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Optical sensing based on multimode Fano resonances in metal-insulator-metal waveguide systems with X-shaped resonant cavities.

A plasmonic metal-insulator-metal (MIM) waveguide system is proposed, which is composed of a symmetrical X-shaped resonant cavity and a bus waveguide with a baffle, and its Fano resonance and optical sensing characteristics are investigated by using the finite element method (FEM). The results show that the system allows easy implementation of up to four Fano resonances, and the maximum refractive index sensitivity and figure of merit are 1303 nm/RIU and 3113, respectively. The influences of the geometric parameters of the system on the Fano resonances are also investigated, and further the independent adjustments of the Fano resonance line shape and wavelength are realized. Moreover, when an additional X-shaped resonant cavity is added to the system, more ultrasharp Fano resonances with considerable performances are obtained, which may enhance the parallel processing capability of the system. The proposed plasmonic MIM waveguide system may have potential applications in integrated photonic devices and nanoscale optical sensing.