Plasmonic RI Sensor Design Based on Fano Resonance with MIM Waveguide Coupled by the Concentric Annulus Cavity

A nano resonator is designed consisting of the metal-insulator-metal (MIM) waveguide coupled with the concentric annulus cavity. The optical transmission characteristics and sensing characteristics of the designed sensor are studied by using finite element method. Experimental results show that the combination of the MIM waveguide and the concentric annulus cavity can produce bright mode and dark mode around the central wavelength of 565 nm. Due to the interaction between these modes, the Fano resonance phenomenon will be produced with a transitional dip around the wavelength of 585 nm. Compared with the impacts of the width of the metal baffle and the coupling gap, the Fano resonance effect is very sensitive to the variation of the radii of the concentric annulus cavity, which is helpful to design the tunable optical devices based on Fano resonance in the future. However the modulation depth is obviously influenced by the width of the metal baffle and the coupling gap. The research also demonstrates the sharp asymmetric Fano transmission spectrum is also sensitive to the surrounding medium. By calculating the shift of the resonant wavelength with the change of the refractive index of the medium in the resonator, the device can achieve the sensitivity of 550 nm/RIU and the figure of merit of 70.