Efficient Topology-Optimized Couplers for On-Chip Single-Photon Sources

Room-temperature single-photon sources (SPSs) are critical for emerging practical quantum applications such as on-chip photonic circuity for quantum communications systems and integrated quantum sensors. However, the direct integration of an SPS into on-chip photonic systems remains challenging due to low coupling efficiencies between the SPS and the photonic circuitry that often involve size mismatch and dissimilar materials. Here we develop an adjoint topology optimization scheme to design high-efficiency couplers between a photonic waveguide and an SPS in hexagonal boron nitride (hBN). The algorithm accounts for fabrication constraints and SPS location uncertainty. First, a library of designs for the different positions of the hBN flake containing an SPS with respect to a Si3N4 waveguide is generated, demonstrating an average coupling efficiency of 78%. Then, the designs are inspected with a dimensionality reduction technique to investigate the relationship between the device geometry (topology) and the performance. The fundamental, physics-based intuition gained from this approach could enable the design of high-performance quantum devices.