On-Chip Nonlinear Optics in Silicon Rich Nitride Photonic Crystal Cavities

Photonic crystal (PhC) nanocavities allow to confine light with ultra high quality (Q) factors to wavelength-sized mode volumes, with a strong enhancement of light-matter interaction. Although these features make PhC cavities a promising platform for integrated nonlinear optical components, the usual materials of photonic fabrication (specifically silicon and gallium arsenide) suffer from parasitic effects such as two-photon absorption (TPA) and free-carrier absorption (FCA), which limit the benefits of integration. In this work we show how a novel material, silicon rich nitride [1], can be successfully employed for the fabrication of air membraned PhC cavities reaching ultra high Q factor at telecom wavelength. We fabricated samples with line-width modulated geometry, approaching a theoretical value (from FDTD simulations) of Qth = 520, 000. We then measured the cavities with resonant scattering technique [2], determining a maximum experimental value Qexp = 122, 000. We later studied the spectral behaviour of the cavity at a regime of optical bistability [3]. The resulting analysis of the thermo-optic resonance shift as a function of input power suggests the absence of TPA and TPA-related FCA. In order to confirm the suitability of this material for nonlinear optical signal processing, we designed and fabricated far-field optimized samples [4] with heterostructure geometry. Far-field optimization allows to dramatically increase the coupling of the cavity to focused laser beams, thus increasing the available intracavity optical power. In this experimental configuration we observed second (SHG) and third (THG) harmonic generation even at low CW input power (as low as a fraction of milliwatt). These results confirm the suitability of silicon rich nitride as a potential platform for efficient integrated nonlinear optical devices based upon PhC nanocavities in a fully CMOS compatible approach.