Fig. 1. Electron micrographs of the sample and experimental system. (a) Top view of metal surface; (b) metal cross section; (c) top view of AAO substrate surface; (d) diagram of Fourier transform Michelson device

Fig. 2. Terahertz spectra radiated by metal film^[40]. (a) Experimental results; (b) theoretical simulation; (c) temperature of sample surface, below which is the temperature distribution along the diameter section

Fig. 3. Terahertz spectra under different pump power^[43]. (a) Experimental results; (b) theoretical simulation; (c) temperature of sample surface, and the right shows the temperature distribution along the diameter section

Fig. 4. Intensity distribution of terahertz wave^[43]. (a) 2D scan diagram; (b) 3D diagram

Fig. 5. Optical characteristics of metal films with different thicknesses^[44]. (a) Scanning electron microscope (SEM) images of porous metal films with different thicknesses; (b) optical absorptivity as a function of the thickness of the porous metal films

Fig. 6. Terahertz waves radiated by metal films with different thicknesses^[44]. (a) Measured THz-to-IR thermal radiation intensity as a function of the incidence angle emitted from nanostructured metal films with different thicknesses; (b) THz-to-IR emission intensity and resonant angle as a function of metal film thickness

Fig. 7. Terahertz waves radiated by different metals^[45]. (a)-(c) Terahertz intensity radiated by Ru, Pt, and Au in different gases varing with atmospheric pressure; (d) terahertz intensity of three metals varing with number of gas atoms at pressure of 105 Pa

Fig. 8. Terahertz wave radiated by gas plasma excited by long-wavelength laser^[68]. (a) Experimental device; (b)(c) simulated and measured terahertz energy varing with incident pulse power

Fig. 9. Power dependence of terahertz wave radiated by plasma excited by long-wavelength laser^[68]. (a) Ionization rate of different wavelength laser varing with the pump power; (b) terahertz energy varing with pump power

Fig. 10. Polarization characteristics of terahertz wave radiation^[68]. (a) Terahertz transmittance varing with the angle of the grid polarizer; (b) terahertz transmittance varing with polarization angle at different wavelengths

Fig. 11. Changes of normalized free electron density (above) and electron current (below) with time excited by 800, 1200, 1600 nm laser

Fig. 12. Terahertz spectra. (a) Normalization of terahertz spectra with different laser wavelengths; (b) normalized simulation of terahertz spectra with different laser wavelengths

Fig. 13. Femtosecond laser modulated plasma^[73]. (a)-(c) Photos of 1200, 1300, 1500 nm laser filament intersection; (d) experimental device

Fig. 14. Terahertz energy varing with time of pre-ionized plasma modulation pulse^[73]

Fig. 15. Wavelength and power dependence of THz modulation depth^[73]. (a) Variation of THz modulation depth with 800 nm pulse power at different pump wavelengths; (b) modulation depth of terahertz wave varing with the pump laser wavelength; (c) relationship between simulated THz modulation depth and 800 nm modulation power; (d) comparison between simulation results and experimental results of terahertz modulation depth changing with pump laser wavelength

Fig. 16. Polarization characteristics of terahertz transmittance^[73]. (a)-(c) With/without pre-ionized plasma, the terahertz transmittance at excitation wavelengths of 1200, 1300, 1500 nm varing with the polarization angle of the linear grid polarizer; (d) relationship between the polarization angle of terahertz wave and the pre-pulse power under 1200, 1300, 1500 nm wavelength excitation

Fig. 17. Terahertz wave generated by two-color field with unusual frequency ratio^[76]. (a) Schematic of experimental device; (b) terahertz autocorrelation signal when the frequency ratio of two-color field is 1∶4 and 2∶3; (c)(d) when one pulse is fixed at the wavelength of λ1=800 nm and λ1=400 nm, the change of terahertz intensity with the other pulse wavelength of λ2

Fig. 18. Polarization characteristics of terahertz wave^[76]. (a)-(d) Horizontal and vertical components of terahertz energy changing with the polarization angle of 1600, 400, 1200, 800 nm pulse respectively

Fig. 19. Power dependence of terahertz wave^[76]. (a)-(d) Terahertz energy changing with the power of 1600, 400, 1200, 800 nm pulse respectively

Fig. 20. Terahertz wave generated by liquid plasma^［84］. (a) Diagram of experimental system; (b) photos of laser focusing on water film; (c)(d) terahertz time domain waveforms from water and air plasma and the corresponding spectra

Fig. 21. Terahertz wave radiated by water film at different positions^［84］

Fig. 22. Changes of terahertz wave energy in horizontal and vertical directions with polarization angle of pump pulse^［84］

Fig. 23. Schematic of experimental device for generating terahertz wave by water line^[85]

Fig. 24. Terahertz wave generated by various media^[85]. (a)-(c) Terahertz pulse generated by water column, water film, air; (d) corresponding spectra

Fig. 25. Theoretical simulation of terahertz wave radiation^[85]. (a) Terahertz pulse at x_L=±60 µm; (b) terahertz intensity is a function of x_L; (c) PIC results of quasi-static current

Fig. 26. THz wave generation enhanced by a preformed plasma^[87]. (a) THz signals individually generated by the P-polarized pre-pump and main-pump, and the THz signal generated by two beams with a certain time delay; (b) similar results are plotted when the pre-pump is S-polarized

Fig. 27. Terahertz generation from liquid nitrogen (LN₂)^[88]. (a) Diagram of the apparatus for guiding a liquid nitrogen line; (b) photo of a flowing liquid nitrogen line; (c) detected THz waveforms from a water line (210 μm) and a liquid nitrogen line (400 μm)

Fig. 28. THz waves generated from air and a 210 μm-diameter line of water and gallium with a single-color excitation^[89]. (a) Comparison of THz field strengths in air plasma, water, and liquid gallium; (b) corresponding comparison in spectra