Broadband refractive index sensor using photonic molecule architecture with subwavelength grating mirrors on silicon nitride

Sensors based on photonic integrated circuits have emerged as a powerful technology for applications such as biochemical analysis, environmental monitoring, medical diagnostics, and pharmaceutical screening. In this work, we present a broadband refractive index sensor based on a photonic molecule architecture, consisting of side-coupled Fabry-Perot resonators on a silicon nitride platform. The design incorporates subwavelength grating (SWG) mirrors to increase the evanescent field interaction with the surrounding analyte leading to higher refractive index sensitivity. Experimental measurements using NaCl solutions demonstrate a sensitivity of 166 nm/RIU with a limit of detection of 4.4 × 10^-4 RIU. Compared to distributed Bragg reflector mirrors, which achieved a sensitivity of 80.8 nm/RIU and a narrower 5 nm bandwidth, the SWG design delivers over two-fold sensitivity improvement and a substantially broader photonic bandgap exceeding 90 nm. The proposed photonic molecule architecture offers two split resonance modes with distinct quality factors and extinction ratios, enabling flexibility in selecting between high-resolution or high-signal readout, depending on application requirements.