Polarization structures generated through metalenses with vectorial foci for high-security optical encryption

Optical encryption technologies show significant potential applications in information security due to their advantages of parallel processing, large capacity, and low power consumption. Polarization, as an important degree of freedom of light, has attracted extensive research interest in optical encryption through polarization manipulation and multiplexing. However, current polarization control methods based on pixelated or interleaved metasurfaces still face significant challenges, including fabrication complexity and inevitable crosstalk caused by coupling between the neighboring structures, which limits the number of achievable multiplexing channels. A novel encryption method featuring longitudinal variability and cascaded polarization structures realized by metalenses with vectorial foci is proposed in this work. The intensity distributions on different observation planes are simulated using the Fresnel–Kirchhoff diffraction integral. Based on the geometric phase principle, the designed metalens consisting of TiO<sub>2</sub> nanopillars with identical dimensions but spatially variant orientation angles, can generate multiple vectorial foci in different observation planes, reconstructing cascaded polarization structures. Here, any two cascaded polarization structures are encoded with mutually orthogonal polarization rotation angles. As the polarization direction of incident linearly polarized light changes, the polarization distribution encoded on the polarization structures can be dynamically modulated, consequently enabling ten-channel information encryption through polarization-dependent intensity redistribution. The encrypted information can only be decoded using the correct keys (incident wavelength, incident polarization state, output light polarization state, and observation position). This method integrates polarization rotation, polarization structure design, and longitudinal/cascaded control, significantly enhancing information capacity and security. It offers promising applications across various fields, such as optical display, encryption, and anti-counterfeiting.