Multiple Fano resonances in metal-insulator-metal waveguide with umbrella resonator coupled with metal baffle for refractive index sensing*

A single baffle metal-insulator-metal (MIM) waveguide coupled with a semi-circular cavity and a cross-shaped cavity is proposed based on the multiple Fano resonance characteristics of surface plasmon polaritons (SPPs) subwavelength structure. The isolated state formed by two resonators interferes with the wider continuous state mode formed by the metal baffle, forming Fano resonance that can independently be tuned into five different modes. The formation mechanism of Fano resonance is analyzed based on the multimode interference coupled mode theory (MICMT). The finite element method (FEM) and MICMT are used to simulate the transmission spectra of this structure and analyze the influence of structural parameters on the refractive index sensing characteristics. And the transmission responses calculated by the FEM simulation are consistent with the MICMT theoretical results very well. The results show that the figure of merit (FOM) can reach 193 and the ultra-high sensitivity is 1600 nm/RIU after the structure parameters have been optimized, and can provide theoretical basis for designing the high sensitive refractive index sensors based on SPPs waveguide for high-density photonic integration with excellent performance in the near future.