Fig. 1. Schematic diagram of the metasurface for beam shaping with dual-channel polarization conversion. (a) Taking the orthogonal linearly polarized terahertz waves as an example, a 45° polarized (a-polarized) or right circularly polarized (RCP) beam can be obtained when the x- or y-polarized wave is incident, respectively. (b) The arrangement of two types of meta-atoms in the metasurface. The eigen-polarizations for the first and second types of meta-atoms are x (y) polarization and ±45°(a,b) polarizations, respectively.

Fig. 2. Amplitudes and phases of the transmitted wave corresponding to meta-atoms with selected geometric parameters. (a) Geometric shape of the two kinds of meta-atoms. Amplitude and phase values for the (b) second and (c)–(f) first types of meta-atoms.

Fig. 3. Simulated and experimental results of sample 1 for the generation of focused vortex beam with polarization conversion. (a) SEM images of sample 1. (b) Demonstration of dual-channel polarization conversion on the Poincaré sphere. (c) Electric field intensity of the transmitted wave in the focal plane when the x- or y-polarized beam is incident. (d) Electric field intensity in the longitudinal section (xoz-plane) at different frequencies. (e) The simulated and measured polarization conversion efficiency of the metasurface in the range of 0.7–1.4 THz at the focal plane, in which the designed working frequency is 1.1 THz.

Fig. 4. Simulated and experimental results of sample 2 for the generation of focused vortex beam with polarization conversion. (a) SEM image and optical photo of sample 2. (b) Demonstration of the dual-channel polarization conversion on the Poincaré sphere. (c) Electric field intensity of the transmitted wave when the x- or y-polarized beam is incident. (d), (e) Electric field intensity in the longitudinal section (xoz-plane) for different polarization components, and values of S3 component of the Stokes parameters.

Fig. 5. General discussions on the available polarization states. (a), (b) The possible polarization realization of the transmitted wave when the linearly polarized wave is incident. (c) Transmitted electric field intensities under circularly polarized wave illumination.

Fig. 6. Transmission amplitudes and phases of the meta-atoms with different geometric sizes.

Fig. 7. Near-field phase distributions of the two samples.

Fig. 8. Phase distributions in the focal plane of the generated vortex beam from sample 1.

Fig. 9. THz imaging system based on two-dimensional electro-optical sampling.