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.
Browse
6 results
Search Results
Item TOPOLOGICAL PHOTONICS: NESTED FREQUENCY COMBS AND EDGE MODE TAPERING(2024) Flower, Christopher James; Hafezi, Mohammad; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Topological photonics has emerged in recent years as a powerful paradigm for the designof photonic devices with novel functionalities. These systems exhibit chiral or helical edge states that are confined to the boundary and are remarkably robust against certain defects and imperfections. While several applications of topological photonics have been demonstrated, such as robust optical delay lines, quantum optical interfaces, lasers, waveguides, and routers, these have largely been proof-of-principle demonstrations. In this dissertation, we present the design and generation of the first topological frequency comb. While on-chip generation of optical frequency combs using nonlinear ring resonators has led to numerous applications of combs in recent years, they have predominantly relied on the use of single-ring resonators. Here, we combine the fields of linear topological photonics and frequency microcombs and experimentally demonstrate the first frequency comb of a new class in an array of hundreds of ring resonators. Through high-resolution spectrum analysis and out-of- plane imaging we confirm the unique nested spectral structure of the comb, as well as the confinement of the parametrically generated light. Additionally, we present a theoretical study of a new kind of valley-Hall topological photonic crystal that utilizes a position dependent perturbation (or “mass-term”) to manipulate the width of the topological edge modes. We show that this approach, due to the inherent topological robustness of the system, can result in dramatic changes in mode width over short distances with minimal losses. Additionally, by using a topological edge mode as a waveguide mode, we decouple the number of supported modes from the waveguide width, circumventing challenges faced by more conventional waveguide tapers.Item Nonlinear and Stochastic Dynamics of Optoelectronic Oscillators(2024) Ha, Meenwook; Chembo, Yanne K.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Optoelectronic oscillators (OEOs) are nonlinear, time-delayed and self-sustained microwave photonic systems capable of generating ultrapure radiofrequency (RF) signals with extensive frequency tunabilities. Their hybrid architectures, comprising both optical and electronic paths, underscore their merits. One of the most notable points of OEOs can be unprecedentedly high quality factors, achieved by storing optical energies for RF signal generations. Thanks to their low phase noise and broad frequency tunabilities, OEOs have found diverse applications including chaos cryptography, reservoir computing, radar communications, parametric oscillator, clock recovery, and frequency comb generation. This thesis pursues two primary objectives. Firstly, we delve into the nonlinear dynamics of various OEO configurations, elucidating their universal behaviors by deriving corresponding envelope equations. Secondly, we present a stochastic equation delineating the dynamics of phases and explore the intricacies of the phase dynamics. The outputs of OEOs are defined in their RF ports, with our primary focus directed towards understanding the dynamics of these RF signals. Regardless of their structural complexities, we employ a consistent framework to explore these dynamics, relying on the same underlying principles that determine the oscillation frequencies of OEOs. To comprehend behaviors of OEOs, we analyze the dynamics of a variety of OEOs. For simpler systems, we can utilize the dynamic equations of bandpass filters, whereas more complex physics are required for expressing microwave photonic filtering. Utilizing an envelope approach, which characterizes the dynamics of OEOs in terms of complex envelopes of their RF signals, has proven to be an effective method for studying them. Consequently, we derive envelope equations of these systems and research nonlinear behaviors through analyses such as investigating bifurcations, stability evaluations, and numerical simulations. Comparing the envelope equations of different models reveals similarities in their dynamic equations, suggesting that their dynamics can be governed by a generalized universal form. Thus, we introduce the universal equation, which we refer to as the universal microwave envelope equation and conduct analytical investigations to further understand its implications. While the deterministic universal equation offers a comprehensive tool for simultaneous exploration of various OEO dynamics, it falls short in describing the stochastic phase dynamics. Our secondary focus lies in investigating phase dynamics through the implementation of a stochastic approach, enabling us to optimize and comprehend phase noise performance effectively. We transform the deterministic universal envelope equation into a stochastic delay differential form, effectively describing the phase dynamics. In our analysis of the oscillators, we categorize noise sources into two types: additive noise contribution, due to random environmental and internal fluctuations, and multiplicative noise contribution, arising from noisy loop gains. The existence of the additive noise is independent of oscillation existence, while the multiplicative noise is intertwined with the noisy loop gains, nonlinearly mixing with signals above the threshold. Therefore, we investigate both sub- and above-threshold regimes separately, where the multiplicative noise can be characterized as white noise and colored noise in respective regimes. For the above-threshold regime, we present the stochastic phase equation and derive an equation for describing phase noise spectra. We conduct thorough investigations into this equation and validate our approaches through experimental verification. In the sub-threshold regime, we introduce frameworks to experimentally quantify the noise contributions discussed in the above-threshold part. Since no signal is present here and the oscillator is solely driven by the stochastic noise, it becomes feasible to reverse-engineer the noise powers using a Fourier transform formalism. Here, we introduce a stochastic expression written in terms of the real-valued RF signals, not the envelopes, and the transformation facilitates the expressions of additive and multiplicative noise contributions as functions of noisy RF output powers. The additive noise can be defined by deactivating the laser source or operating the intensity modulator at the minimum transmission point, given its independence from the loop gains. Conversely, the expression for the multiplicative noise indicates a dependence on the gain, however, experimental observations suggest that its magnitude may remain relatively constant beyond the threshold.Item Wave Chaos Studies and The Realization of Photonic Topological Insulators(2022) Xiao, Bo; Anlage, Steven; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Wave propagation in various complex media is an interesting and practical field that has a huge impact in our daily life. Two common types of wave propagation are examined in this thesis: electromagnetic wave propagation in complex wave chaotic enclosures, where I studied its statistical properties and explored time-domain pulse focusing, and unidirectional edge modes propagating in a reciprocal photonic topological insulator waveguide. Several theories, e.g. the Random Matrix Theory and the Random Coupling Model, have been developed and validated in experiments to understand the statistical properties of the electromagnetic waves inside wave chaotic enclosures. This thesis extends the subject from a single cavity to a network of coupled cavities by creating an innovative experimental setup that scales down complex structures, which would otherwise be too large and cumbersome to study, to a miniature version that retains its original electromagnetic properties. The process involves shrinking down the metal cavity in size by a factor of 20, increasing the electromagnetic wave frequency by the same factor and cooling down the cavity by a dilution refrigerator to reduce its ohmic loss. This experimental setup is validated by comparison with results from a full-scale setup with a single cavity and it is then extended for multiple coupled cavities. In the time domain, I utilized the time-reversal mirror technique to focus electromagnetic waves at an arbitrary location inside a wave chaotic enclosure by injecting a numerically calculated wave excitation signal. I used a semi-classical ray algorithm to calculate the signal that would be received at a transceiver port resulting from the injection of a short pulse at the desired target location. The time-reversed version of this signal is then injected into the transceiver port and an approximate reconstruction of the short pulse is observed at the target port. Photonic topological insulators are an interesting class of materials whose photonic band structure can have a bandgap in the bulk while supporting topologically protected unidirectional edge modes. This thesis presents a rotating magnetic dipole antenna, composed of two perpendicularly oriented coils fed with variable phase difference, that can efficiently excite the unidirectional topologically protected surface waves in the bianisotropic metawaveguide (BMW) structure recently realized by Ma, et al., despite the fact that the BMW medium does not break time-reversal invariance. In addition to achieving high directivity, the antenna can be tuned continuously to excite reflectionless edge modes to the two opposite directions with various amplitude ratios. Overall, this thesis establishes the foundation for further studies of the universal statistical properties of wave chaotic enclosures, and tested the limits of its deterministic properties defined by the cavity geometry. It also demonstrated in experiment the excitation of a unidirectional edge mode in a Bianisotropic Meta-waveguide, allowing for novel applications in the field of communications, for example phased array antennas.Item SPIN-PHOTON INTERFACE USING CHARGE TUNABLE QUANTUM DOTS(2021) Luo, Zhouchen; Waks, Edo; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The unconditional security of quantum networks and unparalleled acceleration of quantum algorithms enabled by “quantum computers” has motivated significant research across scientific communities. Among these different architectures to achieve such quantum information processing paradigms, a promising and straightforward proposal is the use of photons as flying qubits to transfer quantum information as well as local quantum memories to store and process quantum information. Toward this goal, it is very important to develop a type of quantum memory that can efficiently interface with photons while possessing good qubit properties, including a long coherence time and good scalability. To date, researchers have developed promising solid state quantum memory platforms, such as defects in diamond and other group IV compounds, rare earth ions hosted in various materials and self-assembled quantum dots. While each platform has its strengths and challenges, this thesis will focus on charge tunable InAs quantum dots grown inside a GaAs matrix that is doped into a PN junction. Though not long after the first demonstration of optically active self-assembled quantum dots, researchers have already developed the idea to sandwich them inside a PN junction to tune their charge status. The spin manipulation in the strong coupling regime has been mostly using these dots without PN junction doping, which has resulted in limited dot-cavity cooperativity and spin lifetime due to electron tunneling. In this thesis, I will first show the design, fabrication and characterization of several common photonic cavities, with their performance compared. Second I will show strong coupling between a negatively charged quantum dot and photonic crystal cavity, where the resonant cavity reflectivity is strongly dependent on the spin state. Third I will show that the electron spin lifetime (T1) can be significantly shortened by an off-resonant laser that reaches the device surface. While the exact reason for this undesired effect is not clear yet, we did observe the thickness of the electron tunnel barrier of the quantum dot wafer can result in distinct spin properties. I will present electron spin T1 characterization across several different quantum dot samples with different electron tunneling barrier thickness. Lastly, I will present coherent control of electron spin using picosecond laser pulse and sidebands of modulated continuous wave laser with limited spin rotation fidelity due to the off-resonant laser induced deterioration of the spin properties.Item TOPOLOGICAL PHOTONICS AND EXPERIMENTAL TECHNIQUES IN QUANTUM OPTICS(2020) ORRE, VENKATA VIKRAM; Hafezi, Mohammad; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Topological photonics is the study of how the geometries and topologies of devices can be used to manipulate the behavior of photons. Many topological models exhibit edge states, a defining feature of these models, which travel around the perimeter of the lattice and are not affected by disorder. These edge states can help create scalable delay lines, quantum sources of light, and lasers all of which are robust against fabrication-induced disorder and bends in photonic structures. This research proposal is structured into two parts. In the first half of this thesis, we investigate a topological model and study some of its quantum applications. First, we realize the anomalous quantum Hall effect in a photonic platform using a 2D array of ring resonators with zero flux threading the lattice. The lattice implements a Haldane model by using the next-nearest couplings in lattice to simulate a nonzero local gauge flux while having a net flux of zero. The lattice hosts edge states, which are imaged through a CCD camera and show robustness against missing site-defects, 90 degree bends and fabrication-induced disorder. We also demonstrate a topological non-trivial to trivial phase transition by simply detuning the ring resonances. Next, we show degenerate photon pair creation in an anomalous quantum Hall device using a dual-pump spontaneous fourwave mixing process. The linear dispersion in the edge band results in an efficient phase matching and shows up as maximum counts in spectral correlations. The flatness of edge band also allows us to tune the bandwidth of the quantum source by changing the pump frequencies. Furthermore, we verify the indistinguishability of the photons using a Hong-Ou-Mandel (HOM) experiment. Finally, we simulate the transport of time-bin entangled photons in an integer quantum Hall device. The edges states preserve the temporal correlations and are robust against fabrication induced disorder. In contrast, the bulk states in the device exhibit localization, which is manifested in bunching/anti-bunching behavior. In the second part, we explore a few experimental quantum optics techniques developed as a part of investigating quantum transport in topological devices. We demonstrate two experimental techniques: 1. We use an EOM-based time-lens technique to resolve temporal correlations of time-bin entangled photons, which would have been otherwise inaccessible due to the limited temporal resolution of single photon detectors. Our time-lens also maps temporal correlations to spectral correlations and provides a way of manipulating frequency-bin entangled photons. 2. We show frequency-resolved interference of two and three photons distinguishable in time, which would not have interfered in a standard Hong-Ou-Mandel (HOM) setup. Our setup can be extended to implement temporal boson sampling using phase modulators. Furthermore, we demonstrate time-reversed HOM-like interference using time-bin entangled photon pairs and show that the spectral correlations are sensitive to phase between photons. Lastly, we demonstrate some miscellaneous experimental techniques, such as the design of the electronics used for time-lens, optimal spontaneous parametric down conversion parameters, measuring joint spectral intensity using a chirped bragg grating, and simultaneous measurement of Hong-Ou-Mandel interference for different frequencies.Item Topological Edge States in Silicon Photonics(2014) Mittal, Sunil; Hafezi, Mohammad; Migdall, Alan; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Under the influence of a magnetic field, at low temperatures, charged particles confined in two-dimensional systems exhibit a remarkable range of macroscopic quantum phenomena such as the quantum Hall effects. A hallmark of these phenomena is the presence of unidirectional, topologically robust edge states - states which are confined to the edge of the system. It is, in principle, possible to engineer a synthetic magnetic field for photons and hence achieve photonic analogs of the robust electronic edge states. Investigating photonic edge states is interesting from a fundamental perspective of studying photonic transport in the presence of a gauge field and also for its application in classical and quantum information processing. In this thesis, we present the implementation of a synthetic magnetic field for photons and our observation of topological edge states in a two-dimensional lattice of coupled ring resonators, fabricated using CMOS-compatible silicon-oninsulator technology. We qualitatively show the robustness of edge states against deliberately induced lattice defects. We then analyze the statistics of transport measurements (transmission and delay) made on a number of different devices and quantitatively verify the robustness of edge states against lattice disorder. Using Wigner delay-time distribution, we show that localization is suppressed in the edge states. Furthermore, to unequivocally establish the non-trivial topological nature of edge states, we compare their transmission to a topologically trivial one dimensional system of coupled ring resonators and demonstrate that the edge states achieve higher transmission. Moreover, for photonic analogs of the quantum Hall effect, the winding number - a topologically invariant integer which characterizes edge states - is quantized, analogous to quantization of conductivity in electronic systems. We measure the winding number of the edge states in our system. Finally, we investigate the effect of nonlinear interactions in silicon ring resonators, on the stability of edge states. We show that the presence of a strong pump can result in a significant decrease in the transmission through edge states.