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Nonlinear optics in high refractive index contrast photonic crystal microcavities Cowan, Allan Ralph

Abstract

This thesis describes theoretical and experimental research on the nonlinear response of high refractive index contrast (HRIC) optical microcavities. An intuitive, numerically efficient model of second harmonic reflection from two dimensional (2D), planar photonic crystals made of sub-wavelength thick, non-centrosymmetric semiconductors is developed. It predicts that appropriate 2D texture can result in orders of magnitude enhancement of the reflected second order signal when harmonic plane waves are used to excite leaky photonic crystal eigenmodes. Local field enhancement in the textured slab, and other physical processes responsible for these enhancements are explained. A different formalism is developed to treat the Kerr-related bistable response of a 3D microcavity coupled to a single mode waveguide. This model predicts that optical bistability should be observed using only milliwatts of power to excite a cavity fabricated in Al₀.₁₈Ga₀.₈₂ As, having a quality factor of Q = 4000 and a mode volume of 0.05 /μm³. Two-photon absorption is shown to only slightly hinder the performance in Al₀.₁₈Ga₀.₈₂As. By including nonresonant downstream reflections in the model, novel hysteresis loops are predicted, and their stability is analyzed. A coupled waveguide-microcavity structure is fabricated by selectively cladding a silicon ridge-Bragg grating waveguide with photoresist. Threedimensionally localized optical modes are realized with Q values ranging from 200 to 1200, at ~ 1.5 μm. Using 100 fs pulses, the transmission spectra of these structures is studied as a function of input power. The output power saturates when the cavity mode and pulse centre frequencies are resonant, and the output exhibits super linear growth when they are appropriately detuned. These results are explained in terms of the filtering action of the microcavity on the nonlinear spectral distortion of the input pulse as it propagates through the waveguide. PbSe nanocrystals are deposited on a microcavity in order to enhance its nonlinear response. Asymmetrical spectral broadening and saturation of the transmitted power is observed with ~ 10 000 photons in the cavity. A time-dependent third order model of the microcavity's impulse response is developed to describe this behaviour. This work contributes to a quantitative understanding of how HRIC dielectric texture can be engineered to obtain useful nonlinear optical responses at moderate power levels.

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