Fig. 1. Schematic diagram of optical path of biconical axicon

Fig. 2. Intensity distribution of terahertz wave passing through biconical axicon

Fig. 3. Spectrum of terahertz time-domain spectroscopy system

Fig. 4. Refractive index curve of HDPE

Fig. 5. Simulation results of different axicons. (a) Traditional axicon; (b) biconical axicon

Fig. 6. Simulation results of biconical axicons with different γ₁. (a) γ₁=10°; (b) γ₁=15°; (c) γ₁=18°

Fig. 7. Geometric diagram of vertex alignment error. (a) Vertex alignment; (b) vertex unalignment

Fig. 8. Simulation results of different vertex alignment errors. (a)Intensity distribution of Δd=0; (b) intensity distribution of Δd=0.2 mm; (c)intensity distribution of Δd=1.0 mm

Fig. 9. Intensity distribution curves of terahertz Bessel beam at z=40 mm

Fig. 10. Experimental setup. (a) Schematic diagram of optical path of terahertz time-domain spectroscopy system; (b) biconical axicon

Fig. 11. Intensity distribution of generated terahertz Bessel beam in experiment. (a) Intensity distribution on Y-Z plane; (b) cross-sectional strength distribution on X-Y plane at z=40 mm; (c) cross-sectional strength distribution on X-Y plane at z=60 mm; (d) cross-sectional strength distribution on X-Y plane at z=80 mm; (e) cross-sectional strength distribution on X-Y plane at z=100 mm; (f) cross-sectional strength distribution on X-Y plane at z=120 mm

Fig. 12. Terahertz time domain waveforms at different positions after terahertz wave passing through biconical axicon