Fig. 1. Schematic diagram of the experiment. (a) THz is produced by the two-color laser filament. The inset diagram shows the spatiotemporal walk-off compensating by α-BBO and polarization modulation by a double wavelength wave plate (DWP). (b) The Michelson interferometer measures the THz field autocorrelation. Experimental setups of the (c) side fluorescence imaging and (d) time-resolved shadowgraphs. F, L, and C are the filter, wide-angle lens, and CCD, respectively.

Fig. 2. THz radiation modulation by rotating the β-BBO crystal. (a) Schematic diagram of polarization changing by rotating the β-BBO crystal. The red arrow and the blue arrow represent the FW and the SH, respectively. This diagram shows three special situations of (b). (b) The SH intensity, THz radiation, and filament length evolution by rotating the β-BBO crystal. (c) Side fluorescence imaging of the two-color filament at −20° and 0° in (b). (d) THz time domain curves of −20° and 0° points in (c).

Fig. 3. The two-color filament evolution as the focal length changes. The Z-direction is the direction of laser propagation, while the X–Y cross-section is the cross-section of the filament. (a) Side fluorescence imaging of the filament. (b) and (c) show the experimental results of filament diameter and length, respectively.

Fig. 4. Experimental and simulation results of the filament length under focal length modulation.

Fig. 5. Time-resolved shadowgraphs of the f = 300 mm and the corresponding plasma density results. (a) Time-resolved shadowgraphs of femtosecond laser filament when Δt is 0 fs and Δt varies to 6000 fs. (b) Plasma density inside the filament under focal length modulation.

Fig. 6. Experimental and simulation results after precise control of polarization and spatiotemporal walk-off of the two-color laser. (a) THz time domain curves. (b) and (c) THz spectra. (c) is an enlarged image of (b). (d) Comparison of the experimental and simulation results.