Multiply-resonant Waveguide Gratings for Enhanced Second-harmonic Generation

Recent progress in fabrication of low-loss waveguiding structures on complementary metal-oxide-semiconductor (CMOS) compatible material platforms is enabling new applications for integrated photonic circuits. Silicon nitride (SiN) has emerged as a prominent material for nonlinear integrated photonics due to its broad transparency window, high refractive index, lower material losses, and strong third-order optical nonlinearities [1]. The recently found strong effective quadratic nonlinearities of SiN are furthermore extending its potential. For example, three-wave mixing processes could transfer generated frequency combs to new wavelength regimes, e.g. enabling self-referencing of combs [2], [3]. Here, we numerically investigate how a resonant waveguide grating (RWG) with multiple resonances can be used to achieve better conversion efficiency in second-harmonic generation (SHG) via a better spatial overlap of the guided modes at the fundamental and the second-harmonic wavelengths.