High FOM Refractive Index Sensor Based on Quasi-Bound States in the Continuum of Tri-Prism Metasurface

Due to the inevitable ohmic losses and radiation losses from metal materials and nanoparticles, the figure of merit (FOM) of plasmon sensors is usually very low. The all-medium material combined with bound states in the continuum (BIC) can effectively improve the FOM of sensors. In this article, a single-layer tri-prism metasurface sensor based on quasi-BIC (Q-BIC) was first proposed and simulated. Some meaningful results were obtained by adjusting and optimizing the structural parameters. By reducing duty ratio (DR) and dielectric column height, the sensor was sensitive to refractive index (RI) changes and had an excellent spectral performance. The results showed the sensitivity and full width at half maximum (FWHM) of the single-layer tri-prism sensors were 680.7 nm/RIU and
$5\times 10^{-{2}}$
nm, respectively, and the FOM could reach
$1.36\times 10^{{4}}$
RIU−1. On this basis, we proposed a novel double-layer metasurface sensor model to improve the sensing performance and simulate it further. Because of the complete symmetry of the two-layer metasurfaces, the sensor had perfect phase matching condition. Under the same conditions, double-layer sensor had higher sensitivity and narrower FWHM in Q-BIC. The results showed the
${S}$
and FWHM of the sensor were 715.6 nm/RIU and
$7\times 10^{-{5}}$
nm, respectively, and the FOM could reach
$10^{{7}}$
RIU−1, detection limit (DL) could reach
$4.89\times 10^{-{9}}$
RIU. FOM had been improved by three orders of magnitude than the single-layer sensor. It provided ideas for sensors with ultralow DLs in the future.