Terahertz necklace beams generated from two-color vortex-laser-induced air plasma

A method for generating terahertz (THz) necklace beams from two-color vortex-laser-induced air plasma is proposed and theoretically investigated based on the frequency down-conversion process. The numerical demonstrations based on both the four-wave mixing and the photocurrent schemes confirm that the generated THz pulse is a shear necklace beam without orbital angular momentum (OAM), whose phase is manifested as the stepwise jump versus azimuthal angle. The origin of OAM vanishing is attributed to the fact that the phase difference between two-color field components does not contribute to the phase but only to the amplitude of the THz pulse.

Figure 1

Intensity distributions of (a) LG ω beam, (b) Gaussian 2ω beam, and (c) superimposed field; (d) radial intensity distributions of (a–c). Here the intensities of three beams are scaled by the peak intensity of the LG ω beam just for a demonstration.

Figure 2

Transverse spatial distributions of (a,b) THz electric field and (c,d) time-integrated intensity. THz bright lobes of THz electric field and intensity patterns are positioned at different azimuthal angles for these two models; i.e., one rotates 45° relative to the other. The beads in the necklace beam from the PC model are much thinner than those from the FWM model. (e,f) The THz temporal electric field waveforms for four reprentative points of A–D. The black open circles and blue open squares indicate the two opposite field peaks, respectively. (g,h) THz field phases versus azimuthal angle. Both show stepwise variance, indicating the generated THz necklace beams have no OAM. (Upper) FWM model; (lower) PC model.

Figure 3

WVDs caculated from (a) FWM and (b) PC models; (c) corresponding normalized intensity spectra. The traces of time-dependent instantaneous frequency versus time are indicated with white dashed lines in (a,b). The black dots indicate the instantaneous frequencies for the two opposite field peaks in Figs. 2 and 2.

Figure 4

Intensity pattern rotation under different initial relative phases of (a,b) ${\phi }_0=0$, (c,d) ${\phi }_0=\pi /4$, and (e,f) ${\phi }_0=\pi /2$. (Upper) FWM model; (lower) PC model.