Experimental retrieval of metaholographic images via evanescent wave using a single-layer nanostructure for encryption.

Metasurfaces have demonstrated significant potential in optical encryption and anti-counterfeiting due to their incredible capability of manipulating various light properties. However, previous metasurface-encryption methods did not sufficiently explore the spatial frequency aspect, particularly regarding evanescent waves. Here, we propose an encryption scheme by introducing evanescent waves into the encoding and decoding processes. Different parts of the target image are individually encoded at distinct spatial frequencies within the evanescent-wave region. Only when an evanescent wave with a specific transverse wave vector (TWV) serves as the key can the complete target image be retrieved. Our work exemplifies a thorough exploration of the mathematical framework underlying metasurface-based holography. It enables the feasibility of a single metasurface in concealing and retrieving information in the evanescent-wave region, thus opening up a new, to our knowledge, methodology for high-security optical information encryption.