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Dissertation Title: Dissertation Defense: Jiahui Gu, "Harmonic and supercontinuum generation in zinc-blende materials"
Abstract:
Mid-infrared (MIR) laser sources have become an emerging research domain due to diverse applications in spectroscopy and strong field studies. Frequency conversion in nonlinear optical crystals is one of the promising techniques for generating the spectral coverage over one octave in the MIR. In polycrystalline structured semiconductors, relaxed phase matching conditions greatly extend the limits of frequency conversion imposed by acceptance bandwidth. Due to large nonlinearities and broad transparency window in the MIR, zinc-blende materials in their polycrystalline form are uniquely suitable for extremely broadband harmonic and supercontinuum generation.
This dissertation reports on the development of numerical methods for simulating light-matter interactions in polycrystalline zinc-selenide and zinc-sulfide. It is the first, to the best of our knowledge, quantitative and comprehensive numerical investigation on ultrafast mid-infrared laser-solid interactions in disordered materials. Rigorous modeling of random quasi-phase-matching takes into account the transformation of the second-order susceptibility tensor in randomly oriented crystallites, the realistic grain size distribution, and connect them with spatial propagation and polarization analysis. Results obtained by characterizing the coherence properties shed light on the influence of the disordered micro-structures. The model demonstrates its predictive ability in explaining the mechanism of nano-joule level supercontinuum generation in Cr:ZnS oscillator, as a result of complex interplay among material nonlinearities, pulse pre-chirp and thermal optical effects. Taking a step further, we explore the possibility of employing fully microscopic theory. A semiconductor Bloch equations based approach is presented, taking into account contributions from the entire 3D Brillouin zone, and the connection between symmetry of the medium and the generation of high harmonics is illustrated.