Nonlinear Optics in Photonic Crystal Cavities

Harnessing the properties of light for technological applications is perhaps the ultimate objective of photonics, whereas the generation, manipulation and detection of light in chip-based structures has an important impact both on industry and on fundamental research1. The integration to semiconductor-based nanostructures makes possible the application of the photonics paradigm to compact devices which on one hand enable the processing of information at high speed, and on the other hand allow to include, onto a single chip, complex experimental apparatuses, opening the way towards novel technological and physical applications. In this perspective, an effective light-matter interaction is primarily important, as this feature greatly enhances capability of integrated optical and opto-electronic devices, in terms of switching time, energy consumption and spectrum of applications. For these reasons, the development of structures capable to enhance the interaction of light and matter covers great interest in the field of integrated optics. In this thesis work, we investigate on the linear and nonlinear properties of photonic crystal (PhC) microresonators2, a specific type of integrated optical cavity which capabilities to confine the electromagnetic field both in time and space are particularly suited for enhanced photonic devices. In particular, this type of nanostructure benefits from an enhanced nonlinear response with respect to bulk or non-resonant nonlinear devices, and they are characterized by a minimal footprint, a crucial feature in view of integration, thanks to a Bragg-type physical confinement mechanism. Throughout the work, we focus on three specific topics. The first one consists in the design, fabrication and characterization of PhC cavities realized in silicon suspended membranes, designed for the demonstration of integrated optical frequency combs. These rely on a specific cavity design, engineered to provide equally spaced resonances in energy, as a consequence of the effective confinement potential3. We discuss experimental results showing comb-like resonant spectra and we investigate the possibility to use the structures for the implementation of triply-resonant nonlinear processes, such as four-wave mixing (FWM), with unprecedented conversion efficiency. In a second part, we investigate on the suitability of a novel material, silicon-rich silicon nitride (SRSN)4, for the fabrication of PhC cavities and their operation as nonlinear devices. We show how SRSN deposited films can be successfully used to fabricate high-quality factor PhC cavities and we experimentally study the generation of second- and third-harmonic under resonant pumping regime. Finally, we investigate the suitability of otherwise parasitic nonlinear effects, related to two-photon absorption in silicon microcavities, for the implementation of nonlinear properties based on the dynamical thermo-optic response of the material. We show that the PhC platform provides a way to achieve narrow spectral holes and gain windows, associated to a pronounced dispersion. This feature, associated to a dramatic decrease in group velocity, can be exploited to achieve slow-light on a chip exclusively via thermo-optic effect, in a completely novel approach. 1. Thomson, D. et al. Roadmap on silicon photonics. J. Opt. 18, 073003 (2016). 2. Notomi, M. Manipulating light with strongly modulated photonic crystals. Reports Prog. Phys. 73, 096501 (2010). 3. Alpeggiani, F., Andreani, L. C. & Gerace, D. Effective bichromatic potential for ultra-high Q-factor photonic crystal slab cavities. Appl. Phys. Lett. 107, 261110 (2015). 4. Clementi, M. et al. Cavity-enhanced harmonic generation in silicon rich nitride photonic crystal microresonators. Appl. Phys. Lett. 114, 131103 (2019).