Realizing Prominent Fano Resonances in Metal-Insulator-Metal Plasmonic Bragg Gratings Side-Coupled with Plasmonic Nanocavities

Abstract

The generation of Fano resonances usually stems from the interference between a continuum and a discrete state. In this paper, we show that prominent Fano resonances can be realized in plasmonic metal-insulator-metal (MIM) Bragg gratings side-coupled with plasmonic nanocavities, where the Bragg grating provides the continuum state and the nanocavity supports the discrete state. Through tuning the position of the bandgap of surface plasmon polariton (SPP) in the Bragg gratings, the transmission Fano profiles can be modified dramatically. We find that strong coupling between the band edge mode of SPP bandgap and the resonant mode of nanocavity can lead to prominent Fano resonances with obvious transmission peak and valley. When the coupling strength between band edge mode and resonant mode becomes weak, the asymmetric Fano transmission profile vanishes. Additionally, by increasing the refractive index of the insulator, extracted sensitivity of our structures can reach 1425 nm/RIU (refractive index unit, RIU) and the maximal figure-of-merit (FOM) can be as large as 1170. Our proposed structures provide a new feasible solution in realizing Fano resonances and can become one of promising candidates for the Fano resonance-based integrated nanoscale refractive index sensors.