Efficient third-harmonic generation in composite aluminum nitride/silicon nitride microrings

Aluminum nitride and silicon nitride have recently emerged as important nonlinear optical materials in integrated photonics for their quadratic and cubic optical nonlinearity, respectively. A composite aluminum nitride and silicon nitride waveguide structure, if realized, will simultaneously allow highly efficient second-and third-harmonic generation on the same chip platform and therefore assists 2f-3f self-referenced frequency combs. On-chip third-harmonic generation, being a higher-order nonlinear optics effect, is more demanding than second-harmonic generation due to the large frequency difference between the fundamental-and third-harmonic frequencies, which implies a large change of refractive indices and more stringent requirements on phase matching. In this work we demonstrate high-efficiency third-harmonic generation in a high-Q composite aluminum nitride/silicon nitride ring cavity. By carefully engineering the microring resonator geometry of the bilayer structure to optimize the quality factor, mode volume, and modal overlap of the optical fields, we report a maximum conversion efficiency of 180% W-2, corresponding to an absolute conversion efficiency of 0.16%. This composite photonic chip design provides a solution for efficient frequency conversion over a large wavelength span, broadband comb generation, and self-referenced frequency combs. (c) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement