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.