Compact linearly uncoupled resonators for efficient spontaneous parametric downconversion via angular phase matching

We report an integrated platform for efficient second-order nonlinear interactions based on linearly uncoupled resonators and angular phase matching. The proposed architecture confines phase control to a limited section of the device, maximizing field enhancement and effective nonlinear interaction length while simultaneously reducing the overall footprint. As an example we show the results for an AlGaAs-on-insulator structure demonstrating a photon-pair generation rate of 3.16 GHz/mW in the continuous-wave regime and 5.89 MHz under pulsed pumping. The generated biphoton state exhibits a Schmidt number K=1.02, indicating nearly uncorrelated photon pairs. The compact and reconfigurable nature of this approach, together with its independence from material-specific poling techniques, makes it applicable to a broad class of integrated $\chi_2$ nonlinear platforms.