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
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item Chiral Quantum Optics using Topological Photonics(2020) Barik, Sabaysachi; Waks, Edo EW; Hafezi, Mohammad MF; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Topological photonics has opened new avenues to designing photonic devices along with opening a plethora of applications. Recently, even though there have been many interesting studies in topological photonics in the classical domain, the quantum regime has remained largely unexplored. In this thesis, I will demonstrate a recently developed topological photonic crystal structure for interfacing a single quantum dot spin with a photon to realize light-matter interaction with topolog-ical photonic states. Developed on a thin slab of Gallium Arsenide(GaAs) mem- brane with electron beam lithography, such a device supports two robust counter- propagating edge states at the boundary of two distinct topological photonic crystals at near-IR wavelength. I will show the chiral coupling of circularly polarized lights emitted from a single Indium Arsenide(InAs) quantum dot under a strong magnetic field into these topological edge modes. Owing to the topological nature of these guided modes, I will demonstrate this photon routing to be robust against sharp corners along the waveguide. Additionally, taking it further into the cavity-QED regime, we will build a topological photonic crystal resonator. This new type of resonator will be based on valley-Hall topological physics and sustain two counter- propagating resonator modes. Thanks to the robustness of the topological edge modes to sharp bends, the newly formed resonators can take various shapes, the simplest one being a triangular optical resonator. We will study the chiral coupling of such resonator modes with a single quantum dot emission. Moreover, we will show an intensity enhancement of a single dot emission when it resonantly couples with a cavity mode. This new topological photonic crystal platform paves paths for fault-tolerant complex photonic circuits, secure quantum computation, and explor- ing unconventional quantum states of light and chiral spin networks.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.