Fig. 1. Principle of the THz wave generated by two-color femtosecond laser filament formation

Fig. 2. Working principle of the dual-wavelength wave plate. (a) Directions of the optical axis of the fundamental frequency light, frequency doubled light and dual-wavelength wave plate; (b) time delays of the four laser pulses obtained by the fundamental frequency light and frequency doubled light

Fig. 3. Ionization rates under three different conditions

Fig. 4. Curves of the free electron density with time in the three conditions

Fig. 5. Spectra in three conditions. (a) Ionizing radiation spectrum; (b) enlarged view of the THz band

Fig. 6. Time domain diagrams of THz without and with pulse separation

Fig. 7. Pump pulse separation with a pulse width of 50 fs

Fig. 8. Spectra under different conditions. (a) Ionizing radiation spectrum calculated by the dual-wavelength wave plate; (b) enlarged view of the THz band

Fig. 9. THz time domain spectra of dual-wavelength wave plate with different thicknesses

Fig. 10. Change of the THz intensity in different situations