UMD Theses and Dissertations
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Item Quantum Dots in Photonic Crystals for Hybrid Integrated Silicon Photonics(2024) Rahaman, Mohammad Habibur; Waks, Edo Prof.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quantum dots are excellent sources of on-demand single photons and can function as stable quantum memories. Additionally, advanced fabrication techniques of III-V materials and various hybrid integration methods make quantum dots an ideal candidate for integration into fiber- and silicon-based photonic circuits. However, efficiently extracting and integrating quantum dot emissions into fiber- and silicon-based photonic circuits, particularly with high efficiency and low power consumption, presents a continued challenge. This dissertation addresses this challenge by utilizing photonic crystals to couple quantum dot emissions into fiber- and silicon-based photonic circuits. In this dissertation, we first demonstrate an efficient fiber-coupled single photon source at the telecom C-band using InAs/InP quantum dots coupled to a nanobeam photonic crystal. The tapered nanobeam structure facilitates directional emission that is mode-matched to a lensed fiber, resulting in a collection efficiency of up to 65% from the nanobeam to a single-mode fiber. Using this approach, we demonstrate a bright single photon source with a 575 ± 5 Kcps count rate. Additionally, we observe a single photon purity of 0.015 ± 0.03 and Hong-Ou Mandel interference from emitted photons with a visibility of 0.84 ± 0.06. A high-quality factor photonic crystal cavity is needed to further improve the brightness of the single-photon source through Purcell enhancement. However, photonic crystal cavities often suffer from low-quality factors due to fabrication imperfections that create surface states and optical absorption. To address this challenge, we employed atomic layer deposition-based surface passivation of the InP photonic crystal nanobeam cavities to improve the quality factor. We demonstrated 140% higher quality factors by applying a coating of Al2O3 via atomic layer deposition to terminate dangling bonds and reduce surface absorption. Additionally, changing the deposition thickness enabled precise tuning of the cavity mode wavelength without compromising the quality factor. This Al2O3 atomic layer deposition approach holds great promise for optimizing nanobeam cavities, which are well-suited for integration with a wide range of photonic applications. Finally, we propose a hybrid Si-GaAs photonic crystal cavity design that operates at telecom wavelengths and can be fabricated without the need for careful alignment. The hybrid cavity consists of a patterned silicon waveguide that is coupled to a wider GaAs slab featuring InAs quantum dots. We show that by changing the width of the silicon cavity waveguide, we can engineer hybrid modes and control the degree of coupling to the active material in the GaAs slab. This provides the ability to tune the cavity quality factor while balancing the device’s optical gain and nonlinearity. With this design, we demonstrate cavity mode confinement in the GaAs slab without directly patterning it, enabling strong interaction with the embedded quantum dots for applications such as low-power-threshold lasing and optical bistability (156 nW and 18.1 µW, respectively). In addition to classical applications, this cavity is promising for alignment-free, large-scale integration of single photon sources in a silicon chip.Item Tight-binding simulations of random alloy and strong spin-orbit effects in InAs/GaBiAs quantum dot molecules(2023) Lin, Arthur; Bryant, Garnett W; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Self-assembled \ce{InAs} quantum dots (QDs), which have long hole-spin coherence times and are amenable to optical control schemes, have long been explored as building blocks for qubit architectures. One such design consists of vertically stacking two QDs to create a quantum dot molecule (QDM). The two dots can be resonantly tuned to form "molecule-like" coupled hole states with the hybridization of hole states otherwise localized in each respective dot. Furthermore, spin-mixing of the hybridized states in dots offset along their stacking direction enables qubit rotation to be driven optically, allowing for an all-optical qubit control scheme. Increasing the magnitude of this spin-mixing is important for optical quantum control protocols. We introduce the incorporation of dilute \ce{GaBi_xAs_{1-x}} alloys in the barrier region between the two dots, as \ce{GaBiAs} is expected to provide an increase in spin-mixing of the molecular states over \ce{GaAs}. Using an atomistic tight-binding model, we compute the properties of \ce{GaBi_xAs_{1-x}} and the modification of hole states that arise when the alloy is used in the barrier of an \ce{InAs} QDM. We show that an atomistic treatment is necessary to correctly capture non-traditional alloy effects of \ce{GaBiAs}. Additionally, an atomistic model allows for the study of configurational variances and clustering effects of the alloy. We find that in \ce{InAs} QDMs with a \ce{GaBiAs} inter-dot barrier, hole states are well confined to the dots up to an alloy concentration of 7\%. By independently studying the alloy-induced strain and electronic scattering off \ce{Bi} and As orbitals, we conclude that an initial increase in QDM hole state energy at low Bi concentration is caused by the alloy-induced strain. Additionally, a comparison between the fully alloyed barrier and a partially alloyed barrier shows that fully alloying the barrier applies an asymmetric strain between the top and bottom dot. By lowering the energetic barrier between the two dots, \ce{GaBiAs} is able to promote the tunnel coupling of hole states in QDMs. We obtain a three fold increase of hole tunnel coupling strength in the presence of a 7\% alloy. Additionally, we show how an asymmetric strain between the two dots caused by the alloy results in a shift in the field strength needed to bring the dots to resonance. We explore different geometries of QDMs to optimize the tunnel coupling enhancement the alloy can provide, as well as present evidence that the change in tunnel coupling may affect the heavy-hole and light-hole components of the ground state in a QDM. The strong spin-orbit coupling strength of \ce{GaBiAs} allows for the enhancement of spin-mixing in QDMs. A strong magnetic field can be applied directly in the TB Hamiltonian. In order to fit the TB results to a simple phenomenological Hamiltonian, we found it necessary to include second order magnetic field terms in the phenomenological Hamiltonian as a diamagnetic correction to the hole state energies. Fitting to the corrected phenomenological model, we obtain a three-fold enhancement for the spin-mixing strength of offset dots at 7\% \ce{Bi}. Additionally, at higher alloy concentrations, a combination of enhanced spin-mixing and increase resonance change in g-factor results in intra-dot spin-mixing between Zeeman split states of the lower energy dot. A perturbative analysis of the magnetic field shows that both the spin-mixing and resonance g-factor change are effects of the Peierls contribution, or the component of the magnetic field applied to the effective spatial angular momentum of the wavefunction. When spin-orbit coupling is removed from the system, there is no longer a preferred alignment between the spin of the system and the Peierls effective angular momentum, thus removing any magnetic field effects of the Peierls contribution. The analysis of spin-orbit effects can be extended to single dots with in-plane magnetic and electric fields. This thesis concludes with some preliminary results utilizing electric fields, in conjunction with spin-locking effects provided by spin-orbit coupling, to manipulate the spin polarization in single dots. TB calculations with a magnetic field are performed to show the preferred alignment of the effective angular momentum, given by the geometry of the dot, also spatially locks the spin-polarization of hole states. An electric field can then be applied to bias the charge density to either side of the dot, using the spatial texture of the spin to obtain a spin polarized in $z$ while both the magnetic and electric field is in the $xy$-plane. The same perturbative analysis with the QDMs can be applied to show sufficient spin-orbit coupling is needed to generate such an effect. We propose the utilization of spin texture and electric fields as a novel method for rotating the spin in QDs.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 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 Design of Self-Assembling Nanostructures to Promote Immune Tolerance(2018) Hess, Krystina; Jewell, Christopher M; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In autoimmune diseases, which affect more than 23 million Americans, the immune system mistakenly attacks healthy tissue. This occurs when the process that normally controls self-reactive inflammatory cells (i.e. tolerance) fails. In multiple sclerosis (MS), the myelin sheath, which insulates nerves, is recognized as a foreign antigen. Demyelination by immune cells results in serious symptoms of neurodegeneration. Current treatments for MS are not curative, but rather manage symptoms by broadly suppressing the immune system, leaving patients unable to fight infection. New therapies that are more specific and effective could greatly improve the quality of life for patients. Biomaterials offer specific advantages for generating antigen-specific tolerance, such as cargo protection, targeted delivery, and controlled release of signals. Additionally, recent reports demonstrate that materials themselves can be intrinsically immunogenic. Two promising biomaterials-based strategies for combating autoimmunity involve: 1) delivery of self-antigen with a regulatory molecule or 2) delivery of self-antigen alone. Aim 1 of this dissertation focuses on the first strategy, creating a novel delivery system for myelin peptide and GpG, an immunomodulatory oligonucleotide. This approach involves electrostatic self-assembly of the two immune signals, eliminating the need for a carrier that could exacerbate inflammation, while still offering attractive features of biomaterials, such as co-delivery. The goal is for immune cells to encounter both signals simultaneously, biasing the response towards tolerance. This work represents the first studies using self-assembled materials to target toll-like receptor signaling, recently shown to be implicated in many autoimmune diseases. Aim 2 of this dissertation is based on the second strategy above, which relies on evidence that changing the trafficking and processing of a self-antigen can impact the development of inflammation or tolerance. Quantum dots, NPs that are intrinsically fluorescent and rapidly drain to lymph nodes, can be decorated with a large and controllable number of myelin peptides. These key features of QDs were exploited to reveal that parameters of self-antigen display (i.e. dose, density) impact biodistribution and immune cell uptake, and are directly correlated to the level of tolerance induced. Together, the described nanotechnologies offer opportunities to probe important questions towards the design of antigen-specific therapies.