Engineering extinction ratio towards higher-efficiency four-wave mixing process in GaP-OI microresonators.

Cavity-enhanced four-wave mixing lays the foundation for a variety of nonlinear applications such as wavelength conversion, parametric oscillation, Kerr frequency comb generation, etc. While the dramatic enhancement in nonlinear conversion efficiency is commonly attributed to the resonance quality factor, the influence of the resonance extinction ratio has often been overlooked. In this work, we uncover the pivotal role of the extinction ratio in FWM processes through integrated innovations in ring-bus coupler design. Pulley couplers are engineered to selectively excite TE and TM modes within microresonators, enabling precise control over polarization-dependent resonance excitation. We demonstrate that the extinction ratio of the cavity resonance directly dictates FWM efficiency. By optimizing the coupling conditions to achieve high extinction ratios (6.6 dB), we realize an 11.95 dB enhancement in FWM efficiency compared to low-extinction-ratio cavities (3.3 dB), in excellent agreement with theoretical predictions. These findings highlight the importance of designing microring resonators at critical coupling conditions to maximize nonlinear conversion efficiency. To the best of our knowledge, this is the first systematic investigation of FWM conversion efficiency using pulley couplers in the gallium phosphide-on-insulator (GaP-OI) platform. In addition, thermal bistability measurements quantify the material absorption-induced propagation loss, clarifying the dominant loss origin. The results presented here provide valuable insights into the development of highly efficient integrated Kerr nonlinear photonic devices, with significant implications for integrated quantum sources and other nonlinear photonic applications.