Second order nonlinearity in Silicon Nitride waveguides via photo-induced self-organized gratings

Silicon nitride (SiN) has become one of the primary candidates for development of integrated nonlinear photonics owing to its direct CMOS workflow compatibility as well as excellent linear and nonlinear optical properties and its wide transparency from the visible to middle infrared. Within the last decade we have been witnessing the significant progress made towards improving SiN waveguide devices and harnessing the available optical nonlinearities through a wide variety of nonlinear optical phenomena such as supercontinuum generation [1] , soliton formation [2] , and, very recently, three-wave mixing [3] . While the latter is not directly accessible in SiN after fabrication due to its centrosymmetric nature, it can be achieved through inducing the second-order nonlinearity χ (2) via all-optical poling [4] . All-optical poling allows the inscription of a self-organized periodic space-charge grating and subsequent quasi-phase-matching (QPM) of optical fields participating in the three-wave mixing process, such as second-harmonic generation (SHG), sum-frequency generation (SFG) and difference-frequency generation (DFG). The all-optical poling of SiN waveguides was first applied for demonstration of SHG process reaching conversion efficiencies of the order of 1 %/W with wide reconfigurability [5] , hence igniting general interest in the approach.