Dynamic Humidity Modulation of Quasi‐Bound States in the Continuum by Symmetry Breaking in Geometry and Permittivity

Hybrid metal‐dielectric metasurfaces, exhibiting bound states in the continuum (BICs), support high‐quality factor resonances and electric field confinement due to their ability to restrain radiation loss. However, the dynamic transformation between quasi‐BICs and BICs in hybrid nanostructures by symmetry breaking in a multiplicity of ways remains a significant challenge. Herein, a novel humidity‐modulated strategy is demonstrated, which regulates the dynamic transition from quasi‐BICs to BICs based on the symmetry‐breaking in geometry and permittivity for the hybrid photonic‐plasmonic asymmetric gratings (HPAG), which consists of two alternating dielectric gratings deposited on an aluminum oxide and gold film. The regulatory mechanism involves the humidity‐sensitive polyvinyl alcohol, which undergoes volume expansion and refractive index reduction as the relative humidity (RH) increases. Its Q‐factor of a quasi‐BIC in HPAG reached up to 873, primarily due to the breaking of out‐of‐plane symmetry. The HPAG can be used for humidity sensing with a sensitivity of up to 2.63/RH. Moreover, the humidity‐modulated strategy is proven to be suitable for similar nanostructures (e.g., cylinder dimers, tetrameric cubes, and loop antennae). The results indicate that the humidity‐modulated strategy is a promising approach for regulating optical characteristics and is applied in micro/nanophotonic devices, such as humidity sensors, and active components.