Interpretable inverse-designed cavity for nonlinear photon pair generation on-chip

Inverse design has proved to be a powerful tool in photonics for compact, high-performance devices. To date, applications have been limited to linear systems but have rarely been investigated or demonstrated in nonlinear regimes. In addition, the "black box"nature of inverse design techniques hinders the understanding of the optimized structure. Here, we propose an inverse design approach to amplify the efficiency of on-chip photon pair generation. We implement this strategy based on the open-source package EMopt. Our method employs a multi-frequency co-optimization strategy and calculates gradients with respect to the design parameters via the adjoint method. The resulting efficiency enhancement stems not only from the field intensification due to the confinement of light from high-quality factor cavity resonances but also from the improvement of phase-matching conditions, along with coupling between the cavity and waveguide mode considered in the design. We demonstrate the capability of the proposed method by fabricating and characterizing an optimized device that enables the efficient generation of photon pairs. Our design follows the fabrication constraints and can be used for scalable quantum light sources in large-scale computing and communication applications. Interestingly, the shape of the proposed design can also be explained by the effective potential method. The proposed optimization technique can be generalized to other nonlinear processes for compact frequency-mixing devices on-chip.