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 Topological Edge States in Silicon Photonics(2014) Mittal, Sunil; Hafezi, Mohammad; Migdall, Alan; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Under the influence of a magnetic field, at low temperatures, charged particles confined in two-dimensional systems exhibit a remarkable range of macroscopic quantum phenomena such as the quantum Hall effects. A hallmark of these phenomena is the presence of unidirectional, topologically robust edge states - states which are confined to the edge of the system. It is, in principle, possible to engineer a synthetic magnetic field for photons and hence achieve photonic analogs of the robust electronic edge states. Investigating photonic edge states is interesting from a fundamental perspective of studying photonic transport in the presence of a gauge field and also for its application in classical and quantum information processing. In this thesis, we present the implementation of a synthetic magnetic field for photons and our observation of topological edge states in a two-dimensional lattice of coupled ring resonators, fabricated using CMOS-compatible silicon-oninsulator technology. We qualitatively show the robustness of edge states against deliberately induced lattice defects. We then analyze the statistics of transport measurements (transmission and delay) made on a number of different devices and quantitatively verify the robustness of edge states against lattice disorder. Using Wigner delay-time distribution, we show that localization is suppressed in the edge states. Furthermore, to unequivocally establish the non-trivial topological nature of edge states, we compare their transmission to a topologically trivial one dimensional system of coupled ring resonators and demonstrate that the edge states achieve higher transmission. Moreover, for photonic analogs of the quantum Hall effect, the winding number - a topologically invariant integer which characterizes edge states - is quantized, analogous to quantization of conductivity in electronic systems. We measure the winding number of the edge states in our system. Finally, we investigate the effect of nonlinear interactions in silicon ring resonators, on the stability of edge states. We show that the presence of a strong pump can result in a significant decrease in the transmission through edge states.Item Multi-Valley Physics of Two-Dimensional Electron Systemson Hydrogen-Terminated Silicon (111) Surfaces(2010) McFarland, Robert Nicholas; Kane, Bruce E; Drew, Howard D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Recent work on two dimensional electron systems (2DES) has focused increasingly on understanding the way the presence of additional degrees of freedom (e.g. spin, valleys, subbands, and multiple charge layers) affect transport as such effects may be critical to the development of nanoscale and quantum devices and may lead to the discovery of new physics . In particular, conduction band valley degeneracy opens up a rich parameter space for observing and controlling 2DES behavior. Among such systems, electrons on the (111) surface of silicon are especially notable because effective mass theory predicts the conduction band to be sixfold degenerate, for a total degeneracy (spin ×valley) of 12 in the absence of a magnetic field B. Previous investigations of Si(111) transport using Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs) observed a valley degeneracy gv of 2 except in certain specially prepared samples with low mobility. We have developed a novel device architecture for investigating transport on a H-Si(111)-vacuum interface free from the complications created by intrinsic disorder at Si-SiO2 interfaces. The resulting devices display very high mobilities (up to 110,000 cm2/Vs at 70 mK, more than twice as large as the best silicon MOSFETs), enabling us to probe valley-dependent transport to a much greater degree than previously possible. In particular, we observed detailed Integer Quantum Hall structure with hints of Fractional states as well. These devices display clear evidence of six occupied valleys, including strongly “metallic” temperature dependence expected for large gv. Some devices show strong sixfold degeneracy while others display a partial lifting of the degeneracy, resulting in unequal distribution of electrons among the six valleys. This symmetry breaking results in anisotropic transport at low B fields, but other observed anisotropies remain unexplained. Finally, we apply this unusual valley structure to show how corrections to the low-B magnetoresistance and Hall effect can provide information about valley-valley interactions. We propose a model of valley drag, similar to Coulomb drag in bilayer systems, and find good agreement with our experimental data, though a small residual drag in the T→0 limit remains unexplained.