High-sensitivity humidity sensor based on a MIM metasurface with asymmetric gratings: a numerical study of the Fabry–Pérot cavity and bound states in the continuum

In this paper, a high-sensitivity humidity sensor structure based on a metal-insulator-metal (MIM) metasurface is proposed through numerical simulation. The top layer of the structure consists of a periodic array formed by two asymmetric grating units. By combining Fabry–Pérot (F-P) cavity resonance with bound states in the continuum (BIC) modes, and utilizing the swelling characteristics of the intermediate PVA hydrogel layer under different humidity conditions, sensitive humidity monitoring is achieved. Numerical simulation results show that under high relative humidity (>80%), the sensor achieves a peak sensitivity of 12.526 nm/%RH, enabling accurate detection of humidity variations. Furthermore, the distribution of structural colors in the CIE 1931 chromaticity diagram under different humidity conditions covers the full color gamut, indicating that the relative humidity range can be assessed through color comparison. This study provides a feasible theoretical scheme for the design and application of high-sensitivity humidity sensors.