Fig. 1. Schematic of a transmissive metasurface designed to generate vortex beams along the light propagation direction. The metasurface employs a minimal-size array to produce vortex pixel points for the target image and incorporates weight-based calculations to achieve multilayer encryption of characters.

Fig. 2. Schematic diagram of the unit cell structure of the metasurface: (a) dimensional parameters for each metallic and dielectric layer; (b) simulated transmission coefficient of the unit cell; (c) simulated transmission phase of the unit cell.

Fig. 3. Results of near-field numerical simulations of metasurfaces with various topological charges. Panels (a) and (b) display the simulated near-field intensity and phase distributions for vortex beams with a topological charge of l=1, obtained using RCP illumination at 11.7 GHz; panel (c) shows the corresponding simulated purity. With only the topological charge varied, panels (d) and (e) present the simulated near-field intensity and phase distributions for l=2, with panel (f) showing the simulated purity. Similarly, panels (g) and (h) illustrate the near-field intensity and phase distributions for l=3, and panel (i) displays the corresponding simulated purity.

Fig. 4. Phase distribution of vortex beams corresponding to expanded topological charge states. (a)–(i) represent the cases for topological charge values ranging from 1 to 9, respectively.

Fig. 5. Near-field results for metasurfaces of different array sizes. Panel (a) shows a 20×20 metasurface, and panels (b) and (c) display the simulated near-field amplitude and phase distributions under RCP illumination. For a reduced array size of 4×4, panel (d) shows the configuration, and panels (e) and (f) present the corresponding simulated near-field amplitude and phase distributions. For a further reduced array size of 2×2, panel (g) shows the configuration, and panels (h) and (i) display the simulated near-field amplitude and phase distributions.

Fig. 6. (a) Three types of letter arrays; (b) amplitude distribution of letter-shaped metasurface arrays; (c) phase distribution of letter-shaped metasurface arrays.

Fig. 7. (a) Fabricated metasurface board and experimental setup for measuring the near-field results of the designed metasurfaces; (b) simulated amplitude and phase distributions at 10.67 GHz; (c) experimentally measured amplitude and phase distributions at 10.67 GHz.

Fig. 8. Schematic illustration of the encryption principle: (a) optical propagation and wavefront modulation; (b) encryption principle and exemplary demonstration; (c) decryption process.

Fig. 9. Letter mapping rules and example demonstration after varying the weight matrix.