Fig. 1. Schematic diagram of a metasurface capable of generating a longitudinally varying vector vortex beam and parameters of the unit structure. (a) Schematic diagram of the metasurface capable of generating a longitudinally varying vector vortex beam, (b) schematic diagram of the unit structure of the metasurface, (c) positional relationship between the unit structure after rotation and before rotation, (d) transmittance and phase parameters of the selected unit at 0.1 THz, (e) relationship between the phase and rotation angle of the selected unit at LCP incidence, (f) relationship between the phase and rotation angle of the selected unit at RCP incidence.

Fig. 2. Superposition of metasurface phase arrangements. (a) Metasurface phase distribution of vortex beams generated by LCP incidence with topological charges varying from +3 to −3. (b) Metasurface phase distribution of vortex beams generated by RCP incidence with topological charges varying from +4 to −4.

Fig. 3. Characterization of the metasurface that produces a longitudinally varying vortex beam. (a) Intensity distribution of the transmitted cross-polarized field under LCP incidence with propagation distance, (b) phase distribution of the transmitted cross-polarized field under LCP incidence with propagation distance, (c) intensity distribution of the transmitted cross-polarized field under RCP incidence with propagation distance, (d) phase distribution of the transmitted cross-polarized field under RCP incidence with propagation distance, (e) mode purity with propagation distance under LCP incidence (left), and mode purity with propagation distance under RCP incidence (right).

Fig. 4. Characterization of the metasurface evolving from azimuthal to radial polarization. (a) Intensity distribution in the transmitted x-polarized direction under y-linearly polarized light incidence, (b) intensity distribution in the transmitted y-polarized direction under y-linearly polarized light incidence, and (c) polarization state distribution in the xoy plane at different distances.

Fig. 5. Characterization of the metasurfaces capable of generating longitudinally varying vector vortex beams: from azimuthal to second-order radial polarization and from radial to second-order radial polarization. y-linearly polarized light incidence, evolution from azimuthal to second-order radial polarization: (a) intensity distribution in the direction of transmitted x-polarization, (b) intensity distribution in the direction of transmitted y-polarization. y-linearly polarized light incidence, evolution from radial to second-order radial polarization evolution: (c) intensity distribution in the transmitted x-polarization direction, (d) intensity distribution in the transmitted y-polarization direction. (e) Demonstration of polarization states changing from azimuthal polarization to second-order radial polarization, (f) demonstration of polarization states changing from radial polarization to second-order radial polarization.

Fig. 6. Experimental test light path diagram and surface topography characterization of Device I and Device II. (a) Experimental test optical path diagram, (a₁) metasurface array cell distribution, (a₂) refractive index and extinction coefficient of 3D printed alumina. (b₁) 3D surface topography image of laser scanning measurement Device I; (b₂) its top view, with the red arrow showing the direction of the measured surface height, and the inset showing the schematic diagram of the measured height of the metasurface structure, and (b₃) specific height measurement result of the metasurface structure of Device I. (c₁) 3D surface topography image of laser scanning measurement Device II; (c₂) its top view, with the red arrow showing the direction of the measured surface height, and the inset showing the schematic diagram of the measured height of the metasurface structure, and (c₃) specific height measurement result of the metasurface structure of Device II.

Fig. 7. Plots of the results of the experimental tests. (a) From left to right, the initial spot intensity distribution, the intensity and polarization state distribution at the near-focus of Device I, the intensity and polarization state distribution at the far-focus of Device I, the intensity and polarization state distribution at the near-focus of Device II, and the intensity and polarization state distribution at the far-focus of Device II, respectively. (b) Intensity distribution in the transmitted x-polarized direction passing through Device I under the incidence of y-linearly polarized light, (c) intensity distribution in the transmitted y-polarized direction. (d) Intensity distribution in the transmitted x-polarized direction passing through Device II under y-linearly polarized light incidence, (e) intensity distribution in the transmitted y-polarized direction.

Fig. 8. Calculation of Stokes parameters for Device I at 22 mm, from left to right: horizontal linearly polarized light intensity, vertical linearly polarized light intensity, +45° linearly polarized light intensity, right-handed circularly polarized light intensity, and vortex light with polarization direction.

Fig. 9. Calculation of the focusing efficiency position for the metasurface, at 22 mm: (a) the simulated total intensity distribution (left) and experimental total intensity distribution (right) for Device I; (b) the simulated total intensity distribution (left) and experimental total intensity distribution (right) for Device II.

Fig. 10. Quantitative comparison of intensity distribution (the left figure is the simulation result, and the right figure is the experimental result; the white dashed line indicates that the intensity curve is at this cross-section). (a) Quantitative comparison of the intensity distribution of Device I in the x-polarization direction at 22 mm; (b) quantitative comparison of the intensity distribution of Device II in the y-polarization direction at 42 mm.