High-Q spiral-based coupled-resonator device on a Si 3 N 4 platform for ultrasensitive sensing applications

Abstract

We demonstrate high- Q Si 3 N 4 racetrack-spiral and spiral-spiral coupled-resonator devices for sensing applications. The coupled-resonator architecture resolves the confusion caused by small free spectral range (FSR) in conventional long resonators. The sensitivity of the racetrack-spiral coupled-resonator device for sodium chloride (NaCl) and biomolecule detection is numerically achieved at around 199 nm/RIU (bulk sensitivity) and 183 pm/nm (surface sensitivity), respectively. We fabricate a racetrack-spiral coupled-resonator device with intrinsic Q of 560,000 (263,000) at near-infrared wavelengths, at around 1300 nm, for air (water) cladding. We extract an experimental bulk sensitivity of around 131 nm/RIU for this device from the wavelength shift of the device with different concentrations of the NaCl solution. The performance of the spiral-spiral coupled-resonator device for NaCl and biomolecule detection is theoretically calculated. For biomolecular detection, this device is self-referenced, and its FSR increases almost linearly with the refractive index of biomolecular layer in a certain range. A theoretically defined FSR sensitivity of 74 nm/RIU is extracted from the experimental data. We fabricate a spiral-spiral coupled-resonator device with intrinsic Q of 276,000 (112,000) at around 1300 nm for air (water) cladding, and a bulk sensitivity of 167 nm/RIU for NaCl detection.