Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
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
More information is available at Theses and Dissertations at University of Maryland Libraries.
Browse
11 results
Search Results
Item TOPOLOGICAL PHOTONICS: NESTED FREQUENCY COMBS AND EDGE MODE TAPERING(2024) Flower, Christopher James; Hafezi, Mohammad; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Topological photonics has emerged in recent years as a powerful paradigm for the designof photonic devices with novel functionalities. These systems exhibit chiral or helical edge states that are confined to the boundary and are remarkably robust against certain defects and imperfections. While several applications of topological photonics have been demonstrated, such as robust optical delay lines, quantum optical interfaces, lasers, waveguides, and routers, these have largely been proof-of-principle demonstrations. In this dissertation, we present the design and generation of the first topological frequency comb. While on-chip generation of optical frequency combs using nonlinear ring resonators has led to numerous applications of combs in recent years, they have predominantly relied on the use of single-ring resonators. Here, we combine the fields of linear topological photonics and frequency microcombs and experimentally demonstrate the first frequency comb of a new class in an array of hundreds of ring resonators. Through high-resolution spectrum analysis and out-of- plane imaging we confirm the unique nested spectral structure of the comb, as well as the confinement of the parametrically generated light. Additionally, we present a theoretical study of a new kind of valley-Hall topological photonic crystal that utilizes a position dependent perturbation (or “mass-term”) to manipulate the width of the topological edge modes. We show that this approach, due to the inherent topological robustness of the system, can result in dramatic changes in mode width over short distances with minimal losses. Additionally, by using a topological edge mode as a waveguide mode, we decouple the number of supported modes from the waveguide width, circumventing challenges faced by more conventional waveguide tapers.Item Influence of Noise on Response Localizations in Mechanical Oscillator Arrays(2022) Cilenti, Lautaro Daniel; Balachandran, Balakumar; Cameron, Maria; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The dynamics of mechanical systems such as turbomachinery and vibration energy harvesting systems (VEH) consisting of one or multiple cantilever structures is often modeled by arrays of periodically driven coupled nonlinear oscillators. It is known that such systems may have multiple stable vibration steady states. Some of these steady states are localized vibrations that are characterized by high amplitude vibrations of a subset of the system, with the rest of the system being in a state of either low amplitude vibrations or no vibrations. On one hand, these localized vibrations can be detrimental to mechanical integrity of turbomachinery, while on the other hand, the vibrations can be potentially desirable for increasing energy yield in VEHs. Transitions into or out of localized vibrations may occur under the influence of random factors. A combination of experimental and numerical studies has been performed in this dissertation to study the associated transition times and probability of transitions in these mechanical systems. The developments reported here include the following: (i) a numerical methodology based on the Path Integral Method to quantify the probability of transitions due to noise, (ii) a numerical methodology based on the Action Plot Method to quantify the quasipotential and most probable transition paths in nonlinear systems with periodic external excitations, and (iii) experimental evidence and stochastic simulations of the influence of noise on response localizations of rotating macro-scale cantilever structures. The methodology and results discussed in this dissertation provide insights relevant to the stochastic nonlinear dynamics community, and more broadly, designers of mechanical systems to avoid potentially undesirable stochastic nonlinear behavior.Item SUPERCONDUCTING RADIO FREQUENCY MATERIALS SCIENCE THROUGH NEAR-FIELD MAGNETIC MICROSCOPY(2020) Oripov, Bakhrom Gafurovich; Anlage, Steven M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Superconducing Radio-Frequency (SRF) cavities are the backbone of a new generation of particle accelerators used by the high energy physics community. Nowadays, the applications of SRF cavities have expanded far beyond the needs of basic science. The proposed usages include waste treatment, water disinfection, material strengthening, medical applications and even use as high-Q resonators in quantum computers. A practical SRF cavity needs to operate at extremely high rf fields while remaining in the low-loss superconducting state. State of the art Nb cavities can easily reach quality factors Q>2x10^10 at 1.3 GHz. Currently, the performance of the SRF cavities is limited by surface defects which lead to cavity breakdown at high accelerating gradients. Also, there are efforts to reduce the cost of manufacturing SRF cavities, and the cost of operation. This will require an R&D effort to go beyond bulk Nb cavities. Alternatives to bulk Nb are Nb-coated Copper and Nb3Sn cavities. When a new SRF surface treatment, coating technique, or surface optimization method is being tested, it is usually very costly and time consuming to fabricate a full cavity. A rapid rf characterization technique is needed to identify deleterious defects on Nb surfaces and to compare the surface response of materials fabricated by different surface treatments. In this thesis a local rf characterization technique that could fulfill this requirement is presented. First, a scanning magnetic microwave microscopy technique was used to study SRF grade Nb samples. Using this novel microscope the existence of surface weak-links was confirmed through their local nonlinear response. Time-Dependent Ginzburg-Landau (TDGL) simulations were used to reveal that vortex semiloops are created by the inhomogenious magnetic field of the magnetic probe, and contribute to the measured response. Also, a system was put in place to measure the surface resistance of SRF cavities at extremely low temperatures, down to T=70 mK, where the predictions for the surface resistance from various theoretical models diverge. SRF cavities require special treatment during the cooldown and measurement. This includes cooling the cavity down at a rate greater than 1K/minute, and very low ambient magnetic field B<50 nT. I present solutions to both of these challenges.Item Design of a nonlinear quasi-integrable lattice for resonance suppression at the University of Maryland Electron Ring(2018) Ruisard, Kiersten; Koeth, Timothy; Thomas, Antonsen; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Conventional particle accelerators use linear focusing forces for transverse confinement. As a consequence of linearity, accelerating rings are sensitive to myriad resonances and instabilities. At high beam intensity, uncontrolled resonance-driven losses can deteriorate beam quality and cause damage or radio-activation in beam line components and surrounding areas. This is currently a major limitation of achievable current densities in state-of-the-art accelerators. Incorporating nonlinear focusing forces into machine design should provide immunity to resonances through nonlinear detuning of particle orbits from driving terms. A theory of nonlinear integrable beam optics is currently being investigated for use in accelerator rings. Such a system has potential to overcome the limits on achievable beam intensity. This dissertation presents a plan for implementing a proof-of-principle quasi-integrable octupole lattice at the University of Maryland Electron Ring (UMER). UMER is an accelerator platform that supports the study of high-intensity beam dynamics. In this dissertation, two designs are presented that differ in both complexity and strength of predicted effects. A configuration with a single, relatively long octupole magnet is expected to be more stabilizing than an arrangement of many short, distributed octupoles. Preparation for this experiment required the development and characterization of a low-intensity regime previously not operated at UMER. Additionally, required tolerances for the control of first and second order beam moments in the proposed experiments have been determined on the basis of simulated beam dynamics. In order to achieve these tolerances, a new method for improved orbit correction is developed. Finally, a study of resonance-driven losses in the linear UMER lattice is discussed.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.Item Silicon-based terahertz waveguides(2015) Li, Shanshan; Murphy, Thomas E.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, we present the design, fabrication and application of two types of silicon-based terahertz waveguides. The first is anisotropically etched highly doped silicon surface for supporting terahertz plasmonic guided wave. We demonstrate propagation of terahertz waves confined to a semiconductor surface that is periodically corrugated with subwavelength structures. We observe that the grating structure creates resonant modes that are confined near the surface. The degree of confinement and frequency of the resonant mode is found to be related to the pitch and depth of structures. The second is silicon dielectric ridge waveguide used to confine terahertz pulses and study silicon's terahertz intensity-dependent absorption. We observe that the absorption saturates under strong terahertz fields. By comparing the response between lightly-doped and intrinsic silicon waveguides, we confirm the role of hot carriers in this saturable absorption. We introduce a nonlinear dynamical model of Drude conductivity that, when incorporated into a wave propagation equation, predicts a comparable field-induced transparency and elucidates the physical mechanisms underlying this nonlinear effect. The results are numerically confirmed by Monte Carlo simulations of the Boltzmann transport equation, coupled with split-step nonlinear wave propagation.Item Nonlinear Fluid-Structure Interactions in Flapping Wing Systems(2013) Fitzgerald, Timothy; Balachandran, Balakumar; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This work relates to fluid-structure interactions in the context of flapping wing systems. System models of flapping flight are explored by using a coupling scheme to provide communication between a fluid model and a structural model describing a flexible wing. The constructed computational models serve as a tool for investigating complex fluid-structure interactions and characterizing them. Primary goals of this work are construction of models to understand nonlinear phenomena associated with the flexible flapping wing systems, and explore means and methods to enhance their performance characteristics. Several system analysis tools are employed to characterize the coupled fluid-structure system dynamics, including proper orthogonal decomposition, dimension calculations, time histories, and frequency spectra. Results obtained from two-dimensional simulations conducted for a combination of a two-link structural system and a fluid system are presented and discussed. Comparisons are made between the use of direct numerical simulation and the unsteady vortex lattice method as the fluid model in this coupled dynamical system. To enable three-dimensional studies, a novel solid model is formulated from continuum mechanics for geometrically exact finite elements. A new partitioned fluid-structure interaction algorithm based on the Generalized-α method is formulated and implemented in a large scale fluids solver inside the FLASH framework. Consistent boundary conditions are also formulated by using Lagrangian particles. Several examples demonstrating the effectiveness of the methods and implementation are shown, in particular, for flapping flight at low Reynolds numbers. Unique experiments have also been undertaken to determine the first few natural frequencies and mode shapes associated with hawkmoth wings. The computational framework developed in this dissertation and the research findings can be used as a basis to understand the role of flexibility in flapping wing systems, further explore the complex dynamics of flapping wing systems, and also develop design schemes that might make use of nonlinear phenomena for performance enhancement.Item Nonlinear Complexity of Boolean Permutations(2009) Draper, Thomas Gordon; Washington, Lawrence C; Mathematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We introduce the concept of nonlinear complexity, where the complexity of a function is determined by the number of nonlinear building blocks required for construction. We group functions by linear equivalence, and induce a complexity hierarchy for the affine equivalent double cosets. We prove multiple invariants of double cosets over the affine general linear group, and develop a specialized double coset equivalence test. This is used to classify the 16! permutations over 4 bits into 302 equivalence classes, which have a maximal nonlinear depth of 6. In addition, we present a new complexity class defined in terms of nonlinearity.Item Characterization of Linear Electro-Optic Effect of Poled Organic Thin Films(2008-02-29) Park, Dong Hun; Lee, Chi H; Herman, Warren N; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The goal of this thesis is to re-evaluate both Teng-Man and attenuated total reflection (ATR) methods for measuring the linear electro-optic (EO) coefficients of poled organic thin films based on a multilayer structure containing a transparent conducting oxide (TCO) layer. The linear EO properties are often characterized using the Teng-Man reflection method. However, it has been reported that experimental error can result from ignoring multiple reflections and that an accurate determination of the EO effect could be achieved only by a numerical calculation that applies anisotropic Fresnel equations to the multilayer structure. We present new closed-form expressions for analysis of Teng-Man measurements of the EO coefficients of poled polymer thin films. These expressions account for multiple reflection effects using a rigorous analysis of the multilayered structure for varying angles of incidence. The analysis based on plane waves is applicable to both transparent and absorptive films and takes into account the properties of the TCO electrode layer and buffer layers. Methods for fitting data are presented and the error introduced by ignoring the TCO layer and multiple reflections is discussed. We also discuss the effect of Gaussian beam optics and the suitability of a thick z-cut LiNbO3 crystal as a reference to validate the Teng-Man measurement. Simply taking the metal electrode off the Teng-Man sample makes it feasible to use the ATR method using a metal-coated prism. This technique has the capability of measuring anisotropic indices of refraction along with film thicknesses. In addition, it enables measurement of r13 and r33 separately without an assumption for the ratio of r13 to r33 as required in the Teng-Man method. We have found that the ATR analysis based on a three-layer waveguide structure (air/film/substrate) can produce a large error especially when the film supports a single guided mode and the ATR analysis based on a multilayer structure containing a TCO layer gives you a more reliable estimation. We discuss the error introduced by using the three-layer waveguide structure and compare to using the multilayer structure. Finally, we discuss the characterization of the optical property of TCO's using ellipsometeric analysis, which is required for both the rigorous Teng-Man and ATR analysis. Representative experimental results showing that the result from the ATR method based on the multilayer structure shows a good agreement with that from the rigorous Teng-Man analysis are presented. We have measured a very high linear electro-optic coefficient (r33=350 pm/V) from a NLO film (AJ-TTE-II, synthesized by Alex Jen's group at University of Washington) at 1310 nm wavelength, which is ~12 times higher than the best inorganic electro-optic crystal LiNbO3.Item Making Forecasts for Chaotic Processes in the Presence of Model Error(2006-02-20) Danforth, Christopher M; Yorke, James A; Kalnay, Eugenia; Applied Mathematics and Scientific Computation; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Numerical weather forecast errors are generated by model deficiencies and by errors in the initial conditions which interact and grow nonlinearly. With recent progress in data assimilation, the accuracy in the initial conditions has been substantially improved so that accounting for systematic errors associated with model deficiencies has become even more important to ensemble prediction and data assimilation applications. This dissertation describes two new methods for reducing the effect of model error in forecasts. The first method is inspired by Leith (1978) who proposed a statistical method to account for model bias and systematic errors linearly dependent on the flow anomalies. DelSole and Hou (1999) showed this method to be successful when applied to a very low order quasi-geostrophic model simulation with artificial "model errors." However, Leith's method is computationally prohibitive for high-resolution operational models. The purpose of the present study is to explore the feasibility of estimating and correcting systematic model errors using a simple and efficient procedure that could be applied operationally, and to compare the impact of correcting the model integration with statistical corrections performed a posteriori. The second method is inspired by the dynamical systems theory of shadowing. Making a prediction for a chaotic physical process involves specifying the probability associated with each possible outcome. Ensembles of solutions are frequently used to estimate this probability distribution. However, for a typical chaotic physical system H and model L of that system, no solution of L remains close to H for all time. We propose an alternative and show how to "inflate" or systematically perturb the ensemble of solutions of L so that some ensemble member remains close to H for orders of magnitude longer than unperturbed solutions of L. This is true even when the perturbations are significantly smaller than the model error.