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.

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2007 - 20102

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    Visualization of the Vortex Lattice Dynamics in Superfluid Helium
    (2010) Gaff, Kristina Teresa; Lathrop, Daniel P; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    We study the lattice structure and dynamics of the quantized vortices in superfluid helium-4 using a new rotating experiment. This setup includes control of the entire apparatus from the rotating frame, installation of a new EMCCD camera that allows for imaging of nanoscale tracer particles, and the development and implementation of a new isolation cell, which permits investigation into new phenomena such as differential rotation in helium-II. We have observed the vortex lattice dynamics in the (r, &phi) plane (i.e. transverse to the vortices) and present here the first real-time visualization of Tkachenko waves in helium-II from this cross section. Additionally, we present evidence of differential rotation with distinct Stewartson layer boundaries, possible Kelvin-Helmholtz instabilities, and the formation and propagation of superfluid collective vortex eddies. We show that the angular velocity is a function of radius and may be driven by the geometry of the isolation cell. We also document the observation and analysis of gravity-capillary surface waves that demonstrate an interaction between the liquid helium free surface and the bulk of the fluid.
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    Benzocyclobutene Microring Resonators
    (2007-11-26) Chen, Wei-Yen; Ho, Ping-Tong; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Wavelength division multiplexing (WDM) at optical carrier frequencies offers more capacity and flexibility of fiber networks and upgrades conventional point-to-point fiber-optic transmission links to multiuser networks for the demand of high-speed network systems. The microring resonator, which has been seen in action in many photonic devices, is ideal for WDM applications in realization of narrow bandwidth with a wide free spectral range. In this research, microring resonators are fabricated in benzocyclobutene (BCB), a popular polymer in photonics and electronics applications. First, the single-mode BCB undercut-cladding waveguides were designed to reduce bending loss. BCB microring resonators were fabricated based on those principles. BCB single-microring devices were demonstrated as add-drop filters and notch filters with a negative coupling gap. Not only are the microring resonators compact (as small as 5 µm in radius) for photonic VLSI, they also exhibit a high out-of-band rejection (~ 30 dB), high extinction as well as a high finesse (~ 285). In addition, BCB lattices consisting of over one hundred microring resonators were fabricated and demonstrated as bandstop filters. The lattices, despite the large number of resonators, exhibit an extremely low propagation loss. Finally, optical bistability and the field-enhanced all-optical nonlinear switching were demonstrated in BCB microring devices.