Independently tunable Fano resonances in a metal-insulator-metal coupled cavities system.

Herein, multiple Fano resonances with excellent ability to be tuned independently are produced in a sub-wavelength metal-insulator-metal system. The input and output waveguides are separated by a metal gap, and a stub and an end-coupled cavity are placed below and to the right side of the input waveguide, respectively, as discrete states. Owing to the mode interferences, double ultra-sharp and asymmetric Fano resonant peaks are observed in the transmission spectrum. Successfully, the basic structure is extended by two extra rectangular cavities, giving rise to four Fano resonances with high refractive index sensitivity and figure of merit. Due to the discrete modes of Fano resonances from different coupling cavities, their resonant wavelengths can be controlled independently, which can provide greater flexibility for tuning Fano resonances. The performances of the proposed structure are investigated by both the finite-difference time-domain method and the multimode interference coupled-mode theory. It is believed that the research can provide important guidance in designing Fano resonance structures, and the proposed structure has a wide application in sensors, switches, and nano-photonic integrated circuit devices.