Multiple Fano resonance modes in an ultra-compact plasmonic waveguide-cavity system for sensing applications

We propose an ultra-compact plasmonic nanostructure to realize multiple Fano resonance (FR) modes, comprising two separated metal-insulator-metal (MIM) bus waveguides side-coupled with a circular ring, including an air path, and this design is less considered before in the plasmonic MIM-cavity system. The sensing performance can significantly improve by introducing an air path to induce a new coupled plasmonic structure, generating multiple FR modes and unique optical properties. Using the finite element method, we numerically simulate the influences of transmittance spectra on structural parameters of the proposed plasmonic sensor. Results reveal that these multiple resonance modes stem from the interference among two bus waveguides, a circular ring, and an air path. Optimizing the structure parameters, we can obtain ten FR modes in the proposed structure. The calculated maximum refractive index and temperature sensitivities are 2900 nm/RIU and 1.13 nm/degrees C, respectively. Besides, its maximum on/off extinction ratio achieves about 44.03 dB. We find that the proposed all-system structure can offer a high sensitivity application of refractive index and temperature sensing. The research results have more functional and diverse applications for designing high sensitivity to the nextgeneration plasmonic sensor.