Refractive Index Sensing Based on Fano Resonances of a Bus Metal-Insulator-Metal Waveguide with D-Shaped Resonant Cavity.

A plasmonic waveguide coupling cavity structure is proposed, consisting of a bus metal-insulator-metal (MIM) waveguide-coupled single/double D-shaped resonant cavity. The transmission properties, magnetic field distribution, and refractive index sensing performance are simulated using the finite element method (FEM). A clear multi-Fano resonance phenomenon is observed in the transmission spectra. The Fano resonances arise from the interaction between the discrete states of the D-shaped resonant cavity and the continuum state of the bus MIM waveguide. The influence of various structural parameters on the Fano resonance is investigated. Furthermore, the refractive index sensing properties based on the Fano resonance are studied by varying the refractive index of the insulator layer. Maximum sensitivities of 1130 nm/RIU and 1150 nm/RIU are achieved at the fourth Fano resonance dip for the single and double D-shaped resonator cavities, respectively. And a maximum figure of merit of 171 is achieved by the single D-shaped cavity structure. This research can provide new avenues for the design of high-sensitivity sensors as well as novel on-chip sensing structures.