On-chip ultracompact broadband reflector and high-Q Fabry-Pérot cavity based on inverse design.

Photonic integrated circuits require compact broadband passive components. We report an inverse-designed reflector integrated with a high-Q Fabry-Pérot (F-P) cavity on a standard 220-nm silicon-on-insulator (SOI) platform. The 2 µm × 2 µm reflector is incorporated into a 400-µm bus-coupled cavity. The fabricated device exhibits a loaded Q of approximately 1 × 105 and a round-trip-loss-limited effective reflectivity of 0.92-0.96 across 1500-1600 nm, providing a lower bound on the mirror reflectivity. The design includes a 250-nm minimum-feature constraint and ±10 nm lithographic-bias tolerance analysis. These results establish a compact, broadband, and fabrication-aware reflector-cavity building block for wavelength-selective silicon photonic circuits.