Fig. 1. Schematic diagram of the proposed separated information encryption via a cascaded metasurface. The secret information is divided into two metasurfaces; each one of the two metasurfaces contains the information of a nanoprinting image and part of a holographic image. The individual nanoprinting image of each single metasurface is decoded in the near field, under the action of meta-key 1. The holographic image is decoded in the far field under the action of meta-key 2 when the two metasurfaces are cascaded.

Fig. 2. Working principles of cascaded metasurfaces for separated encryption. (a) Working optical path to encrypt the nanoprinting image; the black and red two-way arrows represent the polarization direction of polarizer and analyzer, respectively, and the incident light propagates along the z axis. (b) Orientation distribution generating the equal output light intensity; (c) curves for the output intensity and the generated geometric phase by metasurfaces with different orientation angles; for a given optical intensity, there are four orientation choices to generate different geometric phases. As an example, the intersections of the red dashed line and the output light intensity curve represent for the same output light intensity 0.5, but different output light phase profiles. The output light phase profiles can be chosen to be π/4, 3π/4, 5π/4, or 7π/4.

Fig. 3. Flow chart of designing cascaded metasurfaces for separated information encryption. Combining the orientation degeneracy and geometric phase, different information can be encrypted into different working channels and different metasurfaces.

Fig. 4. Illustration of a unit-cell of a cascaded metasurface and the numerically simulated optical response. (a) Schematic of a unit-cell structure with a TiO₂ nanobrick and a SiO₂ substrate; (b) simulated phase delays between the long and short axes of the nanobrick; (c), (d) simulated transmittances under the illumination of x- and y-polarized incident light. The optimally selected nanobrick parameters are marked with a blue star.

Fig. 5. The target images and simulation results of separated information encryption. (a)–(c) The target images are set to be 120 pixels × 120 pixels. (d)–(f) Simulated results of nanoprinting images and holographic image.