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Phonon-induced dephasing of quantum dot excitons and microcavity-embedded quantum

Morreau, Amy 2018. Phonon-induced dephasing of quantum dot excitons and microcavity-embedded quantum. PhD Thesis, Cardiff University.
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Abstract

Central to the present work is the interaction between a semiconductor quantum dot (QD) exciton and its phonon environment. In the spectral domain, phonon assisted dephasing of the QD exciton presents as a phonon broadband, which is superimposed upon a narrow zero-phonon line (ZPL). The phonon broadband exhibits a high degree of thermal sensitivity, which we exploit in order to measure the temperature of semiconductor QD samples from their respective photoluminescence (PL) spectra. Temperature measurement is achieved through an automated fit procedure based upon the independent boson (IB) model with additional Gaussian and Lorentzian broadening. We find there to be very good agreement between fit temperature and nominal (cryostat-measured) temperature. Further, the fit procedure enables extraction of other key parameters such as the material deformation potential and the QD confinement lengths. Also presented is a semi-analytical exact solution to the problem of phonon decoherence in a QD embedded in an optical microcavity. The approach is based on Trotter’s decomposition theorem and takes into account the effects of the exciton-cavity and exciton-phonon coupling on equal footing, thereby providing access to regimes of comparable polaron and polariton timescales. We show that the emission spectrum consists of two polariton lines, with optical decoherence determined by acoustic phonon-induced transitions between the polariton states. When viewed in the polariton frame, we find the dependence of the polariton line broadening on the exciton-cavity coupling strength to be well described by Fermi’s Golden Rule for real phonon-assisted transitions. For comparison, we additionally calculate the QD-microcavity absorption spectra according to well-known master equation approaches and examine the agreement between the differing methods. We show that there is good agreement between the approaches if the polariton dynamics are slow in comparison to the polaron timescale, but significant deviation at comparable polaron and polariton timescales. We attribute the observed discrepancies to a break-down in the master equation approach within the latter regime.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Physics and Astronomy
Subjects: Q Science > QC Physics
Uncontrolled Keywords: - cavity QED - quantum dot - strong coupling - open quantum system - decoherence - phonon - photoluminescence
Funders: EPSRC, Cardiff University
Date of First Compliant Deposit: 22 May 2019
Last Modified: 22 May 2020 02:05
URI: http://orca.cf.ac.uk/id/eprint/122789

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