Hybrid nonlinear effects in photonic integrated circuits

Nonlinear optics in photonic integrated circuits typically exploits the nonlinearity of a single material platform. We demonstrate hybrid optical nonlinearities that occur in two different materials, enabling the observation of combined Raman scattering and Kerr frequency comb generation in silicon nitride (Si3N4) microresonators with silica cladding. The silica cladding provides Raman gain, whereas the Si3N4 core provides the Kerr nonlinearity for frequency comb generation. In this way, Raman scattering is introduced into an integrated silicon nitride photonic platform, where it has not been observed previously due to insufficient Raman gain. Raman lasing is observed in the silica-clad Si3N4 resonators at an on-chip optical power of 143 mW, in agreement with theoretical simulations. Broadband Raman-Kerr frequency comb generation is achieved through dispersion engineering of the waveguides. The use of hybrid optical nonlinearities in multiple materials opens new functionalities for integrated photonic devices, for example, by combining second- and third-order nonlinear materials for simultaneous supercontinuum generation and self-referencing of frequency combs. Combining materials with low threshold powers for different nonlinearities may enable highly efficient nonlinear photonic circuits for applications such as high-resolution spectroscopy, broadband supercontinuum generation, and telecommunications.