Design of a micro-ring sensor based on silicon nitride in a trapezoidal slot-waveguide structure

Microring resonators are widely used in sensing, but liquid infiltration into slot waveguides remains a bottleneck. This study proposes a silicon nitride (SiN) trapezoidal slot microring resonator sensor, simulated via the finite-difference time-domain (FDTD) method at 1550 nm. By combining a rectangular slot in the straight waveguide with a trapezoidal slot in the microring, the design enhances light-matter interaction and reduces the static capillary entry pressure, expected to ease liquid infiltration. At a 210 μm radius, the device achieves a free spectral range (FSR) of 1.16 nm, a sensitivity of 780 nm/refractive index unit (RIU) and an extinction ratio of 31.979 dB. A Young-Laplace estimate shows the trapezoidal profile lowers the static capillary entry pressure by 25.9%. Although dynamic filling is not modelled and this reduction is only a necessary condition for infiltration, the device shows promising potential for integrated optical sensing.