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Coherent processes in quantum dot systems

Sirkina, Liubov 2023. Coherent processes in quantum dot systems. PhD Thesis, Cardiff University.
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Quantum dots (QD)s are nanoscale crystals embedded in a semiconductor material. There can be different coherent processes in QD systems, which can occur on comparable timescales and will in general lead to complex quantum dynamics. At any temperature, there are lattice vibrations (phonons) in the QD material. An optical excitation of the QD gives rise to the formation of an exciton, which is a bound electron-hole pair, spatially confined in a QD. A sudden exciton formation inevitably causes a deformation of the crystal lattice. However, physical observables in QD systems are typically calculated by partially, or completely ignoring the role of phonons. We show in this thesis that it is often important to study the full coherent exciton-phonon dynamics in QD systems as it can properly describe decoherent processes and may result in interesting non-Markovian behaviour. We use a rigorous path integral based approach to probe interesting regimes of comparable timescales, and compare with various analytical approximations to provide an intuitive understanding of the underlying physical processes. The first QD system we study in this thesis is a single QD embedded in a micropillar cavity. The latter is a semiconductor microcavity, which confines light both in vertical and horizontal directions. In this system there are two comparable interactions: the exciton-cavity and exciton-phonon couplings. We focus on a nonlinear optical response in this system, mainly the four-wave mixing polarisation directly representing the system's coherence. This is also an important physical quantity to study because it can be reliably measured by heterodyne spectral interferometry in experiments. Moreover, it can be used to obtain two-dimensional optical spectra, which provide useful information about the nature of coherent coupling in the system. The second QD system consists of two QDs embedded in a bulk semiconductor. The excitons localised in different QDs, while electronically decoupled, experience a dipolar interaction and are also coupled to a shared phonon environment. These interactions can be of comparable strength. We study phonon-assisted Foerster resonance energy transfer between QDs, while treating the exciton and the phonon dynamics microscopically, on a fully coherent level. Foerster transfer is an important mechanism, which is extensively studied and has many applications. Here we provide a first exact theoretical treatment of this process and compare results with common analytic theories. Applying the rigorous path-integral based approach to the exciton population dynamics, we extract and compare decoherence timescales: population decay, dephasing and pure dephasing times.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Physics and Astronomy
Subjects: Q Science > QC Physics
Uncontrolled Keywords: quantum dots, cavity quantum electrodynamics, Foerster resonance energy transfer, four-wave mixing polarisation, open quantum systems, decoherence, acoustic phonons, non-Markovoan processes, path-integral methods, exact solutions for many-body systems
Funders: EPSRC
Date of First Compliant Deposit: 13 May 2024
Last Modified: 17 May 2024 11:27

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