High figure of merit surface lattice resonance sensor based on double-layer inverted nano-crescent array

Abstract

Plasmonic nano-arrays provides a promising platform for optical devices due to the high degree of freedom in structural design. However, the broadening of full width at half maximum (FWHM) caused by high radiation loss in nanostructure limits its further application because it can only achieve very low quality factor (Q-factor) spectral response. Based on this, a novel double-layer inverted nano-crescent array structure which can excite surface lattice resonance (SLR) with high Q-factor is proposed in this paper. The SLR absorption peak with low radiation loss but low intensity will have better spectral characteristics because of spectral splitting caused by double-layer inverted crescent and the excitation of the out-of-plane mode. The out-of-plane mode is modulated by the height of nano-crescent. The increase of single-layer height can significantly enhance the SLR formed by the coupling between Z-directional electric field in free space and localized surface plasmon resonance; and the increase of multi-layer height can significantly enhance the weak SLR coupling peak dominated by waveguide mode. Both cases achieve FWHM of 10⁻² magnitude (minimum: 0.04 nm) and Q-factor of 10⁴ magnitude (33515.69) while increasing the absorption intensity. Further, the SLR sensor with ultra-high figure of merit (FOM) (22347.5 RIU⁻¹) is demonstrated. Compared with ordinary SLR sensors, the FOM has been improved by two orders of magnitude. This paper provides a new idea for the achievement of extremely narrow linewidth spectral response and the design of sensing structures with ultra-high FOM, further high FOM sensing will promote the development of trace substance detection technology.