Fig. 1. Simulation of electric field, the vector of electric field and phase distribution of terahertz vortex beam.

Fig. 2. Schematic diagram of simulation structure: (a), (b) Metal disk with subwavelength periodic grooves; (c) the vertical incidence terahertz vortex beam to the metal disc.

Fig. 3. Intensity spectra at the monitoring point.

Fig. 4. Simulation of electric field after terahertz vortex beam penetrates the structure.

Fig. 5. Transmissive stepped SPP: (a) Schematic of the structure, where the wavelength 500 μm, the total number of steps , the total thickness , the base thickness 961.54 μm, the rotation azimuth ; (b) the physical map of SPP.

Fig. 6. Experimental measurement device diagram and the physical map of sample (inserted figure).

Fig. 7. Experimental measurement device diagram.

Fig. 8. Experimental diagram of electric field and phase after terahertz vortex beam penetrates the structure: (a) Combination of ; (b) combination of .