Chiral metasurface refractive index sensor with a large figure of merit

Metasurface refractive index (n) sensors aim to detect the ambient n by monitoring their spectral shifts, which have found many applications in biosensing, environmental monitoring, and so on. Traditionally, a resonance peak/valley is used as a sensing signal to track the spectral shifts. To track the spectra with higher accuracy, a larger figure of merit (FoM) is desirable; however, whose values are essentially limited by the finite resonance bandwidth. Here, we demonstrate an approach to radically improve the FoM by exploiting the 90°-crossing point of the optical rotation spectral curve as the sensing signal. Benefitting from the infinitesimal linewidth of spectral curves, we deliver a theoretically unlimited value to the FoM. Our method enables tracking the spectral shifts with explicit convenience and high precision from the raw data without any fitting procedures. The n difference on the level of 10−3 RIU was steadily distinguished in experiments. Our study provides a way for constructing the metasurface refractive index sensors.