Second order nonlinear optical phenomena in strained silicon waveguides

A relevant contribution in the explosion of the silicon photonics derives from its nonlinear optics branch, also called nonlinear silicon photonics. This research area exploits the tight confinement of the light, which is allowed by the high contrast index in the silicon sub-micron-structures, and the nonlinearity of silicon to produce (fabricate, develop) novel active devices on the chip scale. Despite of the plenty of third order nonlinear optical phenomena, silicon lacks the second order nonlinearity, which is an essential component of nonlinear optics. In fact, due to the inversion symmetry of its crystalline structure, silicon is characterized by a zero bulk second order nonlinear susceptibility in electric-dipole approximation. Hence, this thesis had the general goal to demonstrate the possibility to perform an all optical experiment of frequency conversion by making use of the second order nonlinear response induced in strained silicon waveguides. The necessary condition to have a bulk second order nonlinear response in silicon is the breaking of its centrosymmetry. This can be obtained by deforming the crystalline structure, for example, by means of a mechanical strain. Based on this approach, this thesis presents and discusses the results achieved in the characterization of the mechanical properties and the strain-induced second order nonlinearity of silicon-on-insulator (SOI) waveguides mechanically strained by using a stressing cladding layer deposited on the waveguide. In particular, the mechanical characterization has been performed by micro-Raman spectroscopy allowing to reconstruct for the first time the two dimensional spatial distribution of the strain across the waveguide cross-section and study its inhomogeneity by varying the stress applied by the cladding overlayer. The second order nonlinear response and the influence of the strain field on it have been experimentally investigated through Second Harmonic Generation (SHG) experiments in transmission configuration and theoretically analyzed, pointing out the strict dependence of the second order nonlinear susceptibility on the extent and inhomogeneity of the strain field.