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 Dynamics and applications of long-distance laser filamentation in air(2024) Goffin, Andrew; Milchberg, Howard; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Femtosecond laser pulses with sufficient power will form long, narrow high-intensity light channels in a propagation medium. These structures, called “filaments”, form due to nonlinear self-focusing collapse in a runaway process that is arrested by a mechanism that limits the peak intensity. For near-infrared pulses in air, the arrest mechanism is photoionization of air molecules and the resulting plasma-induced defocusing. The interplay between plasma-induced defocusing and nonlinear self-focusing enables high-intensity filament propagation over long distances in air, much longer than the Rayleigh range (~4 cm) corresponding to the ~200 µm diameter filament core. In this thesis, the physics of atmospheric filaments is studied in detail along with several applications. Among the topics of this thesis: (1) Using experiments and simulations, we studied the pulse duration dependence of filament length and energy deposition in the atmosphere, revealing characteristic axial oscillations intimately connected to the delayed rotational response of air molecules. This measurement used a microphone array to record long segments of the filament propagation path in a single shot. These results have immediate application to the efficient generation of long air waveguides. (2) We investigated the long-advertised ability of filaments to clear fog by measuring the dynamics of single water droplets in controlled locations near a filament. We found that despite claims in the literature that droplets are cleared by filament-induced acoustic waves, they are primarily cleared through optical shattering. (3) We demonstrated optical guiding in the longest-filament induced air waveguides to date (~50 m, a length increase of ~60×) using multi-filamentation of Laguerre-Gaussian LG01 modes with pulse durations informed by experiment (1). (4) We demonstrated the first continuously operating air waveguide, using a high-repetition-rate laser to replenish the waveguide faster than it could thermally dissipate. For each of the air waveguide experiments, extension to much longer ranges and steady state operation is discussed.Item APPLICATIONS OF INTENSE MID-INFRARED LASER-PLASMA INTERACTIONS(2020) Woodbury, Daniel; Milchberg, Howard; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Intense laser-plasma interaction, generally characterized by focused laser intensities exceeding ~1 TW/cm2, is a major pillar of plasma physics and nonlinear optics with broad applications, including high energy charged particle and photon sources, the generation and study of high energy density physics conditions, fusion energy sources, remote detection techniques, and self-guided nonlinear propagation. For many important applications, longer wavelength lasers provide favorable scaling for laser-plasma interactions, and in several cases enable entirely new phenomena. In this dissertation we present experimental and computational results for three laser-plasma-based applications using ultrashort mid-infrared (mid-IR or MIR) and long-wave-infrared (LWIR) laser pulses. In the first laser wakefield acceleration (LWFA) experiment at mid-IR wavelengths, we demonstrate acceleration of electron bunches driven by relativistic self-focusing collapse of mid-IR laser pulses in near-critical density gas jet targets, and compare scaling of bunch charge and energy to those from common near-infrared systems. Second, we demonstrate that single-electron-seeded avalanche breakdown driven by picosecond mid-IR lasers is an ultrasensitive technique for measuring extremely low plasma densities in gases. We use this technique in two applications. First, we first demonstrate standoff detection of radioactive materials, with avalanche measurements enabling determination of source location and estimates of the radioactivity level. We then use the technique to measure ionization yield induced by an auxiliary laser in atmospheric pressure range gases over 14 orders of magnitude, a record range achievable with no other technique we are aware of. Finally, we present theory and simulations of nonlinear propagation of high power MIR and LWIR multi-picosecond pulses in air, demonstrating that self-guided propagation at moderate intensity is mediated by an ensemble of discrete avalanche plasmas seeded by aerosols.Item Energy Deposition in Femtosecond Filamentation: Measurements and Applications(2017) Rosenthal, Eric Wieslander; Milchberg, Howard M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)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.