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 NONLINEAR PROPAGATION OF ORBITAL ANGULAR MOMENTUM LIGHT IN TURBULENCE AND FIBER(2024) Elder, Henry; Sprangle, Phillip; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Light that carries orbital angular momentum (OAM), also referred to as optical vortices or twisted light, is characterized by a helical or twisted wavefront ∝exp[imφ]. In contrast to spin angular momentum (SAM), where photons are limited to two states, OAM allows for, in principle, an infinite set of spatially orthogonal states. OAM-carrying light has found applications ranging from quantum key distribution in free space and guided-wave communication systems, particle trapping and optical tweezers, nanoscopy, and remote sensing. Understanding how OAM light propagates through complex environments, and how to efficiently generate particular OAM states, is critical for any such application. In the first part of this dissertation, we describe how OAM light propagates through a turbulent atmosphere. We build analytic models which describe (1) the OAM mode mixing caused by turbulence, (2) the evolution of short, high-power OAM pulses undergoing the effects of self-phase modulation (SPM) and group velocity dispersion (GVD), and (3) the evolution of high-power Gaussian pulses including SPM, GVD, and turbulence. The models are validated against both experimental data and nonlinear, turbulent pulse propagation simulation programs, the latter of which we have made freely available. We also explore how self-focusing can minimize certain deleterious effects of turbulence for OAM light. The second part of this dissertation considers nonlinear effects of OAM light propagating in azimuthally symmetric waveguides. Such waveguides have so-called spin-orbit (SO) modes, which are quantized based on their total angular momentum (TAM). We develop a generalized theory of four wave mixing-based parametric amplification of SO modes and show that these processes conserve TAM, but under certain circumstances can be taken to conserve SAM and OAM independently. Our theory is validated against a nonlinear multimode beam propagation simulation program which we developed and, again, have made freely available.Item Spatiotemporal Optical Vortices(2023) Hancock, Scott; Milchberg, Howard; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Light beams carrying orbital angular momentum (OAM) have become a mainstay of optical science and technology. In these beams, well-known examples of which are the Laguerre-Gaussian (LG_pm ) and Bessel-Gaussian (BG_m ) beams, the OAM vector points parallel or anti-parallel to propagation, and is associated with a phase winding 2πm in the plane transverse to the propagation direction, where integer m is the winding order or the “topological charge”. Such beams can be monochromatic.Recently, our group discovered a new type of OAM structure that naturally emerges from nonlinear self-focusing, which we dubbed the spatio-temporal optical vortex (STOV). Here, the phase winding exists in a spatiotemporal plane, with the OAM pointing transverse to propagation. In this dissertation, we extend the generation of STOV-carrying pulses to the linear regime, demonstrating their generation using a 4f pulse shaper and measuring their free-space propagation using a new ultrafast single-shot space- and time-resolving diagnostic, TG-SSSI (transient-grating single-shot supercontinuum spectral interferometry). We then demonstrate that transverse OAM is a property of photons by experimentally confirming the conservation of transverse OAM in second harmonic generation. Because the field of STOVs is so new, a first principles theory for their transverse OAM was lacking. We developed such a theory for transverse OAM that predicts half integer values of OAM and the existence of a STOV polariton in dispersive media. The surprise of half-integer OAM values launched a debate in the OAM community, which has been resolved in favor of our theory by our most recent experiments. These explore how phase and amplitude perturbations can impart spatiotemporal torques to light. We find that transverse OAM can be imparted to light pulses only for (1) sufficiently fast transient phase perturbations or (2) energy removal from a pulse already possessing transverse OAM.