Electrical & Computer Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2765
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Item INTEGRATION OF CLASSICAL/NONCLASSICAL OPTICAL NONLINEARITIES WITH PHOTONIC CIRCUITS(2023) Buyukkaya, Mustafa A; Waks, Edo; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Recent developments in nanofabrication have opened opportunities for strong light-matter interactions that can enhance optical nonlinearities, both classical and non-classical, for applications such as optical computing, quantum communication, and quantum computing. However, the challenge lies in integrating these optical nonlinearities efficiently and practically with fiber-based and silicon-based photonic circuits on a large scale and at low power. In this thesis, we aimed to achieve this integration of classical and quantum optical nonlinearities with fiber-based and silicon-based photonic circuits.For classical optical applications, optical bistability is a well-researched nonlinear optical phenomenon that has hysteresis in the output light intensity, resulting from two stable electromagnetic states. This can be utilized in various applications such as optical switches, memories, and differential amplifiers. However, integrating these applications on a large scale requires low-power optical nonlinearity, fast modulation speeds, and photonic designs with small footprints that are compatible with fiber optics or silicon photonic circuits. Thermo-optic devices are an effective means of producing optical bistability through thermally induced refractive index changes caused by optical absorption. The materials used must have high absorption coefficients and strong thermo-optic effects to realize low-power optical bistability. For this purpose, we choose high-density semiconductor quantum dots as the material platform and engineer nanobeam photonic crystal structures that can efficiently be coupled to an optical fiber while achieving low-power thermo-optical bistability. For applications that require non-classical nonlinearities such as quantum communication and quantum computing, single photons are promising carriers of quantum information due to their ability to propagate over long distances in optical fibers with extremely low loss. However, the efficient coupling of single photons to optical fibers is crucial for the successful transmission of quantum information. Semiconductor quantum dots that emit around telecom wavelengths have emerged as a popular choice for single photon sources due to their ability to produce bright and indistinguishable single photons, and travel long distances in fiber optics. Here, we present our advances in integrating telecom wavelength single photons from semiconductor quantum dots to optical fibers to realize efficient fiber-integrated on-demand single photon sources at telecom wavelengths. Finally, using the same methodology, we demonstrate the integration of these quantum dots with CMOS foundry-made silicon photonic circuits. The foundry chip is designed to individually tune quantum dots using the quantum confined stark shift with localized electric fields at different sections of the chip. This feature could potentially enable the tuning of multiple quantum emitters for large-scale integration of single photon sources for on-chip quantum information processing.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 INTEGRATED QUANTUM PHOTONIC CIRCUITS WITH QUANTUM DOTS(2019) Aghaeimeibodi, Shahriar; Waks, Edo; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Scalable quantum photonics require efficient single-photon emitters as well as low-loss reconfigurable photonic platforms that connect and manipulate these single photons. Quantum dots are excellent sources of on-demand single photons and can act as stable quantum memories. Therefore, integration of quantum dots with photonic platforms is crucial for many applications in quantum information processing. In this thesis, we first describe hybrid integration of InAs quantum dots hosted in InP to silicon photonic waveguides. We demonstrate an efficient transition of quantum emission to silicon. Quantum nature of the emission is confirmed through photon correlation measurements. Secondly, we present a micro-disk resonator device based on silicon photonics that enables on-chip filtering and routing of single photons generated by quantum dots. The tunability of silicon photonics decreases at low temperatures due to “carrier freeze-out”. Because of a strong electro-optic effect in lithium niobate, this material is the ideal platform for reconfigurable photonics, even at cryogenic temperatures. To this end, we demonstrate integration of quantum dots with thin-film lithium niobate photonics promising for active switching and modulating of single photons. More complex quantum photonic devices require multiple identical single-photon emitters on the chip. However, the transition wavelength of quantum dots varies because of the slightly different shape and size of each dot. To address this hurdle, we propose and characterize a quantum dot device located in an electrostatic field. The resonance wavelength of the quantum dot emission is tuned up to 8 nm, more than one order of magnitude greater than the transition linewidth, opening the possibility of tuning multiple quantum dots in resonance with each other. Finally, we discuss the application of a single quantum dot strongly coupled to a nanophotonic cavity as an efficient medium for non-linear phenomenon of optical amplification. Presence of a strong pump laser inverses the population of the quantum dot and leads to stimulated emission from the cavity-coupled quantum dot. Using this platform, we observe an optical gain of ~ 16%, significantly increased compared to previous demonstrations of gain in single solid-state quantum emitters without cavities or weakly coupled to cavities. These demonstrations are significant steps toward robust control of single photons using linear and non-linear photonic platforms.Item MONOCRYSTALLINE SUPERSATURATED ALUMINUM LAYERS BURIED IN EPITAXIAL SILICON(2019) Kim, Hyun soo; Iliadis, Agis; Pomeroy, Joshua M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quantum information device performance in semiconductors and superconductors is limited by the quality of materials and interfaces, particularly the interface to the oxide layer. By merging semiconductors and superconductors in a single crystal material, the oxide layer can be eliminated, and the advantage of both systems can be realized. To explore interface free circuits in quantum computing, I have synthesized and studied a new two-dimensional hole gas in silicon using aluminum layers sandwiched between single crystal Si layers. At high enough Al density, this system is expected to behave as a superconductor in single crystal Si. The samples were fabricated by low temperature molecular beam epitaxy (MBE) with modulation doping of elemental Al. Scanning tunneling microscope (STM) and scanning transmission electron microscope (STEM) images show epitaxial Si layers with low surface defects and no crystalline defects in the Al enriched region. Electrical measurement shows that holes are the dominant carrier in this system with charge carrier densities of $\approx$ 1.39 x 10$^{14}$ cm$^{-2}$, and Hall mobilities of $\approx$ 20 cm$^{2}$/(Vs). The charge carrier density corresponds to $\approx$ (0.93 $\pm$ 0.1) hole per Al dopant atom. Unfortunately, no superconductivity was observed down to 300 mK. The likely reason for this is found to be re-distribution of Al dopants over $\approx$ (17 to 25) nm due to thermal annealing up to 550 $^{\circ}$C, which decreases the peak Al concentration in Si below the critical density for superconductivity. Al has not been well studied as a dopant in Si due to its low solid solubility, low vapor pressure, and tendency to segregate. To better understand Al as a dopant, the structures and electrical properties of incorporated Al in Si(100) are studied using STM. The scanning tunneling spectroscopy (STS) spectra show shifts of band edges on incorporated Al compared to (2x1) Si(100) dimers. To test the compatibility of elemental Al for STM lithography with a hydrogen resist layer, a standard experimental protocol is tested. Elemental Al is evaluated using 3 different metrics: 1) sticking coefficient contrast, 2) effective enthalpy of sublimation contrast, and 3) surface diffusivity by deposition rates. Elemental Al is shown incompatible with STM lithography and hydrogen masking. Our study suggests that other dopants may overcome this difficulty. Finally, a new method using ion implanted wires is a promising technique for making electrical contacts to devices in Si fabricated with STM lithography. Here, I report a new in situ method for detecting ion implanted wires using STM and STS with a novel lock-in technique. Using the ion implanted wires, a-first-of-its-kind STM-patterned nano-wire made of P dopants is demonstrated.Item Nnanoscale light-matter interactions: fundamentals and applications(2018) Yu, Shangjie; Ouyang, Min; Munday, Jeremy; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Novel phenomena and promising applications have been emerging from nanoscience and nanotechnology research over recent decades. Particularly, people pursue a better understanding of how light and matter interact with each other at the nanoscale. This dissertation will present our work on the relevant topics, including ultrafast optical generation and manipulation of nanoscale phonons, metamaterials for thermal management, and cooperative chirality in inorganic nano-systems. Through an acoustically mismatched nanoscale interface, interfacial phonon coupling may lead to a coherently modulated phonon spectrum, which however has been less studied. We have demonstrated unambiguous experimental evidences of coherent interfacial phonon coupling between the core and shell constituents by employing a well-designed nanoscale core-shell structure with a precisely tunable interface as a model system. Furthermore, the observed phonon modes can be selectively tailored in a highly controllable manner by different ultrafast pulse control schemes. This study represents an important step towards nanoscale phonon engineering with rationally tailored nanostructures as building blocks. Metamaterials, which are artificially patterned micro/nano-structures, are studied for thermal management. For this purpose, we propose patterned arrays in different forms, including micropillar arrays and fiber arrays. We have discovered the structural dependence of the arrays’ characteristic resonance and emission properties, and how the properties are impacted in imperfect patterns which are common in real life. This study provides new perspectives on metamaterials for thermal management and the textile industry. Lastly, chiral light-matter interaction is studied in a novel type of inorganic nanocrystals, consisting of both crystallographic and geometric chirality. We build up a general model for simulating electromagnetic response of chiral objects and extract the materials parameters from experimental data of the achiral-shape nanocrystals. By simulating nanocrystal of different geometries and comparing with experimental circular dichroism spectra, the unique spectral features from the nanocrystals’ intrinsic crystallographic chirality, geometric chirality and their interplay are identified. Besides, an excellent agreement is achieved between the simulation and the experiment. This result opens up the opportunities for new chiroptical devices and chiral discrimination technology.Item Neural and computational approaches to auditory scene analysis(2015) Akram, Sahar; Shamma, Shihab A; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Our perception of the world is highly dependent on the complex processing of the sensory inputs by the brain. Hearing is one of those seemingly effortless sensory tasks that enables us to perceive the auditory world and integrate acoustic information from the environment into cognitive experiences. The main purpose of studying auditory system is to shed light on the neural mechanisms underlying our hearing ability. Understanding the systematic approach of the brain in performing such complicated tasks is an ultimate goal with numerous clinical and intellectual applications. In this thesis, we take advantage of various experimental and computational approaches to understand the functionality of the brain in analyzing complex auditory scenes. We first focus on investigating the behavioral and neural mechanisms underlying auditory sound segregation, also known as auditory streaming. Employing an informational masking paradigm, we explore the interaction between stimulus-driven and task-driven attentional process in the auditory cortex using magnetoencephalography (MEG) recordings from the human brain. The results demonstrate close links between perceptual and neural consequences of the auditory stream segregation, suggesting the neural activity to be viewed as an indicator of the auditory streaming percept. We examine more realistic auditory scenarios consisted of two speakers simultaneously present in an auditory scene and introduce a novel computational approach for decoding the attentional state of listeners in such environment. The proposed model focuses on an efficient implementation of a decoder for tracking the cognitive state of the brain, inspired from neural representation of auditory objects in the auditory cortex. The structure is based on an state-space model with the recorded MEG signal and individual speech envelopes as the input and the probability of attending to the target speaker as the output of the model. The proposed approach benefits from accurate and highly resolved estimation of attentional state in time as well as the inherent model-based dynamic denoising of the underlying state-space model, which makes it possible to reliably decode the attentional state under very low SNR conditions. As part of this research work, we investigate the neural representation of ambiguous auditory stimuli at the level of the auditory cortex. In perceiving a typical auditory scene, we may receive incomplete or ambiguous auditory information from the environment. This can lead to multiple interpretations of the same acoustic scene and formation of an ambitious perceptual state in the brain. Here, in a series of experimental studies, we focus on a particular example of ambitious stimulus (ambitious Shepard tone pair) and investigate the neural correlates of the contextual effect and perceptual biasing using MEG. The results from psychoacoustic and neural recordings suggest a set of hypothesis about the underlying neural mechanism of short-term memory and expectation modulation in the nervous system.Item CHARACTERIZATION OF FLUORESCENCE FROM QUANTUM DOTS ON NANOSTRUCTURED METAL SURFACES(2011) Hwang, Ehren; Davis, Christopher C; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The behavior of fluorescent materials coupled to surface plasmon supporting surfaces and structures is an area of active research due to their fluorescence enhancing properties. The inherent field enhancements present near structures and interfaces where surface plasmons are excited provide great potential for increasing the response of many optical interactions. While many studies focus on the application of plasmonic nanoparticles or finite metallic structures the use of dielectric structures on a continuous metallic film has received little attention. A comprehensive experimental study using dielectric gratings on gold films is presented illustrating the fundamental properties of fluorescence enhancement on such structures. A process for fabrication of samples using Electron Beam Lithography is demonstrated and comparisons between various quantum dot deposition methods are made to determine the best conditions for surface coating. Conditions for optimization of the fluorescence enhancement phenomena for practical application are explored for gratings with square function profile illustrating the influence of gratings on fluorescence behavior and identifying conditions for optimal enhancement. Complementing these results, an understanding of the underlying physical phenomena is developed by differentiation between enhanced emission and enhanced absorption effects using measurements of fluorescence decay lifetime and emission spectra. Using these observations a thorough description of these systems and the requirements for their practical application is illustrated.