Confinement Effects on the Electronic and Optical Properties of Semiconductor Quantum Dots Revealed with Two-Dimensional Coherent Spectroscopy

<p>Confinement of electron-hole pairs (excitons) in semiconductor quantum dots (QDs) leads to novel quantum phenomena, tunable optical properties and enhanced Coulomb interactions, all of which are sensitive to the size, shape and material composition of the QDs. This thesis discusses our pursuit in unraveling the complex interrelation between morphology of a QD and its electronic and optical properties. A series of epitaxially-grown semiconductor nanostructures with different QD sizes and composition is studied using optical two-dimensional coherent spectroscopy (2DCS). With the unique capabilities of unambiguously identifying coupling between resonances, isolating quantum pathways and revealing homogeneous dephasing information in heterogeneous systems, 2DCS is a powerful tool for studying QD ensembles.</p> <p>Of paramount importance is the exciton homogeneous line width, which is inversely proportional to the dephasing time. As the dephasing time sets the duration for which coherence is maintained, knowledge of the principal dephasing mechanisms in QDs is essential. 2D spectra of excitons in weakly confining GaAs QDs reveal that elastic exciton-phonon coupling and intra-dot exciton-exciton interactions are responsible for line width broadening beyond the radiative limit, and the interaction strength of both mechanisms increases for decreasing QD size. These results are compared to those obtained from InAs QDs, which exhibit an order-of-magnitude larger confinement, to illustrate the role quantum confinement plays in exciton dephasing.</p> <p>The lowest energy optical transitions in semiconductor QDs are modifed by confinement-enhanced Coulomb interactions, such as exchange-mediated coupling between excitons and correlation effects that can lead to bound and anti-bound states of two excitons. 2D spectra particularly sensitive to these interactions reveal that the electron and hole wave functions - and therefore the strength of Coulomb interactions - are sensitive to variations in QD size for the GaAs ensemble. In the InAs QDs, however, the wave functions are remarkably independent of the details of confinement, leading to similar electronic and optical properties for all QDs. To provide additional insight, the spectra are modeled using perturbative density matrix calculations, and the results are compared to many-body calculations to reveal the significance of the strength and nature of Coulomb interactions on the optical properties of QDs.</p>
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University of Colorado Boulder
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