Stark Effect of Interactive Electron-hole pairs in Spherical Semiconductor Quantum Dots

We present a theoretical variational approach, based on the effective mass approximation (EMA), to study the quantum-confinement Stark effects for spherical semiconducting quantum dots in the strong confinement regime of interactive electron-hole pair and limiting weak electric field. The respective roles of the Coulomb potential and the polarization energy are investigated in details. Under reasonable physical assumptions, analytical calculations can be performed. They clearly indicate that the Stark shift is a quadratic function of the electric field amplitude in the regime of study. The resulting numerical values are found to be in good agreement with experimental data over a significant domain of validity

Scalable Designs for Quasiparticle-Poisoning-Protected Topological Quantum Computation with Majorana Zero Modes

We present designs for scalable quantum computers composed of qubits encoded in aggregates of four or more Majorana zero modes, realized at the ends of topological superconducting wire segments that are assembled into superconducting islands with significant charging energy. Quantum information can be manipulated according to a measurement-only protocol, which is facilitated by tunable couplings between Majorana zero modes and nearby semiconductor quantum dots. Our proposed architecture designs have the following principal virtues: (1) the magnetic field can be aligned in the direction of all of the topological superconducting wires since they are all parallel; (2) topological $T$-junctions are not used, obviating possible difficulties in their fabrication and utilization; (3) quasiparticle poisoning is abated by the charging energy; (4) Clifford operations are executed by a relatively standard measurement: detection of corrections to quantum dot energy, charge, or differential capacitance induced by quantum fluctuations; (5) it is compatible with strategies for producing good approximate magic states.Comment: 34 pages, 17 figures; v4: minor changes, final versio

Multiple Quantum Well Structures As Optical Waveguides

This thesis is concerned with the design, fabrication and characterisation of semiconductor optical waveguides in which the high index guiding layer is a multiple quantum well structure (MQWS), consisting of alternate layers of high and low band gap semiconductors with the electrons and holes in the MQWS being confined to the low band gap material. This confinement in two dimensions alters greatly the electronic and optical properties of the MQWS in comparison to the bulk properties of the constituent layers. The basic concepts involved in MQW waveguides are introduced using an elementary quantum mechanical analysis of quantum wells together with a brief description of the properties of dielectric waveguides, A more detailed treatment of the electronic and optical properties of MQWS and a review of published experimental work is used to show that the fundamental absorption edge is much more abrupt than that in the corresponding bulk material with strong excitonic characteristics being evident even at room temperature. In addition, the absorption edge is seen to be anisotropic with the fundamental energy gap being larger for light polarised perpendicular to the MQW layers. This anisotropic absorption edge, together with the layered dielectric nature of MQWS, makes them birefringent with a smaller refractive index for light polarised perpendicular to the MQW layers. The quantum confinement of carriers in MQWS also enhances their electroabsorption and electro-optic properties through the quantum confined Stark effect. Standard techniques used in the design, fabrication and analysis of bulk semiconductor waveguides are developed for application to MQW waveguides. These include analytical and numerical techniques for the design of dielectric waveguides; dry etching and metallisation processes for the fabrication of devices; and a laser/optics system to analyse the waveguide devices. To verify these techniques they are first applied to the well-understood case of n/n+ GaAs waveguides and are used to successfully fabricate and analyse single-mode, passive, rib waveguides at l=1.15mum. The electro-optic coefficient is also measured in an active, planar n/n+ waveguide and found to be close to that reported by other workers. The design techniques are then applied to MOWS waveguides resulting in the design of a MQW double heterostructure (MQW-DH), p-i-n diode which was predicted to produce the required Quantum properties (strong, room temperature, excitonic behaviour), waveguide properties (single-mode propagation up to the fundamental absorption edge) and electronic properties (a high reverse bias breakdown voltage and uniform applied electric field). Most of the theoretical work and all the experimental work included is devoted to MQWS in the (Al,Ga)As, III-V semiconductor alloy system. Accordingly, the methods available for growing MQWS in this system are reviewed with Molecular Beam Epitaxy (MBE) being found the most likely method to satisfactorily reproduce the desired structure. MQW-DH were grown at two establishments and are initially studied by photoluminescence and scanning electron microscopy before their planar optical waveguide characteristics are checked using the laser system. Only one sample is found to satisfy all the design requirements, and then only partially. Detailed analysis of the properties of MQW waveguides is therefore limited to this structure. Passive MQW-DH waveguides are demonstrated to exhibit an anisotropic absorption edge as predicted, and it is shown that the design and fabrication techniques developed can be successfully used to obtain single, double and multi-mode strip loaded waveguides. Single-mode waveguides are also used to fabricate passive directional couplers with coupling lengths in good agreement with theoretically predicted values. A semi-empirical model is put forward to describe the band edge electro-absorption of MQWS. Although simple, the model is in qualitative and approximate quantitative agreement with published results. (Abstract shortened by ProQuest.)

Optical Studies of Indium Gallium Nitride Nanostructures

Indium gallium nitride (InGaN) is a semiconductor material that is in widespread use in blue light emitting diodes (LEDs) and blue laser diodes and is being used in solid-state lighting, displays, and scientific applications. The scientific understanding of the physical mechanisms responsible for the performance of these devices is still developing; this includes the description of localization of carriers in this material – a fundamental issue which is believed to be responsible for the origin of luminesce in blue LEDs – as well as that of performance limitations of existing devices, including the so-called “green gap” and “efficiency droop,” which are related in part to the exciton-phonon interaction. This thesis studies top-down etched InGaN quantum disks (QDs) embedded in GaN nanopillars, focusing on the effect of localization and of the exciton-phonon interaction. The exciton-phonon interaction is studied with two experiments which examine the effect of quantum dot size and shape on the strength of the optical phonon replica in the PL. First, we study the effect of asymmetrical strain on the exciton-phonon coupling by examining the optical phonon replica in the PL of nineteen individual elliptical QDs with dimensions of 22nm x 36nm. We show that the effect of strain on the phonon coupling strength should be observable by a reduction in the degree of polarization (DOP) of the optical phonon replica. Measurements confirm that there is a reduction in the DOP of the optical phonon replica, with reasonable agreement with theory for the high DOP dots. Lower DOP dots, which arise to due irregularities in the shape and size of the fabricated nanopillars, also show a reduction in DOP of the phonon replica but are more sensitive to the effect of asymmetrical phonon coupling and warrant further study. Second, we examine the effect of nanopillar diameter on exciton-phonon coupling strength in InGaN quantum disks. We observe an enhancement of the phonon replica as the nanopillar diameter is reduced from 1000nm to 60nm. This effect is explained by a reduction in the lateral Bohr radius of the exciton which accompanies the decrease in vertical electron-hole separation in smaller nanopillar diameters. To quantify this effect, a simple model is used to infer that, based on the measured phonon coupling strengths, the Bohr radius reduces from approximately 2.5nm to 2nm as the diameter is reduced over the observed range. In order to study the effect of localization, we measure the Stokes shift, which is the energy difference between emission and absorption. By measuring this quantity as a function of nanopillar diameter, we demonstrate the ability to separately determine the contributions of the strain-induced quantum confined Stark effect and of localization to the observed Stokes shift. In our case, we find that the two effects have approximately equal contributions for the range of nanopillar diameters studied here. Furthermore, the site control of InGaN/GaN quantum disks using this top-down fabrication method assists the integration of advanced device structures with individual nanopillars. We demonstrate the enhancement of light collimation by a factor of 1.8x from single nanopillar LEDs using an integrated nanolens. Additionally, we report measurements of enhanced QD brightness and radiative emission rate using an open-top plasmonic cavity; this demonstration is tailored for applications in quantum technologies such as quantum cryptography.PHDPhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/147624/1/akatcher_1.pd

Microwaves in Quantum Computing

Quantum information processing systems rely on a broad range of microwave technologies and have spurred development of microwave devices and methods in new operating regimes. Here we review the use of microwave signals and systems in quantum computing, with specific reference to three leading quantum computing platforms: trapped atomic ion qubits, spin qubits in semiconductors, and superconducting qubits. We highlight some key results and progress in quantum computing achieved through the use of microwave systems, and discuss how quantum computing applications have pushed the frontiers of microwave technology in some areas. We also describe open microwave engineering challenges for the construction of large-scale, fault-tolerant quantum computers.Comment: Invited review article, to appear in IEEE Journal of Microwaves. 29 pages, 13 figures, $10^{6}$ to $10^{11}$ H

Structure Property and Prediction of Novel Materials using Advanced Molecular Dynamics Techniques: Novel Carbons, Germaniums and High-Performance Thermoelectrics

By means of advanced molecular dynamic techniques, we predict the stability of novel materials based on carbon, germanium and PbSe. This topological solutions have been studied and characterised at a DFT/DFTB level of theory and interesting optical, mechanical, electronic and heat transport properties have been pointed out

Plasmon-mediated Energy Conversion in Metal Nanoparticle-doped Hybrid Nanomaterials

Climate change and population growth demand long-term solutions for clean water and energy. Plasmon-active nanomaterials offer a promising route towards improved energetics for efficient chemical separation and light harvesting schemes. Two material platforms featuring highly absorptive plasmonic gold nanoparticles (AuNPs) are advanced herein to maximize photon conversion into thermal or electronic energy. Optical extinction, attributable to diffraction-induced internal reflection, was enhanced up to 1.5-fold in three-dimensional polymer films containing AuNPs at interparticle separations approaching the resonant wavelength. Comprehensive methods developed to characterize heat dissipation following plasmonic absorption was extended beyond conventional optical and heat transfer descriptions, where good agreement was obtained between measured and estimated thermal profiles for AuNP-polymer dispersions. Concurrently, in situ reduction of AuNPs on two-dimensional semiconducting tungsten disulfide (WS2) addressed two current material limitations for efficient light harvesting: low monolayer content and lack of optoelectronic tunability. Order-of-magnitude increases in WS2 monolayer content, enhanced broadband optical extinction, and energetic electron injection were probed using a combination of spectroscopic techniques and continuum electromagnetic descriptions. Together, engineering these plasmon-mediated hybrid nanomaterials to facilitate local exchange of optical, thermal, and electronic energy supports design and implementation into several emerging sustainable water and energy applications

New approaches to the realization and identifcation of Majorana qubits in solid state quantum devices

Majorana bound states in topological superconductors exhibit exotic non-Abelian braiding statistics and hold promise for particularly robust qubits with natural built-in mechanisms against decoherence. The theme of this dissertation is the theory of novel approaches to realization and identification of such Majorana qubits. Towards the realization of Majorana qubits, we present architectures based on topological insulator nanoribbons, e.g. made of Bismuth selenide, and proximitized by an s-wave superconductor. Piercing of proximitized nanoribbons with an axial uniform magnetic flux of suitably adjusted strength has been previously predicted to give rise to one-dimensional topological superconductors with robust Majorana bound states. We propose qubit designs that incorporate two such topological superconductors connected by a constricted topological nanoribbon segment. This constriction is non-proximitized and its lesser cross section results in a local gap opening. We present theoretical results showing the possibility to conveniently tune the coupling of a pair of Majorana states localized across the constriction via gating. Moreover, we discuss proof-of-principle experiments for initialization, manipulation, and readout of the floating version of the device, which is dominated by charging effects. We compare the platform to other Majorana qubit proposals and give an outlook on applications such as the Majorana surface code. The experimental identification of Majorana bound states represents one of the outstanding goals of contemporary condensed matter physics. Towards identification, we present the theory of novel transport spectroscopic approaches geared to qubits in the Coulomb blockade regime. In particular, we propose a scheme in which three normal-conducting leads are weakly coupled to three different Majorana bound states of the qubit. The protocol relies on the simultaneous continuous weak measurement of two noncommuting, nonlocal Pauli operators of the Majorana qubit and results in a phenomenon of surprisingly strong current cross-correlations. This is the prime signature containing information that enables to identify the nonlocal Pauli algebra, which is intimately related to the celebrated non-Abelian braiding statistics. The latter is a property notoriously hard to demonstrate and of large attractiveness from the fundamental as well as applied perspective. The conditions under which the pronounced current cross-correlations are observable depend on the device configuration, a fact that leads to several experimentally verifiable predictions that allow to test the authenticity of the Majorana qubit. Beyond that, we put forward two further detection methods in this thesis. First, a shot noise scheme which is viable for a single floating topological Majorana wire. Second, a protocol relying on projective current measurements. Beyond the usefulness of these protocols, we identify the aforementioned protocol of monitoring a nonlocal Pauli algebra as the scheme accessing the most information related to the constitutive nature of Majorana bound states