Nonlinear optics in porous silicon photonic crystals and microcavities

This paper is a survey of our recently published and new results on nonlinear-optical studies of photonic band-gap (PBG) structures formed from mesoporous silicon. The studies are performed on photonic crystals, high-quality-factor microcavities (MC) with a half-wavelength-thick microcavity spacer, and on series of coupled microcavities with two identical microcavity spacers and controllable coupling by intermediate Bragg reflector transmittance. The enhanced second-harmonic generation (SHG) is observed and analyzed for the fundamental wavelength at the PBG edge and in resonance with the microcavity mode. The second-harmonic intensity is enhanced by a factor of approximately 10(2) in comparison with that outside the photonic band gap. The interrelation of two enhancement mechanisms, spatial field localization and phase matching, is determined. The dependence of the shape of SHG spectra on coupling is studied at resonance with one of the splitted modes of coupled microcavities. The experimental results on third-harmonic generation in PBG materials are presented. The resonant enhancement is observed both at the PBG edge and in the MC mode. The intensity achieves 10(3) at the photonic band-gap edge and 5 x 10(3) in the MC mode in comparison with an off-resonant value. A peculiar factor is the interplay of silicon absorption and the cubic susceptibility resonance at the third harmonic wavelength near the E-0/E-1 critical point of silicon.