Fig. 1. The schematic of THz beam focusing and focal shift effect

Fig. 2. Theoretical calculations of focal shift effect （a）intensity distributions along the optical axis at different Fresnel numbers，（b）relative focal shift and peak intensity with Fresnel number

Fig. 3. Focal shift analysis of the THz WARL with a 20 mm aperture and focal length （a）to （c）FEA simulations at 0.1，0.3，and 0.5 THz，respectively，（d）intensity distributions along the optical axis，（e）relative focal shift and peak intensity with working frequency and Fresnel number

Fig. 4. FEA simulation results obtained using different lens apertures，focal lengths，and working frequencies

Fig. 5. FEA simulations at 0.1 THz of the THz WARL with a 20 mm aperture and focal length，（a）without and （b）with focal shift compensation

Fig. 6. Focal shift （Δf/f）in relation to lens parameters （a2/f）and working frequency，the area corresponding to the situation where commercial THz WARLs could be used is marked with a light-yellow background