Enhanced optical encryption via polarization-dependent multi-channel metasurfaces

Abstract

Optical encryption offers a powerful platform for secure information transfer, combining low power consumption, high-speed transmission, and intuitive visualization. Metasurfaces, with their unprecedented ability to manipulate light across multiple degrees of freedom within quasi-two-dimensional nanostructures, are emerging as promising devices for advanced encryption. However, encryption capacity remains constrained by limited information channels. Here, we present a visual secret sharing (VSS) scheme utilizing metasurfaces with multiple polarization-dependent channels and minimized crosstalk. Using a global optimization strategy for nanostructure geometries across the entire metasurface, we successfully realize eight independent polarization channels with negligible crosstalk. By encoding both the key and information into these channels with a modified VSS scheme, we demonstrate the complete recovery of seven plaintexts. This strategy supports scalable, high-capacity encryption, and can incorporate additional optical degrees of freedom, offering advanced solutions for advanced secure communication, information storage, and anti-counterfeiting.

Keywords: VSS encryption; metasurface; optical encryption; polarization-dependent multi-channel.

Figure 1:

The polarization-dependent multi-channel encryption and decryption process with metasurface.

Figure 2:

Enhanced VSS encryption method. (a) Encryption framework of the enhanced VSS encoding. Encoding of corresponding pixels in secret message in both the traditional and the enhanced (2, 2) VSS encryption methods. (Black pixels represent 0, while white pixels represent 1 and detailed explanations of VSS encryption are provided in the supplement.) (b) The numerical simulation of the encrypted hologram images and the corresponding decryption results.

Figure 3:

Design of polarization-dependent multi-channel metasurfaces. (a) The workflow of the optimization methodology. (b) Schematic of the unit cell structure. The period of the unit cell is P = 500 nm and the height is H = 500 nm. (c) SEM image of the metasurface. (d) and (e) Simulated transmission and phase delay for different nanorod dimensions.

Figure 4:

Experimental results for 4-channel encryption. (a) Experimental setup for the optical characterization. (b) and (c) The recorded holograms images of the polarization channels and the binarized images. The solid and dashed black arrows along with the images indicate the polarization direction of the incident light and the analyzing state setting of the output light. The polarization states have an equal interval is 45°. (d) The encrypted recovery results for the 4-channel encrypted images.

Figure 5:

The experiment demonstration of VSS encryption based on 8 polarization-dependent channels. (a) The holograms of the 8 polarization channels. The polarization interval is 22.5°. (b) The encrypted recovery results. (c) The correlation coefficient matrix for the simulated and experimental results for the 8-channel encrypted images.