Compact triple Mach-Zehnder interferometer slot micro-ring resonator with ultra-high sensitivity and ultra-wide detection range for label-free sensing

This paper proposes and analyzes a compact triple Mach-Zehnder interferometer slot micro-ring resonator (MZI-SMRR) with dimensions smaller than 50 µm × 50 µm, suitable for label-free detection sensing in various scenarios. The structure integrates three Mach-Zehnder interferometers (MZIs) with a slot micro-ring resonator (SMRR). A mathematical model for the multi-MZI-based SMRR was established using the transfer matrix method and coupled-mode theory. Numerical simulations were conducted through finite-difference time-domain (FDTD) analysis implemented in MODE Solutions. The integration of SMRR with MZIs significantly enhances the detection range while eliminating the free spectral range (FSR) limitation inherent in conventional micro-ring resonators. The proposed MZI-SMRR demonstrates a quasi-FSR detection range exceeding 140 nm, exceeding 7-fold compared to the traditional SMRRs. Therefore, the designed device has achieved excellent merits of ultra-high sensitivities and ultra-wide detection range. Concentration sensitivities for calcium chloride and D-glucose solutions reach 1199.6 pm/% and 1198.5 pm/% respectively, corresponding to refractive index (RI) sensitivities of 1007.3 nm/RIU and 1009.2 nm/RIU. This investigation into the combined SMRR-MZI configuration provides valuable insights for future development of label-free sensing applications for high concentration variations of analytes.