Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item Modeling strong-field laser-atom interactions with nonlocal potentials(2017) Rensink, Thomas C.; Antonsen (Jr.), Thomas M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)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. However, simulation of atom-field dynamics is time-consuming and computationally expensive. In this thesis, a nonlocal model potential is used in place of the Coulomb potential in the time dependent Schrodinger equation, and examined for suitabil- ity of modeling strong field-atom dynamics while offering significant reduction in computation cost. Nonlocal potentials have been used to model many physical systems, from multi-electron molecular configurations to semiconductor theory. Despite their rel- ative success, nonlocal potentials have been largely unexplored for modeling high field laser-gas interactions in the ionizing regime. This work explores the theory and numerical results of a single state gaussian nonlocal model in intense, femtosecond laser pulses, with the main findings: nonlocal potentials are useful for obtaining the photoionization rate in the tunnel and multiphoton regimes, and qualitatively char- acterize the wavefunction dynamics of irradiated atoms. The model is also examined in the context of the two-color technique for producing Terahertz (THz) frequency radiation.Item Detailed Measurements of Fire-Induced Mixing Phenomena(2014) Layton, Thomas George; Marshall, Andre W; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study successfully validated the use of salt-water analog modeling as an effective diagnostic tool for predicting fire-induced flows. A technique was developed for taking measurements using combined Planar Laser-Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV), and results were analyzed with respect to smoke filling as well as transient ceiling layer dynamics, and turbulent mixing intensity. Data was shown to be in good agreement with theory, further validating the salt-water analogy as a tool for diagnostics, prediction, and scaling of fire phenomena.Item Electroencephalography (EEG) and measures of nociception in cattle(2013) Drnec, Kim Ann; Stricklin, William R; Simon, Jonathan Z; Neuroscience and Cognitive Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The first known bovine laser evoked potential (LEP), an EEG response to noxious laser heat stimuli, was measured in 2-3 year old Holstein cows (n=5). The amplitude of the bovine LEP correlated significantly (P<. 05) with behavior scores, the surrogate for self-reporting in human studies. Importantly, and comparable to human studies, the LEP occurs at a latency within which it is considered that cortical potentials reflect increasingly complex cognitive processes, rather than those that are reflexive and non-conscious. Differences between the bovine and human LEP were also determined, that cannot be fully explained at this time. The lack of standardization for large animal EEG-investigations is problematic regarding data sharing across labs. A proposed standard method, for collecting and processing EEG in cattle was developed and is presented. Compared to human studies, signal processing of bovine data required significantly more stringent rejection criteria for data analysis. For example, while wavelet denoising is often used in human EEG; it was found essential for extracting a bovine LEP. In addition, explicitly addressing whether of not cortical potentials were being recorded was necessary to provide foundational background knowledge of bovine EEG. To this end, EEG was recorded under conditions designed to simulate the suppression and excitation of the primary visual cortex, as is measured in humans using eyes-open and eyes-closed. The simulation contrasted a dark and light environment. I propose this protocol to be used in the future large animal studies to verify that cortical potentials are being measured before EEG data recording. My results demonstrate that bovine EEG is a useful bovine cognitive science method, but more sophisticated signal processing techniques are needed to ameliorate issues of artifact. Lowered signal to noise ratios is considerably problematic for evoked response studies in large animals. Importantly, this research determined that a bovine LEP is measurable, and by analogy to human perceptual studies, I contend this demonstrates the cow experiences both the sensation and perception of noxious stimulus as painful.Item III-V Optoelectronic Devices: Room temperature CW operation of interband cascade laser & High efficiency p-side down InGaN/GaN solar cell(2011) Ryu, Geunmin; Dagenais, Mario; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)During the past two decades, the field of III-V optoelectronic devices has gained widespread interest as a result of advances in the performance and reliability of epitaxial structures. In principle, III-V materials can provide sources, detectors and optoelectronic components over wavelengths from UV to IR. During my Ph.D study, I have focused on two III-V optoelectronic devices: Mid-IR interband cascade lasers and group III-Nitride solar cells. In the first part of this dissertation, we will discuss development of a room temperature CW operation interband cascade laser and in the second part, we will discuss the concept of high efficiency III-N solar cells. Part I Lasers that emit in the mid-IR (3~5um) spectral region can be used in many civilian and military applications such as chemical sensing, free space optical communication and IR countermeasures. There are three types of lasers that can cover the Mid-IR region. First, conventional type-I quantum well (QW) lasers on GaSb substrates, second, inter-subband quantum cascade lasers (QCLs) on InP substrates and finally interband cascade laser with type-II alignment of the conduction and valence bands on GaSb substrates. Gallium Antimonide based type II interband cascade lasers (ICLs) cover the 3~4 um wavelength range, and it is the most natural match to the mid-IR. For most applications, it is required that the laser operates in continuous wave (CW) mode either at room temperature or at temperatures accessible to thermoelectric coolers. Recently, we have been able to operate interband cascade lasers in CW mode at room temperature with 62mW of output power, internal loss of 4.8cm-1, 170mW/A slope efficiency, and a threshold current density as low as 300 A/cm2 which are a significant milestone toward many applications. In the first part of this thesis, we are going to talk about the fundamental principles of operation of the ICLs and their applications. Secondly, we will present the development of a fabrication process. Third, we will discuss the performance characteristics of ICLs. Lasers were characterized by doing series of length dependent pulsed/CW measurements to obtain critical parameters at low temperature and at room temperature; such as wall plug efficiency, threshold current density, internal loss, and thermal impedance. For low temperature CW measurement, a specially designed vacuum chamber was used to prevent water condensation. Finally, we will present ICL optimization processes. For laser optimization, we re-designed the device structure, in particular the lower cladding region, the injection region, and the active region thickness, to achieve a higher confinement factor and lower loss, thus increasing the operating temperature and the output power. Part II Since the 1950s, silicon solar cells have been intensively studied and developed. Solar cell technology has greatly benefited from the maturity of silicon technology developed originally for the IC industry. This has led to the development of high quality single crystal silicon wafers with low dislocation densities. However, because of the poor spectral overlap between the absorption of silicon cells and the spectrum of solar light, silicon solar cells cannot fundamentally produce high efficiency solar cells. In order to achieve high efficiency solar cells, researchers have investigated many alternatives including tandem cells, GaAs, and III-Nitride materials. In the second part of this thesis, we will talk about the development of high efficiency III-Nitride solar cells using novel p-side InGaN/GaN materials, including device background, new solar cell design, fabrication process development, and preliminary device characterizations.Item Interaction of Lasers with Atomic Clusters and Structured Plasmas(2007-11-09) Palastro, John Patrick; Antonsen, Thomas M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We examine the interaction of intense, short laser pulses with atomic clusters and structured plasmas, namely preformed plasma channels. In examining the laser pulse interaction with atomic clusters we focus on the optical response of an individual cluster when irradiated by a laser. Our analysis of the laser pulse interaction with plasma channels focuses on the mode structure of a laser pulse propagating within the channel. We then present a novel application of these channels: quasi-phased match acceleration of electrons. The optical properties of a gas of laser pulse exploded clusters are determined by the time-evolving polarizabilities of individual clusters. In turn, the polarizability of an individual cluster is determined by the time evolution of individual electrons within the cluster's electrostatic potential. We calculate the linear cluster polarizability using the Vlasov equation. A quasi-static equilibrium is calculated from a bi-maxwellian distribution that models both the hot and cold electrons, using inputs from a particle-in-cell simulation [Taguchi, T. et al., Phys. Rev. Lett., 2004. 92(20)]. We then perturb the system to first order in field and integrate the response of individual electrons to the self consistent field following unperturbed orbits. The dipole spectrum depicts strong absorption at frequencies much smaller than omega_p/√2. This enhanced absorption results from a beating of the laser field with electron orbital motion. The properties of pulse propagation within plasma are determined by the structure of the plasma. The preformed plasma channel provides a guiding structure for laser pulses unbound by the intensity thresholds of standard wave guides. In particular, the corrugated plasma channel [Layer et al. Phys. Rev. Lett. (2007)] allows for the guiding of subluminal spatial harmonics. These spatial harmonics can be phase matched to high energy electrons, making the corrugated plasma channel ideal for the acceleration of electrons. We present a simple analytic model of pulse propagation in a corrugated plasma channel and examine the laser-electron beam interaction. Simulations show accelerating gradients of several hundred MeV/cm for laser powers much lower than required by standard laser wakefield schemes.