Multiple Fano resonance refractive index sensor based on a plasmonic metal-insulator-metal based Taiji resonator

We propose and numerically analyze a multiple Fano resonance device with high quality (Q) factor and high modulation depth based on a metal-insulator-metal (MIM) waveguide laterally coupled Taiji resonator in the near-infrared regime. A silver-air-silver groove (SASG) is embedded in the MIM waveguide. The so-called Taiji resonator is an S-shaped additional crossover branch embedded in a ring cavity to selectively couple counterpropagating cavity modes, therefore increasing the propagating modes relative to the traditional ring cavity. Up to septuple Fano peaks rise from the interference between narrow discrete modes generated from the Taiji resonator and broad continuum modes stemming from the SASG, which can be easily tuned by adjusting the different geometric parameters of the proposed structure. The maximal Q-factor is about 317 with a modulation depth of 24.1 dB in transmission. Thanks to a narrow resonance linewidth with strong subwavelength light confinement of multiple Fano resonances, an optical refractive index sensor with a sensitivity of 2016 nm/RIU and figure of merit of 345 is demonstrated. Furthermore, the sensor can be used for sensing biological parameters such as glucose solution concentration with maximal sensitivity of 0.245 nm · L/g. This simple proposed structure may provide a further step for the development of Fano resonance applications in nanosensing, lasing, and nonlinear optics.