA has been speculated to be the mechanism that limits the advance of entanglement, preventing the mutual information from traveling superluminally. As an extension of this phase-insensitive gain assisted anomalous dispersion investigation, we explore the advance and delay of information transmitted through the PSA. We start with a two-mode squeezed state created by the PIA and measure the mutual information shared by the correlated quadratures. We then pass one of these two modes through a PSA and investigate the shift of the mutual information as a function of the PSA phase. In the case of a PSA, it is well known that no extra noise will be added to the quadrature with the correct input phase (e.g., the quadrature with the maximal amplification or the maximal deamplification). We find that there is no dispersion-like behavior at these two phases, however, the peak of mutual information could either be delayed or advanced at any other phase. We also observe an almost identical behavior when we input an amplitude modulated signal to the PSA. We are able to explain the physics of this "quasi-fast-and-slow-light" behavior utilizing a theoretical framework with distributed gain on the carrier and both positive and negative side bands but with distributed loss only on the negative side band. We obtain a good agreement between the experimental results and the theoretical simulations.

Dissertation Title: Classical and quantum dynamics of Bose-Einstein condensates

Date and Time: Thursday, August 10, 1:00 pm

Location: CSS 2115

Dissertation Committee Chair: Prof. Jay Deep Sau

Committee:

Dr. Eite Tiesinga (advisor)
Dr. Jacob Taylor
Dr. William Dorland
Dr. Dionisios Margetis

Abstract:

After the first experimental realization of a Bose-Einstein condensate (BEC) in 1995, BECs have become a subject of intense experimental and theoretical study. In this dissertation, I present our results on the classical and quantum dynamics of BECs at zero temperature under different scenarios.

First, I consider the analog of slow light in the collision of two BECs near a Feshbach resonance. The scattering length then becomes a function of the collision energy. I derive a generalization of the Gross-Pitaevski equation for incorporating this energy dependence. In certain parameter regimes, the group velocity of a BEC traveling through another BEC decreases. I also study the feasibility of an experimental realization of this phenomena.

Second, I analyze an experiment in which a BEC in a ring-shaped trap is stirred by a rotating barrier. The phase drop across and current flow through the barrier is measured from spiral-shaped density profiles created by interfering the BEC in the ring-shaped trap and a concentric reference BEC after release from all trapping potentials. I show that a free-particle expansion is sufficient to explain the origin of the spiral pattern and relate the phase-drop to the geometry of a spiral. I also bound the expansion times for which the phase-drop can be accurately determined and study the effect of inter-atomic interactions on the expansion time scales.

Third, I study the dynamics of few-mode BECs when they become dynamically unstable after preparing an initial state at a saddle point of the Hamiltonian. I study the dynamics within the truncated Wigner approximation (TWA) and find that due to phase-space mixing, the expectation value of an observable relaxes to a steady-state value. Using the action-angle formalism, we derive analytical expressions for the steady-state value and the time evolution towards this value. We apply these general results to two systems: a condensate in a double-well potential and a spin-1 (spinor) condensate.

Finally, I study quantum corrections beyond the TWA in the semiclassical limit. I derive general expressions for the dynamics of an observable by using the van Vleck-Gutzwiller propagator and find that the interference of classical paths leads to non-perturbative corrections. As a case study, I consider a single-mode nonlinear oscillator; this system displays collapse and revival of observables. We find that the interference of classical paths, which is absent in the TWA, leads to revivals.

Dissertation Title: Optical and magnetic measurements of a levitated, gyroscopically stabilized graphene nanoplatelet

Date and Time: Tuesday, August 1, 2:00 pm

Location: PSC 3150

Dissertation Committee Chair: Prof. Frederick Wellstood

Committee:

Dr. Richard Greene
Dr. Christopher Lobb
Dr. Bruce Kane (advisor)
Dr. Ichiro Takeuchi

Abstract:

Levitation of nanoscale particles is an increasingly popular technique in studies ranging from the investigation of material properties to fundamental tests of quantum mechanics. Two-dimensional materials have been extensively studied while attached to substrates, but have rarely been levitated. This work will discuss the design of a system for levitating a charged, micron-scale, multilayer graphene nanoplatelet in a quadrupole electric field trap in high vacuum, enabling sensitive mechanical and magnetic measurements.

The levitated nanoplatelet is gyroscopically stabilized by locking its frequency of rotation to an applied radio frequency (rf) electric field. The stabilized nanoplatelet is extremely sensitive to external torques, and the residual slow dynamics of the direction of the axis of rotation are determined by an applied magnetic field. Optical data on the interaction of the platelet with the magnetic field are presented. Two mechanisms of interaction are observed: a diamagnetic polarizability and a magnetic moment proportional to the frequency of rotation.

Dissertation Title: Left-right symmetric model and its TeV-scale phenomenology

Date and Time: Wednesday, July 26, 1:00 pm

Location: PSC 3150

Dissertation Committee Chair: Prof. Rabindra Mohapatra

Committee:

Dr. Raman Sundrum
Dr. Sarah Eno
Dr. Kaustubh Agashe
Dr. Niranjan Ramachandran

Abstract:

The Standard Model of particle physics is a chiral theory with a broken parity symmetry, and the left-right symmetric model is an extension of the SM with the parity symmetry restored at high energies. Its extended particle content allows us not only to find the solution to the parity problem of the SM but also to solve the problem of understanding the neutrino masses via the seesaw mechanism. If the scale of parity restoration is in the few TeV range, we can expect new physics signals that are not present in the Standard Model in planned future experiments. We investigate the TeV-scale phenomenology of the various classes of left-right symmetric models, focusing on the charged lepton flavour violation, neutrinoless double beta decay, electric dipole moments of charged leptons, and leptogenesis.

Dissertation Title: A NATURAL EXTENSION OF STANDARD WARPED HIGHER-DIMENSIONAL COMPACTIFICATIONS: THEORY AND PHENOMENOLOGY

Date and Time: Thursday, July 13, 11:00 am

Location: PSC 3150

Dissertation Committee Chair: Prof. Kaustubh Agashe

Committee:

Dr. Raman Sundrum
Dr. Rabindra Mohapatra
Dr. Sarah Eno
Dr. Zackaria Chacko
Dr. Richard Wentworth

Abstract:

Warped higher-dimensional compactifications with “bulk” standard model, or their AdS/CFT dual as the purely 4D scenario of Higgs compositeness and partial compositeness, offer an elegant approach to resolving the electroweak hierarchy problem as well as the origins of flavor structure. However, low-energy electroweak/flavor/CP constraints and the absence of non-standard physics at LHC Run 1 suggest that a “little hierarchy problem” remains, and that the new physics underlying naturalness may lie out of LHC reach. Assuming this to be the case, we show that there is a simple and natural extension of the minimal warped model in the Randall-Sundrum framework, in which matter, gauge and gravitational fields propagate modestly different degrees into the IR of the warped dimension, resulting in rich and striking consequences for the LHC (and beyond).

The LHC-accessible part of the new physics is AdS/CFT dual to the mechanism of “vectorlike confinement”, with TeV-scale Kaluza-Klein excitations of the gauge and gravitational fields dual to spin-0,1,2 composites. Unlike the minimal warped model, these low-lying excitations have predominantly flavor-blind and flavor/CP-safe interactions with the standard model. In addition, the usual leading decay modes of the lightest KK gauge bosons into top and Higgs bosons are suppressed. This effect permits erstwhile subdominant channels to become significant. These include flavor-universal decays to SM fermions and Higgs bosons, and a novel channel — decay to a radion and a SM gauge boson, followed by radion decay to a pair of SM gauge bosons.

Remarkably, this scenario also predicts small deviations from flavor-blindness originating from virtual effects of Higgs/top compositeness at ∼ O(10) TeV, with subdominant resonance decays into Higgs/top-rich final states, giving the LHC an early “preview” of the nature of the resolution of the hierarchy problem. Discoveries of this type at LHC Run 2 would thereby anticipate (and set a target for) even more explicit explorations of Higgs compositeness at a 100 TeV collider, or for next-generation flavor tests.

Dissertation Title: HYDRODYNAMIC AND ELECTRODYNAMIC IMPLICATIONS OF OPTICAL FEMTOSECOND FILAMENTATION

Date and Time: Thursday, July 7, 10:00 am

Location: ERF 1207 (IREAP large conference room)

Dissertation Committee Chair: Prof. Howard Milchberg

Committee:

Dr. Phillip Sprangle
Dr. James Drake
Dr. Daniel Lathrop
Dr. Ki-Yong Kim

Abstract:

Filamentation is a nonlinear propagation mode of high peak power optical pulses in dielectric media, where nonlinearities arising from the interaction between light and matter can overcome diffraction, resulting in the self-channeling of the pulse. The large optical field present in filaments enables efficient nonlinear conversion for electromagnetic sources spanning from terahertz to x-rays. Filamentation in air is of particular interest for remote applications, where filaments are unique in their ability to deliver high field intensities (up to 10^14 W/cm^2) at kilometer distances, allowing for ranged applications such as LIDAR and laser induced breakdown spectroscopy. This dissertation presents research exploring two topics: i) how to use filaments to inscribe optical guiding structures into the air with millisecond lifetimes, enabling the generation of line-of-sight atmospheric waveguides, and ii), the discovery of the spatiotemporal optical vortex (STOV), a new and previously unobserved type of optical vortex present in all filamenting beams.

Dissertation Title: COMPACT LASER-DRIVEN ELECTRON AND PROTON ACCELERATION WITH LOW ENERGY LASERS

Date and Time: Thursday, May 25, 3:30 pm

Location: ERF 1207 (IREAP large conference room)

Dissertation Committee Chair: Prof. Howard Milchberg

Committee:

Dr. Gretchen Campbell
Dr. Phillip Sprangle
Dr. Timothy Koeth
Dr. Ki-Yong Kim

Abstract:

Laser-driven particle accelerators offer many advantages over conventional particle accelerators. The most significant of these is the magnitude of the accelerating gradient and, consequently, the compactness of the accelerating structure. In this dissertation, experimental and computational advances in laser-based particle acceleration in three intensity regimes are presented. All mechanisms investigated herein are accessible by "tabletop" ultrashort terawatt-class laser systems found in many university labs, with the intention of making them available to more compact and high repetition rate laser systems. The first mechanism considered is the acceleration of electrons in a preformed plasma "slow-wave" guiding structure. Experimental advances in the generation of these plasma guiding structures are presented. The second mechanism is the laser-wakefield acceleration of electrons in the self-modulated regime. A high-density gas target is implemented experimentally leading to electron acceleration at low laser pulse energy. Consequences of operating in this regime are investigated numerically. The third mechanism is the acceleration of protons by a laser-generated magnetic structure. A numerical investigation is performed identifying operating regimes for experimental realizations of this mechanism.

Dissertation Title: Investigation of Graphene and other Low Dimensional Materials

Date and Time: Thursday, May 11, 4:00 pm

Location: PSC 3402

Dissertation Committee Chair: Prof. Ellen Williams

Committee:

Dr. Min Ouyang
Dr. Theodore Einstein
Dr. Michael Fuhrer
Dr. Janice Reutt-Robey

Abstract:

This thesis describes experiments to characterize defects in two-dimensional materials and understand their effect on electrical conductivity. Defects limit the electrical conductivity through a material by scattering electrons. Understanding the physics of defects is therefore essential to building materials and structures with novel electronic properties. This dissertation has focused on low dimensional materials because they are simple thereby allowing for more advanced theory and they will act as a foundation for understanding higher dimensional systems.

High resolution x-ray photoelectron spectroscopy (XPS) and near edge x-ray absorption fine structure spectroscopy (NEXAFS) were used to determine the character of vacancy defects in graphene. Vacancies were induced in graphene on a thermally oxidized silicon substrate using argon ion bombardment. XPS of the carbon 1s core level of pristine graphene shows a C 1s spectrum consistent with a single C 1s peak broadened both instrumentally and by a Doniach-Sunjic type effect. As defects are created, the resulting spectrum is deconvolved into two peaks. The first retains the same spectral width as that for the pristine graphene but with a reduced intensity. The second peak, which is broader and at a slightly higher binding energy (~200 meV), increases in intensity with increasing defect concentration. This second peak is identified as the experimental XPS signature of defective graphene. The observation is somewhat at odds with theoretical calculations of XPS spectra for graphene with various vacancy arrangements, which generally produce C 1s peaks shifted to lower binding energy. Instead, the emergence of this second peak, together with the emergence of a single sharp resonance seen near the vacuum level in the NEXAFS spectra, is interpreted as a distribution of molecular-like states forming on the surface.

Preliminary efforts were made to characterize defects in semiconducting single layer MoS2 using scanning tunneling microscopy (STM) and spectroscopy (STS). Techniques for obtaining a clean MoS2 surface suitable for ultra-high vacuum STM were developed, and preliminary characterization of the single layer WS2 surface by STM and STS was carried out. The local density of states of MoS2, as measured by STS, shows the semiconducting bandgap as well as signatures of donor and acceptor states within the gap.

Dissertation Title: Becoming a Physicist: How Identities and Practices Shape Physics Trajectories

Date and Time: Thursday, April 27, 2:30 pm

Location: PHY 1305F (the Toll Room)

Dissertation Committee Co-Chairs: Prof. Andrew Elby and Prof. Chandra Turpen

Committee:

Dr. Edward Redish
Dr. Ayush Gupta
Dr. James Williams
Dr. Derek Richardson

Abstract:

This dissertation studies the relationships and processes which shape students' participation within the discipline of physics. Studying this early disciplinary participation gives insight to how students are supported in or pushed out of physics, which is an important step in cultivating a diverse set of physics students. This research occurs within two learning environments that we co-developed: a physics camp for high school girls and a seminar for undergraduate physics majors to get started in physics research. Using situated learning theory, we conceptualized physics learning to be intertwined with participation in physics practices and identity development. This theoretical perspective draws our attention to relationships between students and the physics community. Specifically, we study how students come to engage in the practices of the community and who they are within the physics community. We find that students' interactions with faculty and peers impact the extent to which students engage in authentic physics practices. These interactions also impact the extent to which students develop identities as physicists. We present implications of these findings for the design of physics learning spaces. Understanding this process of how students become members of the physics community will provide valuable insights into fostering a diverse set of successful trajectories in physics.

Dissertation Title: MANIPULATION OF THE QUANTUM MOTION OF TRAPPED ATOMIC IONS VIA STIMULATED RAMAN TRANSITIONS

Date and Time: Monday, April 24, 12:15 pm

Location: CSS 2115 (Atlantic Building)

Dissertation Committee Chair: Prof. Christopher Monroe

Committee:

Dr. Gretchen Campbell
Dr. Luis Orozco
Dr. James Williams
Dr. Andrew Childs

Abstract:

Trapped ions have been a staple resource of quantum simulation for the past several decades. By taking advantage of the spin motion coupling provided by the Coulomb interaction, trapped ions have been used to study quantum phase transitions of highly frustrated spins, many body localization, as well as discrete time crystals. However, all of these simulations involve decoupling the ion motion from spin at the end of the experimental procedure. Here we present progress towards driving bosonic interference between occupied phonon modes of vibration.

This thesis details a tool box for manipulating the motional states of a chain of trapped ions. Taking advantage of spin motion interaction of tightly trapped chains of Yb ions with two photon Raman transition, we will show how to prepare a specific number state of a given normal mode of motion. This is achieved without traditional individual addressing but instead by using composite pulse sequences and ion transport. This will involve a stage of quantum state distillation, and we also show preservation of phonon and spin coherence after this distillation step. This Fock state preparation sets the stage to observe bosonic interference of different phonon modes.

We will use stimulated Raman transitions to create a parametric drive; this drive will couple different normal modes of motion. To observe the bosonic nature of the phonons, we perform a Hong-Ou-Mandel (HOM) interference experiment on two singly occupied normal modes. We use the same spin motion coupling to read out the spin states of individual ions as a witness for this interaction. We also describe a process to use stimulated rapid adiabatic passage (STIRAP) to read out normal mode occupation. The toolbox presented here will be useful for future experiments towards boson sampling using trapped ions.

Dissertation Title: NITROGEN-VACANCY COUPLING IN NANODIAMOND HYBRID NANOSTRUCTURES

Date and Time: Monday, April 10, 2:00 pm

Location: PHY 1305F (John S. Toll Physics Building)

Committee Chair: Prof. Min Ouyang

Committee:

Dr. Steven Anlage
Dr. Christopher Lobb
Dr. Luis Orozco
Dr. John Cumings

Abstract:

Nitrogen-vacancy centers (NVs) are an atomic defect in diamond which possess remarkable fluorescence and spin properties which can be used for multiple metrological applications, particularly when the NV is hosted in nanodiamond, which can be easily integrated with a variety of nanoscale systems. A new class of nanodiamond hybrid nanostructures was developed using bottom-up synthesis methods. In this work, coupling between NV centers and plasmonic, excitonic and magnetic nanoparticles in these nanodiamond hybrid nanostructures is investigated using fluorescence lifetime measurements, spin relaxometry measurements and modeled using finite element method (FEM) and Monte Carlo simulations. This work not only characterizes the properties of these nanodiamond-hybrid nanostructures but also facilitates design guidelines for future hybrid structures with enhanced metrological and imaging capabilities.

Dissertation Title: Topological Superconductivity and Majorana Zero Modes

Date and Time: Friday, April 7, 1:00 pm

Location: PHY 2205 (John S. Toll Physics Building)

Committee Chair: Prof. Sankar Das Sarma

Committee:

Dr. Jay Deep Sau
Dr. Maissam Barkeshli
Dr. Theodore Einstein
Dr. Christopher Jarzynski

Abstract:

Recent years have seen a surge interest in realizing Majorana zero modes in condensed matter system. Majorana zero modes are zero-energy quasiparticle excitations which are their own anti-particles. The topologically degenerate Hilbert space and non-Abelian statistics associated with Majorana zero modes renders them useful for realizing topological quantum computation. These Majorana zero modes can be found at the boundary of a topological superconductor. While preliminary evidence for Majorana zero modes in form of zero-bias conductance peaks have already been observed, confirmatory signatures of Majorana zero modes are still lacking.

In this thesis, we theoretically investigate several signatures of Majorana zero modes, thereby suggesting improvement and directions that can be pursued for an unambiguous identification of the Majorana zero modes. We begin by studying analytically the differential conductance of the normal-metal--topological superconductor junction across the topological transition within the Blonder-Tinkham-Klapwijk formalism. We show that despite being quantized in the topological regime, the zero-bias conductance only develops as a peak in the conductance spectra for sufficiently small junction transparencies, or for small and large spin-orbit coupling strength. We proceed to investigate the signatures of the Majorana zero modes in superconductor--normal-metal--superconductor junctions and show that the conductance quantization in this junction is.not robust against increasing junction transparency. Finally, we propose a dynamical scheme to study the short-lived topological phases in ultracold systems by first preparing the systems in its long-lived non-topological phases and then driving it into the topological phases and back. We find that the excitations' momentum distributions exhibit Stuckelberg oscillations and Kibble-Zurek scaling characteristic of the topological quantum phase transition, thus provides a bulk probe for the topological phase.

Dissertation Title: Quantum Optics in Optical Nanofibers

Date and Time: Friday, March 31, 9:00 am

Location: PSC 2136

Committee Chair: Prof. Luis Orozco

Committee:

Dr. Mohammad Hafezi
Dr. Steven Rolston
Dr. William Phillips
Dr. Jeremy Munday
Dr. Edo Waks

Abstract:

The study of atom-light interaction is the core of quantum optics and a central part of atomic physics. Systems composed of atoms interacting among each other through the electromagnetic field can be used from fundamental research to practical applications. Experimental realizations of these systems benefit from three distinct attributes: large atom-light coupling, trapping an control of atomic ensembles, and engineering and manipulation of the electromagnetic field. Optical waveguides provide a platform that achieves these three goals. In particular, optical nanofibers are an excellent candidate. They produce a high confinement of the electromagnetic field that improves atom-light coupling, guiding the field that mediates the interactions between atoms, while allowing trapping of the atoms close to it.

This thesis uses an optical nanofiber for quantum optics experiments, demonstrating its possibilities for enabling special atom-light interactions. We trap atoms near the optical nanofiber surface, and characterize the trap in a non-destructive manner. We show how the presence of the nanofiber modifies the fundamental atomic property of spontaneous emission, by altering the electromagnetic environment of the atom. Finally, we use the nanofiber to prepare collective states of atoms around it. These states can radiate faster or slower than a single atom (super and subradiance). The observation of subradiance of a few atoms, a rather elusive effect, evidences nanofibers as a strong candidate for future quantum optics experiments. Moreover, we show how the guided field mediates interaction between atoms hundreds of wavelengths apart, creating macroscopically delocalized collective states.

Dissertation Title: Modeling strong-field laser-atom interactions with nonlocal potentials

Date and Time: Friday, January 20, 3:00 pm

Location: PSC 3150

Committee Chair: Prof. Thomas Antonsen

Committee: 

Dr. Phillip Sprangle
Dr. Mohammad Hafezi
Dr. Rajarshi Roy
Dr. Ki-Yong Kim

Abstract:

Atom-field interactions in the ionization regime give rise to a wide range of physical phenomena, and their study continues to be an active field of research. Many process are may be explained by modeling the time-dependent Schrodinger equation; however, simulation in the strong field regime is computationally expensive and time-consuming. Here, a nonlocal model potential replaces the Coulomb potential in the time dependent Schrodinger equation, and examined for suitability of modeling strong field-atom dynamics while significantly reducing computation time.

Nonlocal potentials have been used to model many quantum mechanical systems, from multi-electron molecular configurations to semiconductor theory. Despite their relative success, nonlocal potentials are largely unexplored for modeling high field laser-gas interactions in the ionizing regime. This work examines theory and numerical results of a gaussian nonlocal model atom in intense, femtosecond laser pulses, with main findings: Nonlocal potentials are useful for obtaining the photoionization rate in the tunnel and multiphoton regimes, and qualitatively characterize electron wavefunction dynamics of irradiated atoms. The model is used to explore the two-color technique for producing Terahertz (THz) frequency radiation.