Waveguide Geometry–Driven Trade‐Offs in Resonant Cavity Sensor Performance

ABSTRACT Achieving ultralow detection limits in integrated photonic sensors requires balancing refractive index sensitivity, intrinsic Q‐factor, and wavelength noise, yet the role of waveguide geometry remains poorly understood. Here, we benchmark four silicon nitride resonant cavity designs that primarily differ in waveguide geometry, namely ridge, subwavelength grating (SWG), photonic crystal (PhC), and slot, fabricated on a common platform and optimized for operation at 1310 nm. Using a low‐noise wavelength interrogation system with reduced instrumentation noise, we perform bulk refractive index sensing with calibrated NaCl solutions. Nanostructured waveguides show enhanced sensitivity, with slot at 160 nm/RIU, SWG at 152.5 nm/RIU, and PhC at 131 nm/RIU, but also significantly increased wavelength noise, which scales faster than sensitivity and degrades the limit of detection LOD. In contrast, the ridge resonator exhibits lower sensitivity of 71.5 nm/RIU but achieves the highest intrinsic Q of 3.5 × 10 5 and the lowest wavelength noise, resulting in one of the lowest reported LODs for an integrated resonant cavity sensor at 1310 nm, measured as 4.2 × 10 − 8 RIU using 10‐point averaging. These results show that waveguide geometry governs sensitivity, Q, and noise, creating nonintuitive trade‐offs and guiding resonator selection.