Fig. 1. Experimental setup for the THz generation and detection by one-color femtosecond laser pulse with the gas target^[29]

Fig. 2. Polarization diagram of THz emission^[31]

Fig. 3. Experimental schematic for the two-color scheme of THz generation and THz amplitude versus BBO-to-focus distance^[34]

Fig. 4. Schematics of the laser field shape, electron trajectory and electron drift velocity for the two-color laser pulse at different phases^[35].(a) Laser field shape for the two-color laser pulses with a relative phase 0 and π/2; (b) electron trajectory with a relative phase 0 and π/2; (c) electron drift velocity with a relative phase 0 and π/2

Fig. 5. Mechanism and spectral properties of terahertz generation by ionization two-color pulses^[41]. (a) Schematic diagram; (b) electrical field E(t) and current J(t) of two-color filed; (c) step-wise modulation of free electron density and ionization rate W(t); (d) electromagnetic radiation spectrum on a large frequency scale; (e) the contributions from the nth ionization event for -16<n<16 and corresponding THz spectra; (f) THz spectrum versus input pulse duration

Fig. 6. Relation curve of THz energy with fundamental laser pulse wavelength^[77]. (a) Radiated THz energy dependence on the pump laser wavelength obtained by numerical integration of the transverse photocurrent model and experimental data; (b) recorded THz energy for 12 different pump wavelengths when the laser pump wavelength varies from 0.8 to 2.02 μm

Fig. 7. Experimental setup of THz generation by the femtosecond 800 nm fundamental- and 1600 nm half-harmonic pulses^[78]

Fig. 8. Dependence of the zero-frequency (residual) current density (RCD, determining THz radiation intensity) on the frequency ratio of the two-color laser pulse^[79]

Fig. 9. Schematic of physical mechanism for the sawtooth THz radiation^[86]. (a) Stepwise increase of the electron density and current density for a one-color, two-color, and a sawtooth pulse; (b) sawtooth waveform having the maximal drift velocity at the extrema of the light field; (c) spectrum of the sawtooth waveform containing all harmonics with intensities decreasing ( the inset shows the trajectories of free electrons in phase space for an increasing numbers of colors)

Fig. 10. THz radiation spectrum for the harmonic and nonharmonic three-color laser pulse with detuned third harmonic frequency^[87]

Fig. 11. Experimental setup for the incommensurate three-color laser field based on OPA^[89]

Fig. 12. THz peak to peak electric field strength as a function of signal wavelength and idler wavelength^[89]

Fig. 13. Experimental setup for three-color laser excitation without phase control^[90]

Fig. 14. THz energy of three-color laser pulses without phase control^[90]. (a) Dependence of THz energy of three-color laser pulses on the wavelengths of the signal and idler pulses; (b) dependence of the THz pulse energy of the two-color driver pulses (laser and signal pulses) on wavelength

Fig. 15. Experimental setup for three-color laser field with phase control and time-domain waveform of the three-color laser field^[91]

Fig. 16. Effects of relative phase of three-color laser field on THz radiation^[91]. (a) Energy of THz pulse as a function of the relative phase of 800 and 400 nm laser fields; (b) amplitude of THz pulse as a function of the relative phase of 266 nm laser field

Fig. 17. Schematic experimental setup for the ionization control of three-color laser pulse^[92]

Fig. 18. [in Chinese]