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 CONFINED PHOTOTHERMAL HEATING OF NANOPARTICLE DISPLAYED BIOMATERIALS(2021) Hastman, David A; Medintz, Igor L; Aranda-Espinoza, Helim; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Controlling the temperature of biological systems has long been utilized as a tool for regulating their subsequent biological activity. Recently, photothermal heating of gold nanoparticles (AuNPs) has emerged as an efficient and remote method to heat proximal biological materials. Moreover, this technique has tremendous potential for controlling biological systems at the subcellular level, as specific components within the system can be heated while the larger system remains unaffected. The small size, biocompatiblilty, and optical properties of AuNPs make them attractive nanoscale heat sources for controlling biological systems. While the utility of photothermal heating has significantly advanced through the optimization of AuNP size, shape, and composition, the choice of incident light source utilized has largely been unexplored. One of the more interesting excitation sources is a femtosecond (fs) pulsed laser, as the subsequent temperature increase lasts for only a few nanoseconds and is confined to the nanoscale. However, it is not yet clear how biological materials respond to these short-lived and ultra-confined nanoscale spaciotemporal temperature increases. In this dissertation, we utilize fs laser pulse excitation to locally heat biological materials displayed on the surface of AuNPs in order to understand the corresponding heating profiles and, in turn, interpret how this can be used to modulate biological activity. Due to its unique temperature sensitive hybridization properties, we exploit double-stranded deoxyribonucleic acid (dsDNA) as our prototypical biological material and demonstrate precise control over the rate of dsDNA denaturation by controlling the laser pulse radiant exposure, dsDNA melting temperature, bulk solution temperature, and the distance between the dsDNA and AuNP surface. The rate of dsDNA denaturation was well fit by a modified DNA dissociation equation from which a “sensed” temperature value could be obtained. Evaluating this sensed temperature in the context of the theoretical temperature profile revealed that the ultra-high temperatures near the AuNP surface play a significant role in denaturation. Additionally, we evaluate this technique as a potential means to enhance enzyme activity and report that enhancement is governed by the laser repetition rate, pulse width, and the enzyme’s inherent turnover number. Overall, we demonstrate that the confined and nanosecond duration temperature increase achievable around AuNPs with fs laser pulse excitation can be used to precisely control biological function and establish important design considerations for coupling this technique to more complex biological systems.Item HYDRODYNAMIC AND ELECTRODYNAMIC IMPLICATIONS OF OPTICAL FEMTOSECOND FILAMENTATION(2017) Jhajj, Nihal; Milchberg, Howard; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The propagation of a high peak power femtosecond laser pulse through a dielectric medium results in filamentation, a strongly nonlinear regime characterized by a narrow, high intensity core surrounded by a lower intensity energy “reservoir” region. The structure can propagate over many core diameter-based Rayleigh ranges. When a pulse of sufficiently high power propagates through a medium, the medium response creates an intensity dependent lens, and the pulse begins to focus in a runaway process known as optical collapse. Collapse is invariably mitigated by an arrest mechanism, which becomes relevant as the pulse becomes increasingly intense. In air, collapse is arrested through plasma refraction when the pulse becomes intense enough to ionize the medium. Following arrest, the pulse begins to “filament” or self-guide. In gaseous media, energy deposited in the wake of filamentation eventually thermalizes prompting a neutral gas hydrodynamic response. The gas responds to a sudden localized pressure spike by launching a single cycle acoustic wave, leaving behind a heated, low density channel which gradually dissipates through thermal diffusion. This dissertation presents a fundamental advance in the theory of optical collapse arrest, which is how a pulse transitions from the optical collapse regime to the filamentation regime. We provide experimental evidence, supported by theory and numerical simulation that pulses undergoing collapse arrest in air generate spatiotemporal optical vortices (STOVs), a new and previously unobserved type of optical vortex with phase and energy circulation in a spatiotemporal plane. We argue that STOV generation is universal to filamentation, applicable to all collapsing beams, independent of the initial conditions of the pulse or the details of the collapse arrest mechanism. Once formed, STOVs are essential for mediating intrapulse energy flows. We also study the hydrodynamic response following filamentation, with the intent of engineering the response to construct a variety of neutral gas waveguides. In a proof-of-concept experiment, we demonstrate that a transverse array of filamenting pulses can be used to inscribe two distinct types of waveguides into the air: acoustic and thermal waveguides. These waveguides can be used to guide very high average power laser beams or as remote atmospheric collection lenses.