Printing polarization and phase at the optical diffraction limit: near‐ and far‐field optical encryption

Abstract Securing optical information to avoid counterfeiting and manipulation by unauthorized persons and agencies requires innovation and enhancement of security beyond basic intensity encryption. In this paper, we present a new method for polarization‐dependent optical encryption that relies on extremely high‐resolution near‐field phase encoding at metasurfaces, down to the diffraction limit. Unlike previous intensity or color printing methods, which are detectable by the human eye, analog phase decoding requires specific decryption setup to achieve a higher security level. In this work, quadriwave lateral shearing interferometry is used as a phase decryption method, decrypting binary quick response (QR) phase codes and thus forming phase‐contrast images, with phase values as low as 15°. Combining near‐field phase imaging and far‐field holographic imaging under orthogonal polarization illumination, we enhanced the security level for potential applications in the area of biometric recognition, secure ID cards, secure optical data storage, steganography, and communications.