Fig. 1. Schematic diagram of THz generation from laser-induced plasma in gas, liquid, and solid.

Fig. 2. Schematic diagram of THz generation by the ponderomotive force.J_r, J_z are the radial and axial currents, respectively. λ_p is plasma wavelength. τ_L is laser pulse duration. Figure reproduced with permission from ref.⁴⁸, American Physical Society.

Fig. 3. Schematic of experimental setup for measurement of THz waves from transition-Cherenkov emission. Figure reproduced with permission from ref.⁵⁷, American Physical Society.

Fig. 4. Dependence of THz signal on the phase shift between the FW and SHW. The distance of BBO-to-focus corresponds the variety of the phase shift between the two-color pulses. Figure reproduced with permission from ref.⁶⁰, The Optical Society.

Fig. 5. Dependence of THz field on the electric field strength of (a) FW and (b) SHW. Figure reproduced with permission from ref.⁶¹, American Physical Society.

Fig. 6. (a) Schematic of experimental setup for THz generation based on external electric fields in single color plasma and (b) experimental results. Figure reproduced with permission from: (a) ref.⁷⁰, American Physical Society; (b) ref.⁷¹, The Optical Society.

Fig. 7. THz signals generated from air plasma induced by (a) one-color pulse and (b) two-color pulse. Simulation result is shown by the red line in (a). Figure reproduced with permission from: (a) ref.⁷⁰, American Physical Society; (b) ref.¹¹⁰, under a Creative Commons Attribution licence.

Fig. 8. (a) Schematic of the experimental setup and (b) results of actively modulated THz spectra. Figure reproduced with permission from ref.¹¹⁵, The Optical Society.

Fig. 9. Polarization of THz forward emission measured atθ= 10°. The experimental and simulation THz signals are denoted with black and red dots. Figure reproduced with permission from: ref.⁵⁷, American Physical Society.

Fig. 10. Measured and simulated far-field THz radiation profiles. Experimental results (a–d) were obtained by raster scanning of a pyroelectric detector with a silicon filter. (a) and (b) are from a plasma with the length of 10 mm. (c) and (d) are from a plasma with the length of 40 mm. (a) (c) Teflon and (b) (d) germanium filters are used for low and high frequency transmission. (e) and (f) are simulated THz profiles for a plasma with the length of 40 mm. Figure reproduced with permission from ref.¹³², American Physical Society.

Fig. 11. Intensity image recorded by a THz camera.(a) Unfocused THz intensity profile and (b) focused intensity THz profile. Figure reproduced with permission from ref.¹³⁴, under a Creative Commons Attribution (CC BY) licence.

Fig. 12. Schematic diagram of experimental setup for THz focal-plane imaging. Figure reproduced with permission from ref.¹³⁷, under a Creative Commons Attribution 4.0 International License.

Fig. 13. Intensity distributions of THz waves generated from plasmas with different lengths.THz images in the (a) time domain and (b) frequency domain. Figure reproduced with permission from ref.¹³⁷, under a Creative Commons Attribution 4.0 International License.

Fig. 14. Ponderomotive force–induced dipole model for THz generation from liquid water. (a) Schematic diagram of the diploe model for THz generation from the plasma in the water film. (b) Its simulation result. α, θ_r are the incidence and refractive angle for the laser pulse, β and θ_t are the detective angle and exiting angle for THz pulse. F and B indicate the forward and back propagation THz signals, respectively. Figure reproduced with permission from ref.¹⁴⁴, AIP Publishing.

Fig. 15. Model of ultrashort pulses propagation equation and its simulation results. (a) Visualization of the model description, (b) simulation result of THz signal with different liquid thickness and pump pulse duration, and (c) comparison of experiment results with simulated results abstracted from (b). Figure reproduced with permission from ref.¹⁴⁷, The Optical Society.

Fig. 16. Schematic diagram of the particle dynamics in the unified theoretical model.a_p and ν_d are the acceleration and drift velocity of electrons, respectively. They are related to the laser propagation direction and electron dynamics in the water medium. The electrons are accelerated forward and backward between the ascending and descending edges owing to the ponderomotive force. ν_d is perpendicular to the laser propagation direction. The combined effect of a_p and ν_d results in THz generation in liquid water. Figure reproduced with permission from ref.¹⁵⁰, AIP Publishing.

Fig. 17. THz intensity versus incidence angle α for different detection angle β, (a) β = 0° and (b) β = 55°. The blue dots and black squares denote the data measured by a Golay cell and EO sampling, while the red line denotes the simulation results. F and B represent the forward and back propagation signals. Figure reproduced with permission from ref.¹⁴⁴, AIP Publishing.

Fig. 18. Spectra of THz waves generated from a 120 μm thick water film with one-color (black line) and two-color (red line) excitation schemes in the case of the pulse duration of (a) 50 fs and (b) 300 fs. Figure reproduced with permission from ref.^{155, 156}, AIP Publishing.

Fig. 19. Spectra of THz waves generated from α-pinene and water. Figure reproduced with permission from ref.¹⁵³, under a Creative Commons Attribution 4.0 Unported License.

Fig. 20. (a) Dispersion curves of plasma waves and electromagnetic waves and (b) Cerenkov wake radiation. v_b is the velocity of the disturbance that creates the wake in the medium. Figure reproduced with permission from: (a) ref.¹⁶³, IEEE; (b) ref.¹⁶⁴, American Physical Society.

Fig. 21. Electron current distributions at the surface of solid target obtained from 2D particle-in-cell simulations. (a), (b), and (c) are the cases at the time of 50, 60, and 70 laser cycles, respectively. Figure reproduced with permission from ref.¹⁶⁹, AIP Publishing.

Fig. 22. Relation of the 3/2ω₀ signal intensity of the scattered light with the THz energy. Figure reproduced with permission from ref.¹⁷², AIP Publishing.

Fig. 23. Schematic diagrams of two main mechanisms for THz generation from plasma emission.(a) Coherent transition radiation and (b) sheath radiation. Figure reproduced with permission from ref.¹⁷⁵, under a Creative Commons Attribution (CC BY) licence.