Nonlinear absorption spectroscopy of bulk semiconductors and nanocrystalline silicon quantum dots

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2021-11-09

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Furey, Brandon Joseph

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My Ph.D work is on investigating the nonlinear absorption spectra of bulk semiconductors and nanocrystalline semiconductor quantum dots with the aim of understanding the limitations and potential applications of bulk semiconductors in photonic applications and of nanocrystalline semiconductor quantum dots for biological imaging. I studied two-photon absorption in these materials using three nonlinear optical techniques. First, in Chapter 2, I performed open-aperture Z-scan measurements of gallium phosphide to measure the degenerate two-photon absorption coefficient and the anisotropy parameter. However, as this technique required high light intensities, self-focusing and free-carrier absorption can complicate analysis. This lead to the development of a pump-probe modulation spectroscopy experiment in Chapter 3, which I performed on gallium phosphide, gallium arsenide, and silicon. This allowed complete characterization of the degenerate imaginary part of the third-order nonlinear optical susceptibility tensor and analysis of the response dynamics which was crucial for rejection of anomalous cases due to non-instantaneous responses, phase gratings, or other effects. In Chapter 4, I measured the size-dependence of two-photon excited photoluminescence in nanocrystalline silicon quantum dots at a single excitation wavelength by calibrating to one-photon excited photoluminescence. My collaborators also demonstrated biological imaging of mouse cells using one- and two-photon excited photoluminescence confocal microscopy with nanocrystalline silicon quantum dots encapsulated in liposomes. Lastly, in Chapter 5 I measured two-photon excited photoluminescence spectra of nanocrystalline silicon quantum dots of two different sizes calibrated to a reference standard. I also simulated their efficiency at two-photon excited photoluminescence imaging in biological tissues to compare to other quantum dots and molecular fluorophores to aid in the selection and optimization of biological imaging agents. I measured nonlinear optical properties not previously reported in these materials and developed the experiments required to make these measurements during the course of these projects. This work opens the door to further investigation, especially in semiconductor quantum dots and other low-dimensional structures and their wide-ranging applications.

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