Fig. 1. Two scenarios of methods for THz vortex beam generation (SPPs: spiral phase plates, APEs: achromatic polarization elements, DOEs: diffractive optical elements, THz LC FPG: THz liquid crystal forked polarization grating, CGHs: computer-generated holograms, STM: spatial terahertz modulator, MPs: modulated plasmas, OR: optical rectification, DFG: difference-frequency generation).

Fig. 2. 3D model of an SPP with proper THz materials. Reproduced with permission from Ref. [34].

Fig. 3. Three examples of q-plates. The tangent to the lines shown indicates the local direction of the optical axis. (a) q = 1/2 and α₀ = 0, which can generate helical modes with l = ± 1; (b) q = 1 and α₀ = 0, (c) q = 1 and α₀ = π / 2, which both can generate helical modes with l = ± 2. Reproduced with permission from Ref. [62].

Fig. 4. Schematic of the conversion process by APEs. Reproduced with permission from Ref. [68].

Fig. 5. Phase profiles of two BPSAs. (a) l = ± 1; (b) l = ± 2. Reproduced with permission from Ref. [57].

Fig. 6. Schematic structure of two representative metasurfaces. (a)–(c) V-shaped slit antennas structure; (d)–(e) circular sub-wavelength slits. Reproduced with permission from Refs. [79,83].

Fig. 7. Phase presentation and idea 3D schematic structure of THz LC FPG. (a) A common spiral phase distribution; (b) introduced gradient blazed phase distribution; (c) integrated phase distribution; (d) ideal 3D LC FPG. Reproduced with permission from Refs. [89,90].

Fig. 8. Local grating structures. (a) Amplitude holograms; (b) phase holograms. Reproduced with permission from Ref. [94].

Fig. 9. Prototype of STM. Reproduced with permission from Ref. [97].

Fig. 10. Geometry of 〈110〉 ZnTe crystal. Reproduced with permission from Ref. [103], © The Optical Society.

Fig. 11. Some experimental devices for a pair of 800 nm vortex beams and some experimental results for THz vortex beam generation. (a) Passive and transmissive device of generating collinear vortex pulse pair of 800 nm with conjugated topological charges and orthogonal polarizations. P, polarizer; Q-P, Q-plate; BQWP, broadband quarter-wave plate; WP, wedge-plate pair; (b) experimental spatial amplitude and intensity distributions of the THz vortex pulses at different time delays in Ref. [105]; (c) corresponding phase distributions of the THz vortex pulses at different time delays in Ref. [105]. (d) Experimental far- and near-fields of 4 THz vortex beams with the topological charge of ± 1 in Ref. [10]. Reproduced with permission from Refs. [10,105], © The Optical Society.

Fig. 12. Simulation results of the generation of THz vortex beams via a type of (a) RPs; (b) SVPs; and (c) DVPs. Reproduced with permission from Ref. [107], © The Optical Society.

Fig. 13. Some experimental and simulation results of intensity modulated THz vortex beam generation via two-color or few-cycle vortex pump laser. (a) Experimental fluencies of Gaussian and vortex second harmonics, corresponding simulating and experimentally far-field THz fluencies in two-color vortex pump field case; (b) simulating fluency and electric field of two-color pump and corresponding far-field intensity and phase of two frequency components of THz pulses in two-color vortex pump field case in Ref. [114]; (c) simulating intensity and phase distributions of different frequency components of THz pulses in few-cycle vortex pump field case in Ref. [113]. Reproduced with permission from Refs. [113,114], © The Optical Society.