Dynamic and polarization-dependent bound states in the continuum for optical encryption in hydrogel metagratings

Bound states in the continuum (BIC) offer unparalleled control over light-matter interactions. However, dynamic modulation of BIC and quasi-BIC resonances in the visible spectrum remains challenging, particularly in achieving rapid, reversible, large-area, and precise tuning. Here, we present a moisture-driven tuning strategy that employs hydrogel-based (polyvinyl alcohol, PVA) meta-gratings for dynamic modulation of quasi-BIC in visible light and optical encryption. By exploiting the humidity-responsive swelling behavior of hydrogels, we induce reversible out-of-plane symmetry breaking, enabling active control of the BIC-to-quasi-BIC transition via precise height modulation. Moreover, the quasi-BIC resonance can be selectively activated by the polarization of incident light, allowing dynamic modulation of encrypted printed images with high concealment efficiency. Our strategy provides a practical and versatile platform for dynamic, switchable optical functionalities in active quasi-BIC metasurfaces devices.