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 An All-Sky, Three-Flavor Search for Neutrinos from Gamma-Ray Bursts with the IceCube Neutrino Observatory(2015) Hellauer, Robert Eugene; Sullivan, Gregory; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ultra high energy cosmic rays (UHECRs), defined by energy greater than 10^18 eV, have been observed for decades, but their sources remain unknown. Protons and heavy ions, which comprise cosmic rays, interact with galactic and intergalactic magnetic fields and, consequently, do not point back to their sources upon measurement. Neutrinos, which are inevitably produced in photohadronic interactions, travel unimpeded through the universe and disclose the directions of their sources. Among the most plausible candidates for the origins of UHECRs is a class of astrophysical phenomena known as gamma-ray bursts (GRBs). GRBs are the most violent and energetic events witnessed in the observable universe. The IceCube Neutrino Observatory, located in the glacial ice 1450 m to 2450 m below the South Pole surface, is the largest neutrino detector in operation. IceCube detects charged particles, such as those emitted in high energy neutrino interactions in the ice, by the Cherenkov light radiated by these particles. The measurement of neutrinos of 100 TeV energy or greater in IceCube correlated with gamma-ray photons from GRBs, measured by spacecraft detectors, would provide evidence of hadronic interaction in these powerful phenomena and confirm their role in ultra high energy cosmic ray production. This work presents the first IceCube GRB-neutrino coincidence search optimized for charged-current interactions of electron and tau neutrinos as well as neutral-current interactions of all neutrino flavors, which produce nearly spherical Cherenkov light showers in the ice. These results for three years of data are combined with the results of previous searches over four years of data optimized for charged-current muon neutrino interactions, which produce extended Cherenkov light tracks. Several low significance events correlated with GRBs were detected, but are consistent with the background expectation from atmospheric muons and neutrinos. The combined results produce limits that place the strongest constraints thus far on models of neutrino and UHECR production in GRB fireballs.Item SEMICONDUCTOR AND GLASS MICRO-RESONATORS(2015) KUO, LICHIANG; Davis, Christopher; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis we have demonstrated the cascading of two photonic AND logic gates by using two symmetric semiconductor GaAs microring resonators. In addition, we have developed a new, low-cost method for fabricating glass microring resonators. In the first part of this work, we discuss the properties of microring resonators and describe the fabrication of semiconductor microring resonators by the research team of which I was a member. In the experiments on cascaded logic gates we launched one probe and pump beam into different input waveguides, respectively. The first ring works as a AND logic gate for probe and pump beams. The output beam from the first ring goes to the second ring. The second ring also work as a AND logic gate using the second pump to switch the beam coming from the first ring. We successfully demonstrated cascading two photonic logic gates by using two symmetric semiconductor GaAs microrings. In the second part of this work, we extended our prior work on the fabrication of semiconductor microrings in a clean room to a purely mechanical method of glass microring fabrication. Many laboratories, including ours, lack the expensive facilities needed for the lithographic fabrication of microrings. And, a low-cost, high yield method of fabrication may have significant application in the development of disposable microring sensors. We have built up a complete mass=production capability based on glass capillary pulling and micro-polishing to fabricate glass microrings, because there were no available off-the-shelf systems available from industry at affordable prices. This method of producing highly polished glass micro-resonators has many advantages, such as fast fabrication (≦6 weeks), high yield (≧50%) (percentage of devices w/o cracks on edge), low cost (no need to use costly facilities in a clean room), mass production (800~1200 devices per batch). The surface quality of glass resonators should be excellent because capillaries were made at high temperature ≧1000℃ and devices were polished by suspension slurry of 70 nm colloidal silica. Further measurements that are beyond our current capability are needed for final verification. If some fabrication steps could be optimized in the future, we estimate that the fabrication time could be within 2 weeks, the yield rate would be higher than 90 %, and the number of devices per batch could be more than 1,200. This innovative method opens a new path for microresonator fabrication at low cost and in fast mass production. In sensor applications where low cost and mass production could be important, our work is an important first step to making microring sensors inexpensive, if further work in characterizing them can be done. Glass microresonators can play a key role, for example, in gas sensors, chemical sensors, liquid sensors, biological sensors, and vibration sensors. Two appendices in this thesis list the most significant sub-systems of the whole system we designed and built for producing glass microring resonators. Designs and engineering drawings are also listed in Appendices.Item III-V Optoelectronic Devices: Room temperature CW operation of interband cascade laser & High efficiency p-side down InGaN/GaN solar cell(2011) Ryu, Geunmin; Dagenais, Mario; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)During the past two decades, the field of III-V optoelectronic devices has gained widespread interest as a result of advances in the performance and reliability of epitaxial structures. In principle, III-V materials can provide sources, detectors and optoelectronic components over wavelengths from UV to IR. During my Ph.D study, I have focused on two III-V optoelectronic devices: Mid-IR interband cascade lasers and group III-Nitride solar cells. In the first part of this dissertation, we will discuss development of a room temperature CW operation interband cascade laser and in the second part, we will discuss the concept of high efficiency III-N solar cells. Part I Lasers that emit in the mid-IR (3~5um) spectral region can be used in many civilian and military applications such as chemical sensing, free space optical communication and IR countermeasures. There are three types of lasers that can cover the Mid-IR region. First, conventional type-I quantum well (QW) lasers on GaSb substrates, second, inter-subband quantum cascade lasers (QCLs) on InP substrates and finally interband cascade laser with type-II alignment of the conduction and valence bands on GaSb substrates. Gallium Antimonide based type II interband cascade lasers (ICLs) cover the 3~4 um wavelength range, and it is the most natural match to the mid-IR. For most applications, it is required that the laser operates in continuous wave (CW) mode either at room temperature or at temperatures accessible to thermoelectric coolers. Recently, we have been able to operate interband cascade lasers in CW mode at room temperature with 62mW of output power, internal loss of 4.8cm-1, 170mW/A slope efficiency, and a threshold current density as low as 300 A/cm2 which are a significant milestone toward many applications. In the first part of this thesis, we are going to talk about the fundamental principles of operation of the ICLs and their applications. Secondly, we will present the development of a fabrication process. Third, we will discuss the performance characteristics of ICLs. Lasers were characterized by doing series of length dependent pulsed/CW measurements to obtain critical parameters at low temperature and at room temperature; such as wall plug efficiency, threshold current density, internal loss, and thermal impedance. For low temperature CW measurement, a specially designed vacuum chamber was used to prevent water condensation. Finally, we will present ICL optimization processes. For laser optimization, we re-designed the device structure, in particular the lower cladding region, the injection region, and the active region thickness, to achieve a higher confinement factor and lower loss, thus increasing the operating temperature and the output power. Part II Since the 1950s, silicon solar cells have been intensively studied and developed. Solar cell technology has greatly benefited from the maturity of silicon technology developed originally for the IC industry. This has led to the development of high quality single crystal silicon wafers with low dislocation densities. However, because of the poor spectral overlap between the absorption of silicon cells and the spectrum of solar light, silicon solar cells cannot fundamentally produce high efficiency solar cells. In order to achieve high efficiency solar cells, researchers have investigated many alternatives including tandem cells, GaAs, and III-Nitride materials. In the second part of this thesis, we will talk about the development of high efficiency III-Nitride solar cells using novel p-side InGaN/GaN materials, including device background, new solar cell design, fabrication process development, and preliminary device characterizations.