Dual and narrow-band optical filtering and sensing enabled by asymmetric dielectric metagratings

Abstract

Here, we propose a metagrating structure for dual-and narrow-band optical filtering and sensing in the VIS-NIR regime, comprising two closely spaced, optically thin layers of asymmetric dielectric nanostrip arrays with equal periods but varying nanostrip widths. Based on the numerical simulations, we clarify that the dual-narrow band filtering characteristics in the transmission spectrum is mainly due to the intercoupled antiphase Fano resonance modes in the each nanoslits and guided mode resonance (GMR) bound to top middle and bottom of the a binary metagratings, respectively. The intercoupled antiphase Fano resonance modes suppress the transmission in a way that adjoining nanostrips in the unit cell act as the dipole resonance, efficiently trapping the leaky radiation and enhance reflectance, while GMR modes at neighbouring wavelengths suppress transmission and enhance reflection, so as to shape the near-zero dual-narrow stopband dips. It is important that asymmetry of the nanostrips helps to improve the quality factor of both resonance modes and thus to reduce the stopband width. The sensitivity for this gas sensor structure was estimated to be 58 and 71 nm/RIU, with a transmission bandwidth of 2 and 4 nm for both modes, respectively. These new findings pave the way for subtractive optical wave modulation techniques, multiband filtering, sensing, and detection.