Orbital angular momentum-based spatial encryption using polarization-sensitive metasurfaces

Secure data transmission requires robust encryption strategies to prevent unauthorized access. Although software-based encryption has been extensively explored, advances in high-speed computing and the development of quantum systems increasingly challenge the security of conventional schemes. Hardware-based encryption offers an alternative approach by exploiting physical properties of light and engineered structures. In this work, we present a metasurface-based hardware encryption system that simultaneously leverages polarization encoding and orbital angular momentum (OAM) wavefront encoding. The system operates across three distinct channels. The first channel, with input and output polarization x, generates OAM superpositions L = 1 at the focal point. The second channel, with input and output polarization y, produces superpositions L = 2. The third channel, corresponding to orthogonal input and output polarizations (y to x or x to y), produces OAM superpositions L = 3. By introducing a slant polarization at a specific angle at the input, tailored weightings of the superposition modes are achieved for each output polarization, enabling precise control over the OAM distribution at the focal point. Finally, the system is integrated with the double random phase encryption (DRPE) method, forming a hybrid optical encryption strategy that combines spatial, polarization, and OAM degrees of freedom. Three communication scenarios for secure image and video transmission via encryption and decryption are investigated. This approach provides enhanced security for optical data transmission, demonstrating the potential of metasurface-based devices for dynamic, high-capacity, and secure hardware encryption applications.