Far-infrared laser action from quantum dots created by electric-field focusing

The modeling and comparative analysis of the relaxation processes in four different heterostructures: traditional heterotransistor, heterotransistor with two channels (heterojunctions), heterotransistor with two channels and a system of quantum dots (QDs), as well as heterotransistor with two channels and two systems of quantum dots were made. It is shown that the heating of electrons in high electric fields in heterotransistors with embedded quantum dots with two heterojunctions is less intensive, and the values of the drift velocity is higher than in the other structures under study.

Though electron spins in electrically defined silicon (Si) quantum dot systems have been extensively employed for physical realization of quantum processing units, their application to quantum sensing has not been active compared to the case of photonic qubits and nitrogen-vacancy spins in diamonds. This work presents a comprehensive study on the feasibility of Si quantum dot structures as a physical platform for implementation of a sensing protocol for magnetic fields. To examine sensing operations at a systematic level, we adopt in-house device simulations taking a Si double quantum dot (DQD) system as a target device where the confinement of electron spins is controlled with electrical biases in a Si/Si-germanium heterostructure. Simulation results demonstrate the fairly nice utility of the Si DQD platform for detecting externally presented static magnetic fields, and, more notably, reveal that sensing operations are not quite vulnerable to charge noise that is omnipresent in solid materials. As a rare study that presents in-depth discussion on operations of quantum sensing units at a device-level based on computational modeling, this work can deliver practical insights for potential designs of sensing units with electron spins in Si devices.

The nondestructive observation of correlation between optical characteristic and morphologic feature within capped ZnCdSe/ZnSe quantum dots system

Semiconductor quantum dots are usually compared to artificial atoms, because their electronic structure consists of discrete energy levels as for natural atoms. These artificial systems are integrated in solid materials and can be localized with a spatial precision of the order of nanometers. Besides, they conserve their quantum properties even at quite high temperatures (∼ 10 K). These properties make quantum dots one of the most suitable systems for the realization of quantum devices and computers. However, the energy states and the optical properties of quantum dots are much more complicated than for atoms, because a quantum dot is never an isolated potential well. Instead, its electronic structure depends on the crystallin structure of the semiconductor material and on the Coulomb ion-electron and electron-electron correlations. In particular, the presence of several valence bands and their mixing, induced by quantum confinement, gives rise to novel properties which are still not completely understood and exploited in applications. To get a major advance in this field, a full deterministic control of the spatial shape of the quantum confinement is needed, combined with a deeper understanding of the connections between electronic and optical properties. This thesis work has these two main objectives. We realized and experimentally studied different quantum dot systems, in pyramidal hetero-structures grown with MOCVD techniques. These systems allowed the realization of several different geometries for the carrier confining potential, with a precision in the order of nanometers. The optical characterization has been obtained in particular by means of polarization-resolved microphotoluminescence, magneto-photoluminescence, excitation photoluminescence (PLE), and interferometry techniques. For single quantum dots, we have observed and characterized for the first time new excitonic complexes, arising from excited hole states. This allowed a full caracterizatioon of the valence band hole states in our peculiar system. By means of photon correlation measurements, we have also experimentally demonstrated that, even in presence of a large family of exciton states, these quantum dot systems can emit single photons. We have then realized much more complex quantum dot structures, double dot systems (quantum dot molecules) and a completely new system called Dot-in-Dot (DiD). This latter is composed by a small inner dot surrounded by an electrostatic potential well (which can be considered as an outer elongated dot). Such a composite system is characterized by a strong valence band mixing. This state superposition is however very sensitive to small variations of the confining potential. Therefore the degree of valence band mixing can be easily switched by the introduction of a week external field. Since the valence band mixing determines the polarization properties of the emitted light, the DiD changes the polarization properties of its emission spectrum under the action of an external field. In particular, we have experimentally demonstrated this effect for an external static magnetic field, while we have numerically predicted a very similar effect for a static electric field. In the latter case, the polarization switching is a direct consequence of the quantum confined Stark effect induced in the DiD. Hence the DiD appears to be an ideal candidate for realizing emitters of single photons with tunable and controllable polarization.

Polarization and excitation power-dependent photoluminescence of magnetic/non-magnetic coupled quantum dots

We discuss the effect of quantum interference on transport through a quantum dot (QD) system. We introduce an indirect coherent coupling parameter α, which provides constructive/destructive interference in the transport current depending on its phase and the magnetic flux. We estimate the current through the QD system using the non-equilibrium Green's function method as well as the Bloch equation method under a large bias voltage condition. The visibility of the Aharonov–Bohm oscillation is evaluated. For a large inter-dot Coulomb interaction, the current is strongly suppressed by the quantum interference effect, while the current is restored by applying an oscillating resonance field with the frequency of twice the inter-dot tunnelling energy.

Quantum dot systems have shown numerous applications in random lasing and light localization phenomena. Conventional simulations of radiation in quantum dot systems rely heavily on a differential formulation of Maxwell's equations to compute dynamics. Unfortunately, the computational costs incurred by the underlying differencing mechanism inherent in such models renders them infeasible for simulations in three dimensions. We propose an integral-equation framework to self-consistently describe the dynamics of a disordered system of quantum dots driven by an incident laser pulse. By adopting an integral formulation of Maxwell's equations, we remove the “radiation grid” in favor of highly optimized point-to-point communication of fields through integral equations. By adopting a semiclassical approximation of the quantum dot system, we partition the computational problem into two distinct parts: (1) determination of source polarizations through evolution of the optical Bloch equations, and (2) evaluation of radiation patterns with electric field integral equation techniques. The solution of each part proceeds explicitly in time, facilitating a complete description of both quantum dots and electromagnetic fields at each timestep, giving rise to phenomena such as localization, photon echo effects, and nonlinear response patterns.

This thesis is devoted to theoretically study the systems of single quantum dots (QDs) embedded in various photonic nanostructures, in close relation with experimental data. The word single is essential: due to their nanometer size, QDs have mainly been studied as ensembles. It is only recently that the experimental progress, in particular in the domain of QDs growth, has allowed to address single QDs. This helped to observe that the confined states in a QD resemble those of an atom, leading in turn to the macro-atom picture for QDs. It opened the path to the study of cavity quantum electrodynamics (CQED) in the solid state, as the analogous of a system of an atom in an electromagnetic cavity. However, already at the level of the QD (i.e. without a solid state cavity), the macro-atom picture has been proven to be oversimplified. Various effects related to the solid state environment of the QD have been experimentally and theoretically evidenced, like the influence of acoustic and optical phonons. These are channels of decoherence for the otherwise well isolated and confined QD levels. When adding the cavity, these effects are possibly enhanced, which can lead to very large deviations from the CQED ideal picture. Therefore the clear understanding of these effects is of essential importance, especially in view of the application of solid state systems to quantum information science. In Chapter 2, we address the radiation-matter coupling between two QDs in a nanocavity. This field of research combines the high quality of light resonators to the nonlinearities of the semiconductor material. It is promising in view of manipulating and transferring single long-lived excitations, an essential goal for quantum information science. Then, we study the deviations of a QD from the idealized macroatom picture. First, in Chapter 3, we include the influence of acoustic phonons on the light emitted by a QD-cavity system. We show that the QD modified susceptibility leads to the qualitative explanation of the cavity feeding effects, which is one of the most striking deviations of the QD-cavity system from the ideal CQED picture. In addition to phonons are extended two-dimensional states populated by the non resonant pumping laser in the QD's wetting layer (WL). In Chapters 4 and 5 we build a Monte-Carlo model of the excitation-decay mechanism in a QD-cavity system. We use it to describe the influence of WL states on the dynamics of the QD, leading in turn to a non-CQED behavior. This approach is particularly successful in the case of large QD-cavity detuning, where it predicts both time-dependent and spectral characteristics of the emitted light. Finally, in Chapter 6, we discuss our conclusions, emphasizing in particular that the knowledge of the actual solid state environment of the QD leads to a better control over the parameters allowing to reach the CQED regime. We also discuss the possible extensions of this work, in particular in relation to the analysis of the stimulated emission regime.

A new concept of energy resonance absorption for photocurrent quenching was proposed using a system of quantum dots (QDs) and the matched dye. The QDs were used as the photocurrent producer, and the dye had an absorption band overlapped with that of the QDs, which led to the resonance absorption of the excitation energy and thus decreased the photocurrent of QDs. By using porphyrin and fluorscein isothiocyanate isomer I as the resonance absorption dyes, the proposed mechanism was proved by UV-Vis spectra, photoluminescence spectra and photocurrent-to-wavelength response, respectively. The interaction of the absorption-matched dye with biomolecule could be conveniently used to introduce it into the photocurrent quenching system, leading to a simple switch-off biosensing method for detection of the biomolecule. As example, a label-free method was proposed for photoelectrochemical detection of target DNA. This method showed a detection range from 6.0 to 600 nmol/L with a detection limit of 2.5 nmol/L. The result demonstrated that the photocurrent quenching via energy resonance absorption not only contributed to the theoretical study of photoelectrochemistry, but also provided a universal tool for photoelectrochemical biosensing.

Spin-flip mesoscopic transport through a carbon nanotube quantum dot system

The influence of strain waves traveling across a semiconductor quantum dot on the optical spectra of the lowest quantum dot transition is analyzed. Pure dephasing interactions between electronic and phononic degrees of freedom in quantum dot systems are considered, which represent the most important type of electron-phonon interaction in strongly confined quantum dots. For the case of excitation by ultrafast laser pulses, a generating function formalism provides analytical results, which are exact within the framework of the model. Two situations are compared: (i) a phonon wave packet is generated by the optical excitation of a single-quantum dot near a surface, which after reflection at the surface reenters the quantum dot and (ii) a phonon wave packet is generated by the excitation of a nearby second dot and then travels across the quantum dot. Although the displacement fields passing the dot are almost identical in these two situations, we find that the real time responses as well as the corresponding spectra exhibit qualitative differences and thus allow for a discrimination of phonon wave packets from different origins.

We discuss photo-luminescence characteristics of CdSe core/shell quantum dots at cryogenic temperatures using a hybrid system of a single quantum dot and an optical nanofiber. The key point is to control the emission species of quantum dot to charged excitons, known as trions, which have superior characteristics to neutral excitons. We investigate the photocharging behavior for the quantum dots by varying the wavelength and intensity of irradiating laser light, and establish a method to create a permanently charged situation which lasts as long as the cryogenic temperature is maintained. The present photocharging method may open a new route to applying the CdSe quantum dots in quantum photonics, and the hybrid system of photocharged quantum-dot and optical nanofiber may readily be applicable to a fiber-in-line single-photon generator.

GaAs-based quantum dot (QD) systems, especially InAs/InGaAs/GaAs QDs, have demonstrated superior device performances as compared with higher dimensional systems. However, to realize high-speed optical interconnects for Si-based electronics, one will need to grow the QDs on Si substrates. While it is promising to integrate the InAs/InGaAs/GaAs QDs on Si with the use of germanium-on-insulator-on-silicon (GeOI) substrates, reported results exhibit bimodal QD sizes and double emission peaks, i.e. unsatisfactory for realistic applications. In this paper, we showed that with an optimized GaAs buffer, single-peak 1.33 µm room-temperature emission can be obtained from InAs/InGaAs/GaAs QDs on GeOI substrates.

Terahertz (THz) emission by difference frequency generation (DFG) technique has attracted vast interests recently. Quantum dot (QD) system is proposed to be the best candidate for THz emission by DFG technique due to its large optical nonlinearity. For efficient QD THz emission, the QD transition energies must lie within the THz range (0.4-41 meV). However, QD transition energies generally fall outside the critical energy difference, i.e. more than 41 meV. In this work, we investigated the effects of QD structures and growth conditions on the energy states. Consequently, we determined the required QD structures and growth conditions needed to obtain energy difference in the THz range.

Molecular beam epitaxy (MBE) enables the growth of semiconductor nanostructures known as tensile-strained quantum dots (TSQDs). The highly tunable nature of TSQD properties means that they are of interest for a wide variety of applications including for infrared (IR) lasers and light-emitting diodes (LEDs), improved tunnel junction efficiency in multijunction solar cell technology, quantum key encryption, and entangled photon emission. In this project, I focus on one of the most technologically important materials, germanium (Ge). Ge has a high gain coefficient, high electron mobility, and low band gap: all excellent properties for optoelectronic applications. Until recently, these technological advantages were unattainable for light-emitting purposes due to Ge’s indirect band gap. Placing Ge under tensile strain changes this semiconductor’s fundamental electronic structure by turning its indirect band gap into that of either a direct band gap semiconductor or a semimetal, depending on the choice of surface orientation. However, it is extremely difficult to use bulk Ge, because the propensity for dislocation formation and strain relaxation is high under the tensile strains required for this band gap transition. In contrast, we can store large amounts of tensile strain in TSQDs without detrimental effects on the crystal quality. The primary objective of this dissertation is therefore to explore whether we can use tensile-strained self-assembly to synthesize Ge TSQDs under large tensile strains, and in doing so, transform the fundamental properties of this technologically important element. In this project I used TSQD self-assembly to create Ge TSQDs on two non-traditional (i.e. non-(001)) surface orientations: (111)A and (110). My research explores the first known Ge TSQDs on these systems. Because of this, I wrote much of this dissertation trying to understand the impact of MBE parameters on Ge TSQD growth/formation and how the properties of Ge TSQDs compare to other quantum dot (QD) systems. I focus on this, because TSQD self-assembly is one of the only ways in which we can induce the very large tensile strains needed for dramatic changes to Ge’s band structure without producing crystalline defects, making it a new and exciting area of study. Additionally, TSQD self-assembly is a recent advancement, leaving this area of science relatively unexplored. For the first time, I am able to report light emission from tensile-strained Ge(110) TSQDs, suggesting that we have transformed this important semiconductor into a direct band gap material with efficient light emission. In Chapter 1, I provide the background needed to understand the work within this dissertation. I describe the motivation for the work, the basics of MBE growth, the characterization tools I employed, the relevant crystallography of these structures, the mechanisms for TSQD self-assembly, and finally the optoelectronic background needed to better understand Ge TSQDs. In Chapter 2, I expand on many of the concepts from Chapter 1. Chapter 2 was an invited paper, wherein we wrote a tutorial-style guide for quantum dot (QD) growth by MBE and provide methodologies for many different QD self-assembly systems commonly investigated in literature. We discuss the premise for QD development, where it fits into quantum applications, how QD self-assembly works, how to grow various compressively strained systems, and finally how to grow tensile-strained systems. This particular paper was designed as an introduction to new QD growers, helping them by giving all the information they would need to start growing QDs in general, but particularly TSQDs. Since TSQDs are still a relatively new field, gathering all the information about how to grow several different materials systems all in one place and providing clear, step-by-step instructions about how to grow them is valuable. This Chapter is thus a guidebook for other researchers. In Chapter 3, I apply this background information and use it to create the first self-assembled Ge TSQDs on InAlAs(111)A. I provide a comprehensive study of how the structural properties of Ge/InAlAs(111)A TSQDs change with growth parameters, providing a robust platform for future work in embedded, low-resistivity tunnel junctions and contacts. I discovered an extremely unusual phase transition for these Ge/InAlAs(111)A TSQDs from a Stranski-Krastonov (SK) growth mode at low temperatures, to a Volmer-Weber (VW) growth mode at higher temperatures. This characteristic is highlighted in the paper, because this work provides the clearest evidence to-date of the ability to switch between different growth modes for quantum dot self-assembly based simply on MBE parameters. Being able to choose between mixed one-and three-dimensional (3D) quantum confinement (e.g., from the wetting layer and QDs in the SK growth mode) or just 3D quantum confinement (e.g., QD-only VW growth) with a high degree of tunability opens up the door to new electronic device applications. In Chapter 4, I compare Ge/InAlAs(111)A TSQDs to an analogous purely III-V TSQD system: namely GaAs/InAlAs(111)A TSQDs. These two TSQD systems, while seemingly similar from both a surface (InAlAs(111)A) and a strain perspective (both have ~3.7% tensile strain), have entirely different shapes, nucleation behaviors, and areal densities. We use potential energy surfaces, radial distribution scaling, and island scaling analyses to compare the two TSQD systems. In the process, we obtain a much deeper understanding of the kinetic behavior during self-assembly for both Ga and Ge adatoms on an InAlAs(111)A surface. This will allow us to more effectively tailor these TSQDs for specific optoelectronic applications. In Chapter 5, I demonstrate growth of the first Ge(110) TSQDs grown on InAlAs. The (110) surface is essential to this project, because theory suggests that tensile strain should produce a direct band gap transition in the Ge, transforming it into an efficient light-emitting semiconductor. I use a variety of experimental techniques and surface symmetry/diffusion anisotropy arguments to explain the unusual shapes of the resulting TSQDs. Initial photoluminescence data indicates strong light emission from Ge(110) TSQDs for the first time, indicating that a strain-driven indirect-to-direct transition has occurred. This breakthrough could enable the future use of these nanostructures for an entirely new type of IR light emitter. In Chapter 6, I discuss several avenues for researchers to continue this project. The light emission I report in Chapter 5 needs to be investigated in greater detail. Once we learn more about the light emission properties of Ge(110) TSQDs, we can make LED and lasing devices out of them, optimizing their growth for these purposes and test their overall performance. We can also investigate the possible topological insulating characteristics of Ge/(111)A TSQDs and test how they change tunnel junction efficiency. Additionally, there are several other surface orientations that could work for defect-free tensile-strained Ge growth, either from the point of known, successful surface orientations (i.e. (111)B) or from the perspective of antiphase-domain (APD)-free capping on (211) surfaces. Tensile-strained self-assembly on miscut (111)B surfaces often leads to quantum wire formation, which in the case of Ge would enable novel quantum wires for embedded semimetallic tunnel junctions, contacts, and even topological insulators. Meanwhile, the tensile-strained self-assembly on the (211) surface is entirely untouched, remaining a vast area for exploration in tunable, tensile-strained self-assembly for the development of novel electronic devices. I discovered proper growth conditions for the first-ever reported Ge TSQDs grown on InAlAs(111)A and (110). During this process, I not only found an unusual growth mode transition for Ge/(111)A TSQDs, but I also increased the body of knowledge on the fundamental kinetics behind their growth. This knowledge will be a boon for future electronic device development using semimetallic Ge TSQDs. I also proved efficient, direct band gap light-emission from Ge(110) TSQDs and provided a robust methodology for their successful growth.