Trapezoidal Subwavelength Grating Micro-Ring Resonator with Envelope-Spectrum Tracking for Ultra-Wide-Range Refractive Index Sensing

This paper proposes a trapezoidal subwavelength grating micro-ring resonator refractive index sensor that integrates subwavelength grating structures with a micro-ring resonator to achieve highly efficient mode conversion, ultra-high sensitivity, and an ultra-wide detection range. The proposed device is numerically investigated using the 2.5-dimensional finite-difference time-domain method. The mode converters, including strip-to-slot, slot-to-double-slot, and double-slot-to-SWG, exhibit a simulated average mode conversion efficiency exceeding 95%. The double-slot structure enhances the light-analyte interaction, thereby improving the sensing sensitivity, while the trapezoidal SWGs in the rings further improve sensing performance. Simulations yield an RI sensitivity of 1127 nm/RIU and a concentration sensitivity of 2780 pm/%. Moreover, the simulated detection range of the TSWG-MRR is extended to 0.45 RIU, overcoming the free spectral range limitation of conventional MRRs. The proposed TSWG-MRR achieves a quality factor of 1885, an extinction ratio of 15.7 dB, and a theoretically calculated limit of detection of 8.87 × 10−6 RIU and demonstrates strong potential for lab-on-a-chip sensing applications.