Browsing by Subject "Quantum dots"
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Hannigan, Justin Michio, 1977 (University of Oregon, December , 2009)[more][less]Hannigan, Justin Michio, 1977 20100726T21:41:01Z 20100726T21:41:01Z 200912 http://hdl.handle.net/1794/10548 xv, 204 p. : ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. This thesis reports on progress made toward realizing strong cavity quantum electrodynamics coupling in a novel microcavity operating close to the hemispherical limit. Microcavities are ubiquitous wherever the aim is observing strong interactions in the lowenergy limit. The cavity used in this work boasts a novel combination of properties. It utilizes a curved mirror with radius in the range of 4060 µm that exhibits high reflectivity over a large solid angle and is capable of producing a diffraction limited mode waist in the approach to the hemispherical limit. This small waist implies a correspondingly small effective mode volume due to concentration of the field into a small transverse distance. The cavity assembled for this investigation possesses suitably low loss (suitably low linewidth) to observe vacuum Rabi splitting under suitable conditions. According to best estimates for the relevant system parameters, this system should be capable of displaying strong coupling. The dipole coupling strength, cavity loss and quantum dot dephasing rates are estimated to be, respectively, g = 35µeV, κ = 30µeV, and γ = 15µeV. A survey of two different distributed Bragg reflector (DBR) samples was carried out. Four different probe lasers were used to measure transmission spectra for the coupled cavityQED system. The system initially failed to display strong coupling due to the available lasers being too far from the design wavelength of the spacer layer, corresponding to a loss of field strength at the location of the quantum dots. Unfortunately, the only available lasers capable of probing the design wavelength of the spacer layer had technical problems that prevented us from obtaining clean spectra. Both a Ti:Al 2 O 3 and a diode laser were used to measure transmission over the design wavelength range. The cavity used here has many promising features and should be capable of displaying strong coupling. It is believed that with a laser system centered at the design wavelength and possessing low enough linewidth and singlemode operation across a wide wavelength range strong coupling should be observable in this system. Committee in charge: Hailin Wang, Chairperson, Physics; Michael Raymer, Advisor, Physics; Jens Noeckel, Member, Physics; Richard Taylor, Member, Physics; Andrew Marcus, Outside Member, Chemistry en_US University of Oregon University of Oregon theses, Dept. of Physics, Ph. D., 2009; Optical microcavity Strong coupling Quantum dots Quantum physics Optics Hemispherical optical microcavity for cavityQED strong coupling Thesis

Hoffmann, Eric A., 1982 (University of Oregon, December , 2009)[more][less]Hoffmann, Eric A., 1982 20100728T23:32:25Z 20100728T23:32:25Z 200912 http://hdl.handle.net/1794/10552 xi, 193 p. : ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. State of the art semiconductor materials engineering provides the possibility to fabricate devices on the lower end of the mesoscopic scale and confine only a handful of electrons to a region of space. When the thermal energy is reduced below the energetic quantum level spacing, the confined electrons assume energy levels akin to the coreshell structure of natural atoms. Such "artificial atoms", also known as quantum dots, can be loaded with electrons, onebyone, and subsequently unloaded using source and drain electrical contacts. As such, quantum dots are uniquely tunable platforms for performing quantum transport and quantum control experiments. Voltagebiased electron transport through quantum dots has been studied extensively. Far less attention has been given to thermoelectric effects in quantum dots, that is, electron transport induced by a temperature gradient. This dissertation focuses on the efficiency of direct thermaltoelectric energy conversion in InAs/InP quantum dots embedded in nanowires. The efficiency of thermoelectric heat engines is bounded by the same maximum efficiency as cyclic heat engines; namely, by Carnot efficiency. The efficiency of bulk thermoelectric materials suffers from their inability to transport charge carriers selectively based on energy. Owing to their threedimensional momentum quantization, quantum dots operate as electron energy filtersa property which can be harnessed to minimize entropy production and therefore maximize efficiency. This research was motivated by the possibility to realize experimentally a thermodynamic heat engine operating with nearCarnot efficiency using the unique behavior of quantum dots. To this end, a microscopic heating scheme for the application of a temperature difference across a quantum dot was developed in conjunction with a novel quantumdot thermometry technique used for quantifying the magnitude of the applied temperature difference. While pursuing highefficiency thermoelectric performance, many mesoscopic thermoelectric effects were observed and studied, including Coulombblockade thermovoltage oscillations, thermoelectric power generation, and strong nonlinear behavior. In the end, a quantumdotbased thermoelectric heat engine was achieved and demonstrated an electronic efficiency of up to 95% Carnot efficiency. Committee in charge: Stephen Kevan, Chairperson, Physics; Heiner Linke, Member, Physics; Roger Haydock, Member, Physics; Stephen Hsu, Member, Physics; David Johnson, Outside Member, Chemistry en_US University of Oregon University of Oregon theses, Dept. of Physics, Ph. D., 2009; Thermoelectric efficiency Quantum dots Nanowires Indium arsenide Indium phosphide Solid state physics Materials science The thermoelectric efficiency of quantum dots in indium arsenide/indium phosphide nanowires Thesis

Nakpathomkun, Natthapon, 1973 (University of Oregon, December , 2010)[more][less]Nakpathomkun, Natthapon, 1973 20110408T21:20:10Z 20110408T21:20:10Z 201012 http://hdl.handle.net/1794/11060 xii, 106 p. : ill. (some col.) Quantum dots are systems in which all three spatial sizes are comparable to the Fermi wavelength. The strong confinement leads to a discrete energy spectrum. A goal of thermoelectric research is to find a system with a high thermoelectric figure of merit, which is related to the efficiency of solidstate heat engines. The deltalike density of states of quantum dots has been predicted to boost this figure of merit. This dissertation addresses some thermoelectric properties relevant to the thermaltoelectric energy conversion using InAs/InP quantum dots embedded in nanowires. In thermoelectric experiments, a temperature difference must be established and its value needs to be determined. A novel technique for measuring electron temperature across the dot is presented. A strong nonlinearity of the thermocurrent as a function of temperature difference is observed at a small ratio of temperature gradient and cryostat temperature. At large heating currents, a sign reversal is observed. Numerical calculations explore the contribution of the energy dependence of the transmission function to this effect. Depending on the relative contributions from sequential tunneling and cotunneling, thermovoltages of quantum dots generally have one of two different lineshapes: a sawtooth shape or a shape similar to the derivative of the conductance peak. Here a simple picture is presented that shows that thermovoltage lineshape is accurately predicted from the energy level spacing inside the dot and the width of the transmission function. An important figure of merit of all heat engines is the efficiency at maximum power. Here the thermoelectric efficiency at maximum power of quantum dots is numerically compared to that of two other lowdimensional systems: an ideal onedimensional conductor (1D) and a thermionic power generator (TI). The numerical calculations show that either 1D or TI systems can produce the highest maximum power depending on the operating temperature, the effective mass of the electron, and the effective area of the TI system. In spite of this, 1D systems yield the highest efficiency at maximum power. Committee in charge: Dr. Richard Taylor, Chair; Dr. Heiner Linke, Research Advisor; Dr. Dietrich Belitz; Dr. David Johnson; Dr. David Strom en_US University of Oregon University of Oregon theses, Dept. of Physics, Ph. D., 2010; Nanowires Quantum dots Thermoelectrics Low temperature physics Nanotechnology Thermoelectric properties of quantum dots and other lowdimensional systems Thesis
Now showing items 13 of 3