Metasurface-empowered optical cryptography

Information security is of great importance in every aspect of life, spawning numerous modern cryptographic strategies to protect data from stealing and deciphering. Among them, optical encryption is an excellent candidate with regard to its unique features such as parallel processing, high speed, low-power consumption and the abundant degrees of freedom (DoFs) of the photon for information encoding and decoding. However, the optical cryptographic systems based on conventional optical components are commonly restricted to exploiting a limited number of photon dimensions. The advent of the metasurface, which is capable of manipulating photons with full DoFs at a subwavelength scale, provides a new generation of versatile platforms for optical encryption techniques. Having almost exhausted the existing DoFs for information multiplexing, considerable attention has been paid to exploring new encryption algorithms and encoding optical information with the time domain. Here, we review recent research efforts devoted to metasurface-empowered photon cryptography in the DoF space, the algorithm space, and the spatiotemporal domain. The design principles, typical encryption strategies and potential applications are introduced in detail. We also discuss the challenges and provide an outlook on this emerging field.