Fig. 1. Phase diagram. (a) Right-handed fourth-order vortex beam; (b) right-handed fourth-order off-axis vortex beam

Fig. 2. Schematic of optical path

Fig. 3. Position of phase singularity in beam and terahertz wave intensity. (a) Terahertz wave intensity generated by different order off-axis vortex beam when phase singularity is located at (0.6, 0); (b) relationship between maximum terahertz wave intensity generated by off-axis vortex beam and topological charge number of vortex beam

Fig. 4. Relationship between plasma volume and terahertz wave intensity. (a) Plasma volume profile of left-handed third-order vortex beam; (b) fluorescence map corresponding to shaded area in Fig. 4(a); (c) terahertz wave intensity of left-handed third-order vortex beam; (d) terahertz wave intensity of right-handed third-order vortex beam

Fig. 5. Intensity of terahertz waves generated by phase singularities at various positions of beam under different wavelengths of left-handed third-order vortex beam. (a) 1550 nm; (b) 1530 nm; (c) 1510 nm; (d) 1490 nm; (e) 1470 nm; (f) 1450 nm

Fig. 6. Intensity of terahertz waves generated by phase singularity at various positions of left-handed third-order vortex beam under different laser pulse energies. (a) 0.02 mJ; (b) 0.018 mJ; (c) 0.016 mJ; (d) 0.014 mJ; (e) 0.012 mJ

Fig. 7. Intensity, spectrum, and polarization characteristics of terahertz wave generated by vortex beam. (a) Energy amplification of terahertz wave generated by Gaussian beam or third-order vortex beam versus laser pulse energy, inset: filaments induced by third-order vortex beams with different laser pulse energies; (b) energy amplification of terahertz wave generated by Gaussian beam or third-order vortex beam versus laser wavelength, inset: filaments induced by third-order vortex beams with differen