Investigation of χ(2)-translated Optical Frequency Combs Tunability in Gallium Phosphide-on-Insulator Resonators

We describe a synergistic optimization approach that enables highly efficient frequency translation of a Kerr optical frequency comb (OFC) from 1550 nm to 775 nm in a gallium phosphide-on-insulator (GaP-OI) microresonator. Key distinctions from previous GaP-OI works which focused on individual optical nonlinearity are that this work not only emphasizes the interaction between the second- and third-order nonlinearity, but also explores the tunability of the χ-translated OFC through geometric and temperature tuning. We apply this approach to the burgeoning GaP-OI platform and demonstrate that a 50 μm-radius ring resonator with a cross-section of 555 nm × 600 nm has an intracavity second harmonic (SH) generation efficiency as high as 71.5%/W, 3 times larger compared to the state-of-the-art designs. The sum-frequency (SF) comb at 775 nm has a geometric tuning sensitivity of 354 GHz/nm, and a thermal tuning sensitivity of 24.8 GHz/K, paving the way for post-fabrication trimming and in-situ spectral shaping, with a broader potential to realize highly efficient, wide-spectrum, and tunable on-chip nonlinear sources.