In recent years, a great deal of progress has been made in understanding the ionization processes that result from the interaction of an intense laser pulse with multielectron atoms. However, due to experimental limitations, the effect of the laser field's spatial dependence on strong-field processes has rarely been investigated. Presented in this work is a theoretical analysis of this spatial dependence including a proposal for an experimentally observable result of the phenomenon. We begin by outlining the elements of the laser field that will vary as a function of position and show their effects on simple free electron trajectories. We then develop a classical, three-dimensional simulation of the entire process of double ionization of helium in an intense laser field using realistic, non-paraxial focal conditions. The existence of an out-of-phase electric field component in the laser propagation direction is determined, which produces an effective longitudinal ellipticity, resulting in a reduction in the double ion yields as a function of position in the laser focus. It is found that under conditions of tight focusing, the effective focal volume for non-sequential double ionization is significantly reduced.

The recent prospect of efficient, reliable, and secure quantum communication relies on the ability to coherently and reversibly map nonclassical states of light onto long-lived atomic states. A promising technique that accomplishes this employs Electromagnetically Induced Transparency (EIT), in which a strong classical control field modifies the optical properties of a weak signal field in such a way that a previously opaque medium becomes transparent to the signal field. The accompanying steep dispersion in the index of refraction allows for pulses of light to be decelerated, then stored as an atomic excitation, and later retrieved as a photonic mode. This dissertation presents the results of investigations into methods for optimizing the memory efficiency of this process in an ensemble of hot Rb atoms. We have experimentally demonstrated the effectiveness of two protocols for yielding the best memory efficiency possible at a given atomic density. Improving memory efficiency requires operation at higher optical depths, where undesired effects such as four-wave mixing (FWM) become enhanced and can spontaneously produce a new optical mode (Stokes field). We present the results of experimental and theoretical investigations of the FWM-EIT interaction under continuous-wave (cw), slow light, and stored light conditions. In particular, we provide evidence that indicates that while a Stokes field is generated upon retrieval of the signal field, any information originally encoded in a seeded Stokes field is not independently preserved during the storage process. We present a simple model that describes the propagation dynamics and provides an intuitive description of the EIT-FWM process.

At the Thomas Jefferson National Accelerator Facility, glass target cells containing a high density of highly polarized 3He nuclei are used in electron scattering experiments studying the substructure of the neutron. In addition to 3He, these cells contain a small amount of rubidium (Rb), potassium (K), and nitrogen (N2), which facilitate the polarization process. The work presented here represents studies of the interactions between the alkali vapor and 3He nuclei when both are polarized and unpolarized.;Our investigations into the mechanisms responsible for the relaxation of the 3He polarization have measured unusually large polarization losses. In addition, most cells studied exhibited polarization lifetimes much shorter than those typically observed in cells used for scattering experiments. These results suggest there are relaxation mechanisms that depend on whether the cell contains polarized or unpolarized alkali vapor, solid alkali, or no alkali. Previous cell studies have assumed these relaxation mechanisms are independent of the presence of alkali in any form. Modication of the polarization rate equations to include these new relaxation mechanisms are given. Further studies are needed to fully understand the origin of these additional relaxation mechanisms.;Studies of the interactions between 3He and alkali vapor, when both are unpolarized, were motivated by the need to determine the number density of 3He inside sealed cells. The system we have implemented to measure the number density examines the broadening of the absorption profiles of the D1 and D2 lines of Rb and K due to collisions with 3He and N2. However, in order to relate this broadening to the gas density, the value of the velocity-averaged collisional cross-section (broadening coefficient) for the interacting pair of atoms must be known. While the value of the coefficient has been measured for Rb, no data have been published for K interacting with 3He at the high number densities required for scattering experiments. Furthermore, pressure broadening theory predicts a temperature dependence for the coefficients, but very little experimental data has been published. In addition to broadening, a shift in the central frequency is also predicted and has been experimentally verified. We have measured both the broadening and shift of the D1 and D2 lines of Rb and K in the presence of 3He and N2 over a range of number densities and temperatures.

This experiment measured the time distribution of muonic hydrogen atoms which were formed when negative muons were brought to rest in H{dollar}\sb2{dollar} gas, containing Au target foils, at five pressures (750 mbar, 375 mbar, 188 mbar, 94 mbar and 47 mbar at 4.6 mm foil spacing). A Monte Carlo method is applied for deducing the initial velocity distribution, and preliminary results are obtained. The initial velocity distribution of {dollar}\mu{dollar}H atoms is reasonably well described as a 'Maxwellian' velocity distribution with a mean energy E = 3.4 eV. The corresponding muon mean capture energy is obtained: E{dollar}\sb{lcub}\rm c{rcub}{dollar} {dollar}\approx{dollar} 34 eV for {dollar}\mu{dollar}H atom and E{dollar}\sb{lcub}\rm c{rcub}{dollar} {dollar}\approx{dollar} 68 eV for {dollar}\mu{dollar}H{dollar}\sb2{dollar} molecules. We also find the negative muon capture energy distribution is exponential.;In addition, a significant improvement of the negative muon mean life {dollar}\tau{dollar} in Au is abtained in this experiment.: {dollar}\tau\sb{lcub}\rm Au{rcub}{dollar} = 69.716 {dollar}\pm{dollar} 0.144 ns. The "full decay curve fitting method" which we use in this experiment has an advantage over the previous method in three aspects: (1) We have measured the mean life and determined the time resolution {dollar}\sigma{dollar}(E) of a detector at a particular energy level; (2) We have determined the effective zero time of the decay curve; (3) We have provided a possible way to measure the mean life {dollar}\tau{dollar} when {dollar}\tau{dollar} is less than the time resolution {dollar}\sigma{dollar}(E) of the detector ({dollar}\tau{dollar} {dollar}<{dollar} {dollar}\sigma{dollar}(E)).

Negative muons were brought to rest in a gas mixture of 30 torr CH$\sb4$ and 570 torr D$\sb2$, using the cyclotron trap at PSI. The muons formed muonic deuterium atoms which diffused through the mixture and transferred their muons to the carbon of the CH$\sb4$ molecules. A planar germanium detector and a silicon detector were used to observe x-rays from the initial muon cascade in the deuterium, and from subsequent cascade in the muonic carbon after transfer. A transfer rate of (4.5 $\pm$ 1.8) $\times$ 10$\sp{10}$/sec was found which agrees well with a previous result measured at 50 bar of (5.1 $\pm$ 1.0) $\times$ 10$\sp{10}$/sec. Transfer was found to occur predominantly to the n = 4 state in $\mu$C. The initial angular momentum state distribution in the $\mu$C was constructed using the cascade program of V. Markushin, and found to be consistent with any combination of two possible initial distributions (I 0.252 (4s) + 0.409 (4f) + 0.339 (4p)) and (II 0.284 (4d)+ 0.377 (4f) + 0.339 (4p)). The transfer theories of Gershtein and that of Holtzwarth and Pfeifer both agree well with the measured transfer rate and initial energy state, but not with the initial angular momentum distributions. The 2S population in $\mu$C was also determined to lie between 5% and 11%, which is higher than the 3% population in direct capture.

Ubiquitous in nature, light-matter interactions constitute the pervasive foundation for many physical phenomena with application in optical and atomic physics, electrical and communication engineering, medicine, and biology. In many situations of interest, only the aftermath of light-matter interaction is experimentally accessible. The properties of light and matter are both encoded in the characteristics of this secondary source of radiation. In most cases, these field and intensity characteristics carrying information about the initial source of radiation or the specific material system are statistical in nature. A typical light-matter interaction experiment involves an initial source of radiation that interacts with a material system from which a secondary emission occurs. Measurements are usually performed on this secondary radiation, whose properties depend on both the characteristics of the primary source and the specifics of light-matter interaction. When the scope is to determine certain material properties, different approaches can be taken. For instance, one can actively modify the properties of the primary source of radiation to control different aspects of the interaction. This active modality offers great versatility in sensing applications. In many practical situations, however, accessing directly the primary source is simply not possible. In such circumstances, one is limited to so-called passive approaches where the sole source of information lies in the measurable properties of the secondary radiation. Contingent upon the intrinsic structural properties, the material system can be static or dynamic during the process of light-matter interaction and the experimental approaches may vary accordingly. In this thesis, we explore three different passive sensing scenarios based on different types of light-matter interactions. First, we examine a situation where the secondary radiation is the result of the continuous interaction between optical radiation and material systems with random structures that do not vary in time. In this case, we will discuss how the spatial coherence of the secondary emission can be used to extract information about either the static matter or the initial light source. Next, we study the cases when the secondary radiation originates from dynamic material systems in both steady-state and transient conditions. In the first case, when the matter is continuously excited, the intensity fluctuations are used to quantify the structural dynamics of the medium and retrieve its complex mechanical properties. As a particular application, we address the viscoelastic properties of blood, a typical example of a dynamic, optically-dense random medium. In the case of transient interactions, a low-intensity decaying signal is measured after the primary source shuts off. We discuss how subtle structural properties of matter can influence, even in these conditions, the rate of secondary emission.

This study investigates the minority carrier transport properties of wide bandgap semiconductors, primarily gallium oxide (Ga2O3) and gallium nitride (GaN). Ga2O3 is an emerging ultra-wide bandgap semiconductor with applications in high temperature electronics and sensors for use in extreme environments. Ga2O3 is a suitable material for devices deployed in the lower Earth satellite orbits due to its intrinsic radiation hardness, applications in solar-blind ultraviolet (UV) detection, and high power/high frequency electronics. The main factor limiting Ga2O3 technology so far is the reliable high mobility p-type Ga2O3; however, recent advances have shown a promising future for developments in this direction. Minority carrier transport properties such as minority carrier diffusion length (L) and lifetime (t) are of vital importance with the advent of p-type conductivity, as they are the limiting factor in the performance of bipolar devices. In this thesis, a comparison of the temperature dependence of L, t, and CL emission in n-type Si-doped Ga2O3 Schottky rectifiers, exposed to 18 MeV alpha particles and 10 MeV protons is presented. Additionally, the effect of electron injection, a countermeasure to in-situ mitigates the radiation damage, is studied in these structures. Electron injection has also been found to enhance L and t in unintentionally doped GaN. Lastly, the temperature dependence of minority carrier diffusion length and CL emission is presented in the novel p-type Ga2O3.

Optical beam steering is a key component in any application that requires dynamic (i.e. realtime control) of beam propagation through free-space. Example applications include remote sensing, spectroscopy, laser machining, targeting, Lidar, optical wireless communications (OWC) and more. The pointing control requirements for many of these applications can be met by traditional mechanical steering techniques; however, these solutions tend to be bulky, slow, expensive, power hungry and prone to mechanical failures leading to short component lifetimes. Two emerging applications, Lidar imaging and OWC, truly need improved beam-steering capabilities to flourish and support the advancement of self-driving cars or relieve the congestion in radio-frequency wireless networks, respectively. We consider the novel requirements of these applications during development of a new beam-steering technology. We introduce imaging-based beam steering (IBBS) that uses an imaging transform between spatial and directional domains to implement a new method of electronic beam-steering. We introduce this concept while focusing on transmitters (Tx) for OWC but the pointing control mechanism is bi-directional supporting both transmit and receive functionality, even out of the same aperture; likewise, features that make this solution compelling for OWC are also great for Lidar imaging. In IBBS, an array of high-speed sources are positioned at the focal plane of a lens and the lens passively collects, collimates and steers the beam into a conjugate direction. "Steering" is accomplished by selecting which source to use for an OWC link. This gives a coarse, pixelated beam-steering control that is well-suited for short-range OWC such as indoor communications and we present a prototype bulb for this application. Notably, multiple sources can be utilized at once with each steered into its conjugate directions and this presents the first beam-steering technology that supports multiple beams out of a single aperture; this feature uniquely supports multiplexed communications and fast, high-resolution Lidar imaging.

Fundamental studies of material surfaces are of continued interest to the development and improvement of many modern technologies, e.g. catalysis, energy efficient electronics, and high-capacity batteries etc. This dissertation targets two distinct sets of molecule-surface interactions relevant to the continued development of structure-property correlations using tools from Density Functional Theory with added verification from ultrahigh vacuum surface-science experiments. These include Haber-Bosch interactions at molybdenum-nitride surfaces and separation-dependent interactions between simple aromatics and Ru(0001) used to model a metal contact of Organic Electronic Devices (OEDs). In the first study, we focus on computational modelling of nitrogen fixation reactions on Mo- and N-terminated δ-MoN(0001). A comparative analysis to analogous predictions reported for Mo-terminated γ-Mo2N(111) sites demonstrates a near-total dependence on the atomic surface-structure with little to no impact from changes in sub-surface stoichiometry. Changing from Mo- to N-terminated surface drastically changes the reaction barriers such that the rate-limiting-step in the overall ammonia evolution reaction changes from NHx hydrogenation to N2 dissociative adsorption. In the second one, we explored the effect of changing metal-organic molecule separation on charge-transfer across the interface and the electronic properties of organic matter pertinent to OEDs. We studied various computational models of benzene and pyridine molecules held at fixed distances from Ru(0001) by introducing two-dimensional hexagonal SiO2 thin-films between molecules and the metal. Substantial metal-to-molecule charge-transfer is noted when molecules bind directly to the Ru interface, but virtually no interaction is noted when increasing metal-molecule separations up to ~12 Å. An analogous series of experiments investigating pyridine-Ru interactions introduced after exposing SiO2/Ru(0001) thin-films to varied doses of pyridine exhibits behavior similar to that predicted by theory.

The last two decades have witnessed the emergence of attosecond science, a new discipline that leverages light pulses with duration at the same characteristic time scale of electronic motion in atoms, molecules, and condensed matter. Attosecond spectroscopy has proven a particularly useful tool to monitor and control the evolution of transiently bound electronic states. These states, also known as autoionizing states, play a fundamental role in the ionization and charge transfer processes in matter. This thesis is a theoretical study on the role of such autoionizing states in the ionization of polyelectronic atoms by ultrashort pulses, which has been instrumental to four joint experimental and theoretical studies: i) with the group of Zenghu Chang, at UCF, concerning the core ionization of the argon atom at the L2,3 edge; ii) with the group of Arvinder Sandhu, at the University of Arizona, on the control of the lifetime of autionizing states in the argon atom, as well as iii) on the measurement of the lifetime of argon's dark autoionizing states via non-colinear four-wave mixing; iv) with the group of Louis DiMauro, at the Ohio State University, on the verification of the Kramers-Krönig relations in the ionization of laser-dressed argon. We have adopted several complementary theoretical approaches. In solving the time-dependent Schroedinger equation, we introduced a novel "essential states" procedure, which drastically reduces the cost of ab initio calculations. We developed a model that shows how destructive interference between autoionization and radiative ionization channels can stabilize transient states, explaining experimental observations and providing the first evidence for a phenomenon first predicted four decades ago. We have devised the formalism needed to extrapolate, from the single-atom response, the off-axis dipolar emission from an extended sample, and implemented it to theoretically reproduce, for the first time and with semi-quantitative accuracy, four-wave mixing experimental spectra in argon. Lastly, we developed a model for propagating light through a laser-dressed sample, demonstrating its use by predicting the pressure dependence of transient absorption spectra in model systems.