Room-Temperature Visible-Light Single-Photon Avalanche Detection in Grating-Coupled Post-CMOS Photonic Circuits

Abstract The integration of photonic circuits with on-chip single-photon detectors has emerged as a key enabling technology for highly scalable visible-wavelength photonic integrated circuits, underpinning advanced applications in quantum information processing and biophotonics. However, detecting weak optical signals down to the single-photon regime at specific wavelengths requires efficient interconnections between photonic waveguides and detectors. Here, we present an interconnect from silicon nitride photonic circuits to single-photon avalanche diodes (SPADs) on a standard 200 mm CMOS wafer integrated using post-CMOS integration. To realize this efficient interconnect, we optimized the interface by introducing metal reflector-assisted grating couplers for the target wavelengths 633 nm and 785 nm. To investigate the influence of the reflector on the efficiency of the interface, we conducted simulations and subsequent optimizations. In simulation, the grating-assisted interconnect achieves a coupling efficiency of 2.0 dB at 785 nm, representing a 6.4 dB improvement over the baseline design. To validate this improvement, we fabricated this structure with and without reflector and experimentally demonstrate detection of single photons at the target wavelengths. After post-CMOS integration, the utilized Fraunhofer IMS SPADs were operated at room temperature with an off-chip passive quenching circuit. Optical characterization showed an improvement by up to 4.5 dB by implementing reflectors. We show venues for improving the photon detection probability by correlating it with loss sources encountered in the fabricated devices. With these results, we demonstrate the applicability of post-CMOS integration and offer a photonic-integrated solution for detecting single photons from red to near-infrared wavelengths.