Analysis of dense quantum dot systems using a self-consistent maxwell-bloch framework

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

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

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.

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.

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.

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.

The proposal of a far-infrared laser employing intraband transitions in the system of quantum dots is presented. The model for electrically defined Gaussian quantum dots in a narrow Ga(Al)As quantum well is examined and the conditions for inversion of population of the dot matrix are discussed. The laser is planned to be pumped by periodically repeated rapid creation and destruction of the quantum dot matrix allowing for repeated filling of the dot levels with electrons from a quantum well. The kinetics of the electron-photon system is analyzed in order to determine the characteristics of the pumping signal and other laser parameters.

The absorption spectra of semiconductor quantum dots (QDs) are expected to be a series of δ-function-like discrete lines due to the nature of the density of states. In a realistic III–V QD system, the absorption spectra is the superimposition of the contribution from each individual dot and the overall behavior is modeled by considering a Gaussian size distribution. In this paper, we study and present the dependence of the Gaussian nature of the absorption spectra of In X Ga 1−X N/GaN QD systems on the dot size distribution. The dots were approximated as elongated rectangular boxes having finite potentials at the boundaries. The optical transitions and absorption spectra of In X Ga 1−X N/GaN QDs that have a square base and the variation of the height is Gaussian, are computed and analyzed

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

Variability of heterostructure type with thickness of barriers and temperature in the InAs/GaAsSb 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.

Within the density response and density matrix formalisms, employing the self-consistent field approach in the quasistatic limit it is considered the interlevel infrared electromagnetic response of individual infinite deep quantum dots (QDs) as well as of two-dimensional infinite square lattices of the identical QDs of different shapes with uniaxial rotation symmetry: semispherical, lens, slightly ellipsoidal, and cylindrical. It is shown that the QD shape can critically affect the interlevel optical properties of the considered QD systems, in particular, the dependence of the QD polarizability upon the polarization of the incident radiation. To facilitate consideration of the dipole coupled interlevel transitions and of the formation of (interacting) modes of the electron interlevel collective excitation, and calculation of the absorption spectra for the QDs, illustrative maps of the transitions are utilized and the concept of the ``$m$ family'' for the eigenstates is found to be fruitful, where $m$ is the magnetic quantum number labeling the eigenstates of the QDs. It is shown that the shape and size of QDs can impact the depolarization effect considerably. Numerical calculations show that the dipole-dipole interaction approximation very well describes the depolarization shift caused by the interdot electron-electron interaction for each of the considered QD shapes and for any reasonable values of the size parameters of the lattices.

Fano effect in an AB interferometer with a quantum dot side-coupled to a single Majorana bound state