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 STUDYING MANY-BODY PHYSICS WITH QUANTUM DOT QUBITS(2022) Buterakos, Donovan Lewis; Das Sarma, Sankar; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quantum dot spin qubits are a promising platform for realizing quantum information technologies, which can theoretically perform calculations such as factoring large integers that are otherwise intractable using classical computing methods. However, quantum dot qubit technology is still in its developmental phases, with current experimental devices capable of holding only a few (less than 10) noisy qubits. Even with only a small number of quantum dots, interesting experiments can be performed, simulating physical systems and observing many-body phenomena which are otherwise difficult to study or numerically simulate classically. In the first part of this thesis, we analytically examine valley states in Silicon, which is one obstacle which can potentially lead to information loss in Silicon qubits. Using a perturbative method, we calculate the dynamics of two exchange-coupled quantum dots in which there is a valley degree of freedom. We find that the spin states can become entangled with the valley states of the system if the electrons are not initialized to the correct valley states, which can adversely affect quantum computations performed on these systems. In the second part of this thesis, we detail how quantum dot plaquettes can simulate the Hubbard model and give many analytic results for different magnetic phenomena that arise under this model. These results include examples of Nagaoka ferromagnetism, violations of Hund's rule, and situations where flatband ferromagnetic ground states are necessarily degenerate with nonferromagnetic states. These phenomena all require only a few quantum dots, and are observable with current experimental technologies.Item Nanophotonic quantum interface for a single solid-state spin(2016) Sun, Shuo; Waks, Edo; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The ability to store and transmit quantum information plays a central role in virtually all quantum information processing applications. Single spins serve as pristine quantum memories whereas photons are ideal carriers of quantum information. Strong interactions between these two systems provide the necessary interface for developing future quantum networks and distributed quantum computers. They also enable a broad range of critical quantum information functionalities such as entanglement distribution, non-destructive quantum measurements and strong photon-photon interactions. Realizing spin-photon interactions in a solid-state device is particularly desirable because it opens up the possibility of chip-integrated quantum circuits that support gigahertz bandwidth operation. In this thesis, I demonstrate a nanophotonic quantum interface between a single solid-state spin and a photon, and explore its applications in quantum information processing. First, we experimentally realize a spin-photon quantum phase switch based on a strongly coupled quantum dot and photonic crystal cavity system. This device enables coherent light-matter interactions at the fundamental limit, where a single spin controls the polarization of a photon and a single photon flips the spin state. Furthermore, we theoretically propose a way to deterministically generate spin-photon entanglement based on the spin-photon quantum interface, which is an important step towards solid-state implementations of quantum repeaters and quantum networks. Next, we show both theoretically and experimentally, a new method to optically read out a solid-state spin based on the same cavity quantum electrodynamics (QED) system. This new method achieves significant improvement in spin readout fidelity over typical approaches using fluorescence light detection. In the end, we report efforts to realize tunable and robust quantum dot based cavity QED systems. We present a technique for tuning the frequency of a quantum dot that is strongly coupled to a photonic crystal cavity by applying strain. This tuning technique enables us to accurately control the detuning between a quantum dot and a cavity without affecting other emission properties of the dot, which is essential for lots of applications associated with cavity QED systems, including non-classical light generation, photon blockade, single photon level optical switch, and also our major focus, the spin-photon quantum interface.Item PHOTONIC ENGINEERING OF ABSORPTION AND EMISSION IN PHOTOVOLTAICS(2016) Xu, Yunlu; Munday, Jeremy N; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)As modern society advances, the demand for clean and renewable energy resources becomes more and more important. The sun is by far the most abundant source of renewable energy and is indirectly responsible for many other energy resources on earth (e.g. sunlight enables photosynthesis, biofuels, wind, and even carbon-based fuels). A solar cell directly converts the energy of solar illumination into electricity through the photovoltaic effect and is expected to play a crucial role in the future total power generation globally. Our work has focused on photonic approaches to improving the conversion efficiency of solar cells. Toward this goal, we present results describing the use of quantum dot emission to redirect light within a solar cell, as well as the modification of absorption and emission of light from a solar cell using nanostructures and thin films to increase the efficiency to approach (or possibly surpass) the currently understood efficiency limits for traditional devices. The Shockley-Queisser (SQ) limit describes the maximum solar power conversion efficiency achievable for a p-n junction composed of a particular material and is the standard by which new photovoltaic technologies are compared. This limit is based on the principle of detailed balance, which equates the photon flux into a device to the particle flux (photons or electrons) out of that device. Based on this theory, we describe how the efficiency of a photovoltaic cell is altered in the presence of new anti-reflection coatings, nanotexturing (e.g. plasmonic nanoparticle, nanowire), and more advanced photonic structures (e.g. photonic crystals) that are capable of modifying the absorption and emission of photons. Nanostructured solar cells represent a novel class of photovoltaic devices. By careful selection of materials, as well as particle shapes and positions, the device performance can be improved by increasing the optical path length for scattered light, improving the modal distribution of the light within the absorber, and increasing light concentration (or angle restriction). For example, nanowires can yield microscale concentration effects to improve device performance; however, it has been unclear whether or not they can exceed the Shockley-Queisser limit. We show that single-junction nanostructured solar cells have a theoretical maximum efficiency of ∼ 42% under AM 1.5 solar illumination. While this exceeds the efficiency of a non-concentrating planar device, it does not exceed the Shockley-Queisser limit for a planar device with optical concentration. For practical devices, we include the effect of diffuse illumination and find that with the modest optical concentration available from nanostructures (× 1,000), an efficiency of 35.5% is achievable even with 25% diffusive solar radiation. Finally, we discuss how photon emission modification offers an approach for low bandgap materials to achieve higher efficiencies. By incorporating specifically designed photonic structures that restrict the absorption and emission of above bandgap photons, the bandgap of materials can be effectively tuned. Similarly, restriction of the emission angle leads to increased optical concentration. For realistic devices, we consider how both of these effects are affected by non-ideal materials and photonic structures. We find that the photonic crystal bandgap required to achieve maximum efficiency depends critically on the reflectivity of the photonic crystal. We experimentally demonstrated that the semiconductor bandgap of a material need not be an intrinsic property of that material but can be changed through photonic structuring of the surrounding layers. GaAs has a natural bandgap of 1.43 eV; however, we show that optical reflectors can be used to induce photon-recycling effects, which result in a bandgap shift of 0.13 eV. When a p-n junction is created within the GaAs, we find that its electrical properties are also shifted resulting in a 1.71 mV improvement in the open-circuit voltage of the device under 0.6 suns equivalent illumination. These results show that both the optical and electrical properties of a semiconductor can be modified purely by photonic manipulation, which enables a fundamentally new method for designing semiconductor structures and devices. We anticipate that our result will enable a range of optoelectronic devices.Item Self-Assembled InAs/GaAs Quantum Dot Solar Cells(2015) Li, Tian; Dagenais, Mario; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Our work focuses on experimental and theoretical studies aimed at establishing a fundamental understanding of the principal electrical and optical processes governing the operation of quantum dot solar cells (QDSC) and their feasibility for the realization of intermediate band solar cell (IBSC). Uniform performance QD solar cells with high conversion efficiency have been fabricated using carefully calibrated process recipes as the basis of all reliable experimental characterization. The origin for the enhancement of the short circuit current density (Jsc) in QD solar cells was carefully investigated. External quantum efficiency (EQE) measurements were performed as a measure of the below bandgap distribution of transition states. In this work, we found that the incorporation of self-assembled quantum dots (QDs) interrupts the lattice periodicity and introduce a greatly broadened tailing density of states extending from the bandedge towards mid-gap. A below-bandgap density of states (DOS) model with an extended Urbach tail has been developed. In particular, the below-bandgap photocurrent generation has been attributed to transitions via confined energy states and background continuum tailing states. Photoluminescence measurement is used to measure the energy level of the lowest available state and the coupling effect between QD states and background tailing states because it results from a non-equilibrium process. A basic I-V measurement reveals a degradation of the open circuit voltage (Voc) of QD solar cells, which is related to a one sub-bandgap photon absorption process followed by a direct collection of the generated carriers by the external circuit. We have proposed a modified Shockley-Queisser (SQ) model that predicts the degradation of Voc compared with a reference bulk device. Whenever an energy state within the forbidden gap can facilitate additional absorption, it can facilitate recombination as well. If the recombination is non-radiative, it is detrimental to solar cell performance. We have also investigated the QD trapping effects as deep level energy states. Without an efficient carrier extraction pathway, the QDs can indeed function as mobile carriers traps. Since hole energy levels are mostly connected with hole collection under room temperature, the trapping effect is more severe for electrons. We have tried to electron-dope the QDs to exert a repulsive Coulomb force to help improve the carrier collection efficiency. We have experimentally observed a 30% improvement of Jsc for 4e/dot devices compared with 0e/dot devices. Electron-doping helps with better carrier collection efficiency, however, we have also measured a smaller transition probability from valance band to QD states as a direct manifestation of the Pauli Exclusion Principle. The non-linear performance is of particular interest. With the availability of laser with on-resonance and off-resonance excitation energy, we have explored the photocurrent enhancement by a sequential two-photon absorption (2PA) process via the intermediate states. For the first time, we are able to distinguish the nonlinearity effect by 1PA and 2PA process. The observed 2PA current under off-resonant and on-resonant excitation comes from a two-step transition via the tailing states instead of the QD states. However, given the existence of an extended Urbach tail and the small number of photons available for the intermediate states to conduction band transition, the experimental results suggest that with the current material system, the intensity requirement for an observable enhancement of photocurrent via a 2PA process is much higher than what is available from concentrated sun light. In order to realize the IBSC model, a matching transition strength needs to be achieved between valance band to QD states and QD states to conduction band. However, we have experimentally shown that only a negligible amount of signal can be observed at cryogenic temperature via the transition from QD states to conduction band under a broadband IR source excitation. Based on the understanding we have achieved, we found that the existence of the extended tailing density of states together with the large mismatch of the transition strength from VB to QD and from QD to CB, has systematically put into question the feasibility of the IBSC model with QDs.