Security-enhanced optical image authentication based on cascaded geometric-phase metasurfaces.

In this paper, a security-enhanced optical image authentication method is proposed based on cascaded geometric-phase metasurfaces. In the encryption process, an original plaintext image is first encoded into an authentication amplitude by using a sparse constraint encoding algorithm. Subsequently, the Fourier phase-only hologram of the authentication amplitude is calculated by employing an iterative Fourier transform algorithm, and then it is decomposed into a ciphertext phase and a key phase. Finally, the ciphertext phase and the key phase are, respectively, constructed as the ciphertext metasurface and the key metasurface through a geometric-phase metasurface unit structure, thus forming two physically separated metasurfaces. During authentication, the ciphertext metasurface and the key metasurface need to be cascaded to achieve the authentication of user identity, overcoming a common problem that the current metasurface-based authentication methods lack physical security keys. Numerical simulations are performed to demonstrate the proposed method, and the simulation results show that the proposed method exhibits high feasibility and strong security-enhanced effect as well as large key space.