High-Efficiency InGaP-on-Insulator Microresonator Nonlinear Conversion and Entanglement Generation.

InGaP-on-insulator (InGaP-OI), with its intrinsically high χ 2 optical nonlinearity, has emerged as an efficient and bright integrated photonic platform for frequency conversion and on-chip entanglement generation, but high waveguide propagation loss in the visible wavelength range has limited its overall performance. Here, we identify the dominant loss mechanisms through mode-profile analysis and effectively mitigate the loss using a surface treatment method. Statistical analysis of the resonator quality factor and propagation loss reveals the optimal ring radius that maintains a strong nonlinear interaction while suppressing significant bending related loss, resulting in loss as low as 0.49 dB/cm (4.31 dB/cm) at 1560 nm (780 nm). The method provides a 3.5-4 × quality factor enhancement at 780 nm, enabling a second-harmonic generation efficiency of 3.01 × 10 5  %/W and a degenerate photon-pair generation rate of 4.27 MHz / μ W and coincidence-to-accidental ratio as high as 10 000. The quasi-phase matching condition is experimentally verified, and nonlinear conversion is systematically characterized across the entire parameter space. This work establishes a scalable pathway for classical and quantum photonics in a low-loss, highly nonlinear, and wafer-scale integration platform.