Fig. 1. Experimental setup for THz wave generation by tightly focusing the optical laser beam into a wire-guided gravity-driven free-flowing water film

Fig. 2. Measurement results of the THz fields when the water film is at different places along the direction of laser propagation^[22]. (a) THz waveforms when the water film is at different places (A before the focal point, B near the focal point, C after the focal point, D without water film); (b) THz waveforms when the water film is moved near the focal point; (c) comparison in the frequency domain

Fig. 3. Normalized THz energy with different laser pulses^[22]. (a) Pulse duration; (b) polarization; (c) pulse energy

Fig. 4. Two kinds of water films with different structures designed by Zhang Xicheng’s team. (a) Wire-guided gravity-driven free-flowing water film; (b) water film created by a flat liquid nozzle^[7]

Fig. 5. Experimental setup for THz wave generation from a water film under two-color laser’s excitation^[24]

Fig. 6. Relation curve of THz energy generated from water film and laser pulse (including ω and 2ω) energy^[24]

Fig. 7. Photograph of the water line produced by a syringe needle

Fig. 8. Experimental setup for THz wave generation using a water line, the inset means the geometric diagram of the laser interacted with the water line^[26]

Fig. 9. Effect of the diameter of water line on THz wave generation from a water line^[25]. (a) Best pulse duration of the pump beam; (b) THz wave energy

Fig. 10. Effects of α on THz wave generation from a water line^[29]. (a) Geometric diagram of α; (b) relationship between α and THz energy when water lines with different diameters are used

Fig. 11. Geometric diagram of the pump laser interacted with the water line, while Δx means the distance between laser propagation axis and the center point O' of water line^[26]. (a) Δx=0;(b) Δx≠0