Dual Fano resonance control and refractive index sensors based on a plasmonic waveguide-coupled resonator system

A compact plasmonic waveguide-coupled resonator system, which consists of a metal-insulator-metal (MIM) waveguide with a double stub side-coupled to a ring cavity, is designed and numerically investigated using finite element method (FEM). Simulation results show that dual Fano resonances with sharp asymmetric line shape are observed in the transmission spectrum. One of the Fano resonances originates from the coupled-resonator effect of the two stub cavities, whereas the other is induced by the ring resonator. The two resonance peaks exhibit different dependencies on the parameters of the structure because of varying mechanisms and can be easily tuned. Moreover, the maximum refractive index sensitivity and Q-factor of the proposed structure are 1000 nm/refractive index unit (RIU) and 41, respectively. The maximum figure-of-merit (FOM∗) is approximately 4200 at 0.71 μm, which is due to the sharp asymmetric Fano line shape with an ultra-low transmittance at this wavelength. The proposed structure is expected to provide guidelines for the design of nano-sensors.