Terahertz Metasurfaces for Thermally Controlled Optical Encryption

Terahertz (THz) metasurfaces have emerged as powerful tools to modulate the wavefronts of THz radiation fully. Smart designs and fabrication are essential for enhancing the flexibility and encryption security of THz metasurfaces. In addition to digital coding metasurfaces and microelectromechanical systems, one method to realize dynamic THz metasurfaces is to utilize an active material. In this paper, a dynamic THz metasurface, which is combined with the phase-change material VO₂ and can be thermally controlled to achieve optical encryption, is proposed. Based on the electromagnetically induced transparency effect and by arranging the antennas in advance according to a specific hologram, a secret image can be encoded into the metasurface. At room temperature, the transmitted light field is an irregular light spot with no useful information. If the temperature increases above the phase-change temperature, the encrypted hologram can be reconstructed. Moreover, owing to the distinct characteristics of VO₂, the phase-change temperature required during decryption is not very high, and the entire process is reversible. It is expected that, in combination with updated processing technology, such metasurfaces can be practically applied to the next generation of optical encryption or optical anticounterfeiting in the future.