Fig. 1. Longitudinally encoding and decoding principles. (a) According to the ASCII hexadecimal encode, at the transmitter side, Bob encodes the message “N”, “P”, “U” into two beams, which orderly present distinct superposition states of OAM modes with azimuthal orders of 4 and 14, 5 and 0, 5 and 5 at different propagation distances. At the receiver side, Alice decodes the message by measuring these beams orders at z₁, z₂, and z₃ planes. (b) Normalized intensity distributions of superposition states of OAM modes with different azimuthal orders and the corresponding ASCII hexadecimal code elements

Fig. 2. Tetratomic macro-pixel polycrystalline silicon metasurface and the experimental setup. (a) The upper image shows meta-atom of poly-Si nanopillar deposited on SiO₂ substrate with geometric parameters of P₀ (period), H₀ (height), L₀ (length), and W₀ (width), and lower image is the top view of the tetratomic macro-pixel structure, where θ_A and θ_B are the rotation angles of the rectangular nanopillars with respect to the reference coordinate system; (b) transmitted phases of 17 selected geometries; (c) SEM images of metasurface and its local area, and a tetratomic macro-pixel is exhibited in the dashed box; (d) experimental setup schematic (the metasurface is placed on a linear displacement stage that moves along the z-direction)

Fig. 3. Simulated and measured intensity distributions and azimuthal orders (top left) of longitudinally modulated beam array encoded with message “Northwestern Polytechnical University” in the planes of z₁=0.1 mm, z₂=0.5 mm, and z₃=0.9 mm, where the symbols m and n denote row and column numbers of the beam array, respectively. (a) Simulated results; (b) measured results