Refractive index sensing based on multiple Fano resonances in a plasmonic defective ring-cavity system

Abstract

A type of coupled plasmonic resonant system using a metal–insulator -metal (MIM) waveguide is proposed and investigated for generating multiple Fano resonances in this paper. The multi-mode interference coupled-mode theory (MICMT) is mainly employed to analyze the system theoretically, and these results agree well with the results numerically obtained with the finite-difference time-domain (FDTD) method. In this proposed system, multiple asymmetrical Fano-resonant peaks are achieved in the spectra by employing two waveguides separated by a metallic baffle and coupled with a defective ring cavity at one side of the MIM waveguide. According to standing wave theory, we can easily manipulate these Fano peaks by adjusting the main parameters of the defective ring cavity, such as the radius and the notch range. As a result, the structures with three and nine ultra-sharp Fano resonances are obtained and discussed, and the sensitivities and figures of merit (FOMs) are also represented. Furthermore, the phase shifts and the group delays that are induced by the mode interactions are investigated. The designed structures show great potential for the high integrated optical circuits of chip-scale optical sensors, slow light devices or splitters.