Fig. 1. Cherenkov radiation THz model[33]. (a) Experimental setup of femtosecond laser filament radiated terahertz wave; (b) polarization of radiated terahertz wave

Fig. 2. Relationship between terahertz wave intensity and distance from BBO crystal to lens[19]

Fig. 3. Terahertz electric field intensity versus fundamental light intensity and frequency-doubled light intensity (the solid circle shows the experimental data, and the solid line is the fitting curve)[34]. (a) Relationship between terahertz amplitude and fundamental light intensity; (b) relationship between terahertz amplitude and frequency-doubled light intensity

Fig. 4. Projection of the instantaneous polarization state of the terahertz electric field vector on the X-Y cross section[35]

Fig. 5. Photocurrent model explains terahertz wave by two-color field radiation[36]. (a) Laser electric field distribution with relative phase of θ=0 and θ=π/2; (b) electronic motion trajectories at different phases; (c) electron drift velocity (solid line) and electric field (dashed line) corresponding to different phases

Fig. 6. THz time domain waveforms detected by the THz-TDS system at different transmission distances z (different tones represent different polarities, and the degree of saturation shows the intensity)[46]

Fig. 7. THz beam diameter distribution along z-direction in the plasma filament and forward region (upper left: lateral plasma filament image captured by a digital camera; lower right: partial enlargement of the THz beam diameter in the region from z=0 mm to z=5 mm[47]

Fig. 8. Experimental setup[48]

Fig. 9. Terahertz wave intensity versus (a) external electric field and (b) pumping energy[48]

Fig. 10. Schematics of terahertz generation and detection[49]. (a) Terahertz generation; (b) terahertz detection

Fig. 11. Experimental setup (QWP, quarter wave plate; WP, Wollaston prism; BD, balanced detector; LIA, lock-in amplifier)[54]

Fig. 12. Terahertz wave form from the plasma generated by the signal pulse and enhanced by the control pulse[54]. (a) The control pulse is blocked with the signal pulse energy of 160 J; (b) the control pulse is 11 ps after the signal pulse with both pulse energies of 160 J; (c), (d) the control pulse is 22 ps and 175 ps, respectively, before the signal pulse with both pulse energies of 160 J

Fig. 13. Effect of phase plate on radiated terahertz wave (SCPP, semi-circular phase plate; HC, half-covered) [58]. (a) Time domain waveform; (b) spectrogram

Fig. 14. Influence of different pumping wavelengths on the intensity of terahertz electric field[59]

Fig. 15. Relationship between radiated terahertz energy and pumping wavelength[60]. (a) Radiated THz energy dependence on the pump laser wavelength obtained by numerical integral of the transverse photocurrent model for different pump wavelengths (solid curve), the dashed curve shows the calculated plasma density (right axis), the energy scale is normalized, and the experimental data are overlapped for clarity (solid circles); (b) recorded THz energy for 12 different pump wavelengths between 0.8 μm and 2

Fig. 16. Terahertz energy from two-color filamentation in air (square) and single-color filamentation in various liquids [ethanol (up-triangle), methanol (down-triangle), acetone (circle), dichloroethane (diamond), deionized water (right-triangle), and carbon disulfide (left-triangle)], as a function of laser energy[63]

Fig. 17. Relationship between terahertz intensity and pulse width. (a) Relationship between terahertz signal and pulse width under constant power[62]; (b) relationship between terahertz energy and pulse width for different filters in argon with a fixed pump energy of 23.5 mJ[64]

Fig. 18. Effect of laser chirp on the intensity of terahertz radiation[65]. (a) THz pulse amplitude as a function of the chirped laser amplitude for two different chirping parameters, where the electron density is 2.5×10-5nc(nc is the critical density); (b) spatial distribution of THz pulses simulated by 2D PIC, where the electron density is 4×10-4nc and a chirped laser pulse with a0=0.1, transverse radius of 8λ0, and C=-0.024 is used

Fig. 19. Frequency doubling efficiency and terahertz intensity versus crystal rotation angle[19]. (a) Relationship between frequency doubling efficiency and crystal rotation angle; (b) relationship between plasma radiated terahertz intensity and crystal rotation angle; (c) relationship between terahertz intensity from BBO and crystal rotation angle

Fig. 20. Variation in terahertz energy with lens focal length and laser energy[66]

Fig. 21. Relationship among filament length, terahertz amplitude, and polarity[38]. (a) THz time-domain waveforms with the filament lengths of 3 mm (solid line) and 10 mm (dotted line); (b) THz time-domain waveforms corresponding to different filament lengths; (c) THz amplitudes corresponding to different filament lengths (different tones represent different polarities, and the degree of saturation shows the intensity)

Fig. 22. Polarization state of terahertz wave under different electric field intensity[70]. (a) Unbiased; (b)-(d) polarization of terahertz waves obtained at bias voltages of 0.25, 1, 5 kV/cm, respectively; (e)-(g) result by subtracting (a) from (b)-(d), respectively

Fig. 23. Experimental setup and result analysis[72]. (a) Experimental setup; (b), (d) experimental data by varying the phase difference between the fundamental and the frequency doubling light at different filament lengths; (c), (e) corresponding simulation results; (f) terahertz time-domain waveforms and (g) corresponding spectra of 23 mm long filament radiation. EX and EY represent the polarization of X- and Y-component of electric field; a is the angle at which the probe light is incident on Si; DWP,

Fig. 24. Remote terahertz generation and detection device[73]. (a) Schematic of remote THz generation; (b) THz wave and remote fluorescence detection from a two-color femtosecond laser-induced filament in air. KDP, potassium dihydrogen phosphate crystal; W, wedge; M3, lidar mirror; DWP, dual wavelength half-wave plate; PMT, photomultiplier tube; filters consist of an 800 nm high reflectivity mirror and a 337 nm interference filter; PM, parabolic mirror; PD, pyroelectric detector

Fig. 25. Optical path of typical laser filament radiated terahertz time-domain spectroscopy system

Fig. 26. Principle of equivalent time sampling

Fig. 27. Principle schematic of terahertz detection by electro-optic sampling

Fig. 28. Terahertz (a) time domain signal and (b) spectrogram by ZnTe detection[76]

Fig. 29. Experimental setup for detecting terahertz wave by the THz-ABCD method[77]

Fig. 30. Typical experimental results of THz-ABCD detection. (a) Comparison of THz-ABCD detection and traditional ZnTe crystal detection[79]; (b) ultra-broad spectrum measured by THz-ABCD method[80]

Fig. 31. Experimental setup of terahertz detection by plasma fluorescence enhancement (UV, ultraviolet; PMT, photomultiplier tube; DWP, dual-band waveplate; PM1 and PM2, two parabolic mirrors; M1 and M2, two concave mirrors)[24]

Fig. 32. Performance comparison of THz-REEF method and traditional electro-optic crystal detection (EO represents the result by electro-optic detection)[24]