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Title: Optical spectroscopy of excitons confined in two-dimensional materials and semiconductor heterostructures
Author: Scarpelli, Lorenzo
ISNI:       0000 0004 7962 1925
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2019
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In this thesis we show experiments of optical spectroscopy of excitons confined in two-dimensional materials and semiconductor heterostructures. We study the exciton and trion density dynamics in monolayers of molybdenum diselenide by resonant three-pulse four-wave mixing spectroscopy, at temperatures from 300 K to 77 K. A multiexponential third-order response function for amplitude and phase of the heterodyne-detected four-wave mixing signal including four decay processes is used to model the data. We provide a consistent interpretation within the intrinsic band structure, not requiring the inclusion of extrinsic effects. We study the coupling of indium arsenide quantum dot excitons to photonic crystal waveguide modes. The coupling efficiency into guided modes, the beta factor, is deter-mined by direct spectral imaging. We find beta factors above 90% over a wide spectral range of 40 meV in the fast light regime, reaching a maximum of 0.99 ± 0.01. All the reported measured beta factors are not corrected for the collection efficiency of free-space and coupler emission, which would increase the beta factor, since the free space emission, according to our simulation, is collected typically twice as efficiently depending on the QD position inside the lattice unit cell. Direct spectral imaging is also used to measure the directional emission of the circularly polarised transitions in a magnetic field, and deduce the mode circularity at the quantum dot sites. We use the emission energy, mode circularity and beta factor to determine the quantum dot position inside the photonic crystal waveguide unit cell, by comparison with finite-difference time-domain simulations. Finally, we study the spontaneous emission lineshape of indium gallium arsenide quan-tum dot excitons by photoluminescence spectroscopy. The lineshape is characterised by a spectrally sharp peak, the zero-phonon line, superimposed to a broad emission due to phonon-assisted transitions. We perform measurements of power dependence and fine-structure splitting to individuate exciton, trion and biexciton emission lines. To fit the corresponding lineshapes, we develop a fitting model based on the independent boson model, from which we extract a set of underlying parameters: deformation potentials, QD confinement lengths and phonon temperature. Additionally, using an observed thermal offset around the zero-phonon transition, we are able to separate the homogeneous from the inhomogeneous broadening.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics