UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
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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 Structured Plasma Waveguides and Deep EUV Generation Enabled by Intense Laser-Cluster Interactions(2012) Layer, Brian; Milchberg, Howard M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Using the unique properties of the interaction between intense, short-pulse lasers and nanometer scale van-der-Waals bonded aggregates (or `clusters'), modulated waveguides in hydrogen, argon and nitrogen plasmas were produced and extreme ultraviolet (EUV) light was generated in deeply ionized nitrogen plasmas. A jet of clusters behaves as an array of mass-limited, solid-density targets with the average density of a gas. Two highly versatile experimental techniques are demonstrated for making preformed plasma waveguides with periodic structure within a laser-ionized cluster jet. The propagation of ultra-intense femtosecond laser pulses with intensities up to 2x1017 W/cm2 has been experimentally demonstrated in waveguides generated using both methods, limited by available laser energy. The first uses a `ring grating' to impose radial intensity modulations on the channel-generating laser pulse, which leads to axial intensity modulations at the laser focus within the cluster jet target. This creates a waveguide with axial modulations in diameter with a period between 35 μm and 2 mm, determined by the choice of ring grating. The second method creates modulated waveguides by focusing a uniform laser pulse within a jet of clusters with flow that has been modulated by periodically spaced wire obstructions. These wires make sharp, stable voids as short as 50 μm with a period as small as 200 μm within waveguides of hydrogen, nitrogen, and argon plasma. The gaps persist as the plasma expands for the full lifetime of the waveguide. This technique is useful for quasi-phase matching applications where index-modulated guides are superior to diameter modulated guides. Simulations show that these `slow wave' guiding structures could allow direct laser acceleration of electrons, achieving gradients of 80 MV/cm and 10 MV/cm for laser pulse powers of 1.9 TW and 30 GW, respectively. Results are also presented from experiments in which a nitrogen cluster jet from a cryogenically cooled gas valve was irradiated with relativistically intense (up to 2x1018 W/cm2) femtosecond laser pulses. The original purpose of these experiments was to create a transient recombination-pumped nitrogen soft x-ray laser on the 2p3/2→1s1/2 (λ = 24.779 Å) and 2p1/2→1s1/2 (λ = 24.785 Å) transitions in H-like nitrogen (N6+). Although no amplification was observed, trends in EUV emission from H-like, He-like and Li-like nitrogen ions in the 15 - 150 Å spectral range were measured as a function of laser intensity and cluster size. These results were compared with calculations run in a 1-D fluid laser-cluster interaction code to study the time-dependent ionization, recombination, and evolution of nitrogen cluster plasmas.Item Asymptotic problems for stochastic processes with reflection and related PDE's(2009) Spiliopoulos, Konstantinos; Freidlin, Mark I; Mathematical Statistics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Asymptotic problems for stochastic processes with reflection and for related partial differential equations (PDE's) are considered in this thesis. The stochastic processes that we study, depend on a small parameter and are restricted to move in the interior of some domain, while having instantaneous reflection at the boundary of the domain. These stochastic processes are closely related to corresponding PDE problems that depend on a small parameter. We are interested in the behavior of these stochastic processes and of the solutions to the corresponding PDE problems as this small parameter goes to zero. In particular, we consider two problems that are related to stochastic processes with reflection at the boundary of some domain. Firstly, we study the Smoluchowski-Kramers approximation for the Langevin equation with reflection. According to the Smoluchowski-Kramers approximation, the solution of the equation μ\ddot{q}μt=b(qμt)-\dot{q}μt+sigma(qμt)\dot{W}t, qμ0=q, dot{q}μ0=p converges to the solution of the equation \dot{q}t=b(qt)+σ(qt)dot{W}t, q0=q as μ← 0. We consider here a similar result for the Langevin process with elastic reflection on the boundary of the half space, i.e. on partial R+n={(x1,...,xn) in Rn: x1= 0}. After proving that such a process exists and is well defined, we prove that the Langevin process with reflection at x1=0 converges in distribution to the diffusion process with reflection on the boundary of R+n. This convergence is the main justification for using a first order equation, instead of a second order one, to describe the motion of a small mass particle that is restricted to move in the interior of some domain and reflects elastically on its boundary. Secondly, we study the second initial boundary problem in a narrow domain of width ε<< 1, denoted by Dε, for linear second order differential equations with nonlinear boundary conditions. The underlying stochastic process is the Wiener process (Xεt,Yεt)$ in the narrow domain Dε with instantaneous normal reflection at its boundary. Using probabilistic methods we show that the solution of such a problem converges to the solution of a standard reaction-diffusion equation in a domain of reduced dimension as &epsilon→0. This reduction allows to obtain some results concerning wave front propagation in narrow domains. In particular, we describe conditions leading to jumps of the wave front. This problem is important in applications (e.g., thin waveguides).