1.1 Introduction
Following extensive study in recent decades, quantum optics has received renewed interest recently owing to its sensational applications. Quantum information has especially garnered considerable attention which can further extend to the areas such as of single-photon source, and quantum cryptography.
The most important aspect of quantum optics is the use of quantum information in quantum computation and communication, quantum cryptography and single-photon source. Quantum information can replace the conventional computation signal 0 and 1 by entanglement states, information can also be presented by a linear combination of the two states rather than a long chain of 0 and 1, thus reducing the computational speed and storage space. Semiconductor quantum dot is especially noteworthy for its quantum confinement effect and atom-like energy level. Achieving the above applications depends on the ability to fabricate the quantum entanglement for spin excitons in quantum dots. The entanglement can be destroyed by the electron-hole exchange interaction and subsequent the fine structure splitting (FSS), which is induced by factors such as shape deformation, and build-in strain effect.
1.2 Motivation
Eliminating the fine structure splitting between the bright exciton states is priority concern to successful
This study investigates
GaAs/AlGaAs hierarchical QDs.
InAs/GaAs self-assembled QDs have GaAs lattice mismatching
with a very close lattice constant and strain free in
(b)
Fig. 1.2.1 (a) InAs/GaAs self
and GaAs lattice mismatch. (b) Shape of InAs/GaAs
hierarchical GaAs/AlGaAs QDs is strain free since the materials near by the QDs have similar lattice constant.
Although the photon (a)
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Eliminating the fine structure splitting between the bright exciton states is successfully generate of entangled photon pair from a QD.
s two quantum dots InAs/GaAs self-assembled QDs and GaAs/AlGaAs hierarchical QDs.
assembled QDs have a built-in initial strain due to InAs and ing[1]. For hierarchical QDs, GaAs is grown on AlGaAs very close lattice constant and strain free in-dot[2].
[21]
InAs/GaAs self-assembled QDs has initial strain due to InAs and GaAs lattice mismatch. (b) Shape of InAs/GaAs self-assembled QDs. (c) hierarchical GaAs/AlGaAs QDs is strain free since the materials near by the QDs have similar lattice constant.
gh the photon-pair emission is generated successfully (c)
Eliminating the fine structure splitting between the bright exciton states is of of entangled photon pair from a QD.
assembled QDs and
in initial strain due to InAs and ierarchical QDs, GaAs is grown on AlGaAs
[2]
assembled QDs has initial strain due to InAs assembled QDs. (c) hierarchical GaAs/AlGaAs QDs is strain free since the materials near by the
successfully and FSS can be
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further eliminated by applying a magnetic field, electric field and external stress, a preliminary yet detailed understanding of geometrical configuration and materials is necessary. Analytical and numerical studies reveal the essential role of the optically active light-hole components of an exciton state in the fine structures and optical polarization properties of an elongagted dot, which introduces additional short-range e-h exchange interactions to the exciton states and significantly changes the magnitude of fine structure splitting between bright exciton levels.
A previous work [6] presented a reduced Hamiltonian and simple analytical solution. By further simulating the exciton energy spectrum by a one-band four-band model with 3-D parabolic model, this study investigates the size effect of fine structure splitting as well as optical polarization. As is well known, optical polarization is induced by heavy- and light-hole coupling. The extent of heavy- and light-hole coupling is studied as well.
To eliminate the FSS value, this study also investigates how stress affects GaAs bulk. The quantity and direction of applied stress that works on the quantum dot cannot be measured or observed directly. However, stress that is induced by an electric field can be transformed by measuring the transition energies of bulk and observing the extended direction of polarizations, subsequently providing insight into exactly how stress is applied to quantum dots during experimentation.[2] Based on the microscopic Bloch function, exactly how the stress affects the distribution of electrons can be understood.
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1.3 Contents
This thesis is organized as follows.
Chapter 2 introduces the fundamental theory of optical anisotropy and the fine structure of QD. Numerical computation of the excitonic structures of the dots is undertaken by combining the Luttinger-Kohn k⋅p model for the valence hole and single-band model for conduction electron within the parabolic potential model of QD. Analytical and numerical results indicate the essential role of optically active light-hole components of an exciton state in the fine structures and optical polarization properties of an elongagted dot, which introduces additional short-ranged e-h exchange interactions to the exciton states and transforms the magnitude of the fine structure splitting between bright exciton levels. This chapter also introduces strain theory.
Chapter 3 describes how FSS and Pol of GaAs/AlGaAs QDs and InAs/GaAs self-assembled QDs vary with size.
Chapter 4 summarizes the computation results of band structures, photoluminescence spectra, optical polarization and microscopic Bloch function of GaAs bulk subjected to uni-axial stresses.
Conclusions are finally drawn in Chapter 5, along with recommendations for future research.
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