Dynamic BIC in Customizable Hydrogel Metagratings for Advanced Information Security

Abstract Optical cryptography leveraging responsive photonics harbors immense potential within encryption markets. Nonetheless, the current reliance on single inherent information encryption/decryption methods hinders advancements in achieving high‐precision and multistage data security, particularly for military‐grade encryption. Here, a water‐triggered encryption and multistage decryption approach in the visible regime is proposed, exploiting the precise dynamic regulation of quasi‐bound states in the continuum (q‐BIC) in hydrogel metagratings. By exploring the thermal interactions between a scanning probe and hydrogels, a direct and one‐step grayscale patterning of hydrogels has been realized, allowing the flexible design of delicate 1D and 1.5D metagratings. Utilizing a water‐triggered mechanism, the sensitive q‐BIC resonance can be switched among ON, OFF, and particularly intermediate states depending on humidity levels, thus facilitating optical encryption and multistage decryption through the dynamic properties of hydrogels. Finally, a Morse code is designed and achieved by arranging the hydrogel metagratings, which can be concealed under water immersion. During the decryption process (from high humidity to low humidity), the active shifting of q‐BIC resonance camouflaged information and enhance data security. The water‐triggered strategy, along with the hydrogel direct thermal patterning, offers a promising platform for multistage optical encryption, security labels, dynamic displays, and anti‐counterfeiting measures.