Resonant cavity structure based on Silicon Nitride platform for refractive index sensing applications

We report the design, fabrication, and experimental demonstration of a high-sensitivity refractive index (RI) sensor based on a silicon nitride (SixNy) resonant cavity structure. The device employs a loop-shaped resonator coupled to a bus waveguide via directional couplers and is fabricated using CMOS-compatible processes. To enhance sensing performance, a subwavelength grating (SWG) waveguide segment is introduced into the resonant cavity, enabling stronger optical confinement and increased light–matter interaction. The standard resonant structure exhibits a sensitivity of ~201.5 nm/RIU, while the incorporation of the SWG segment improves the sensitivity to ~341.5 nm/RIU representing a 69.48% enhancement. The device achieves a high intrinsic quality factor (Q ~ 3.9 × 10⁴), ensuring sharp resonance features and precise wavelength tracking. RI variations from 1.395 to 1.403 were tested using standard index liquids, and the transmission spectra showed consistent redshifts in resonance. These results confirm the efficacy of the SWG-enhanced SixNy platform for compact, label-free, and high-resolution biosensing applications.