Fig. 1. Energy of terahertz pulse and focal spot

Fig. 2. Intense terahertz sources based on femtosecond laser pumping

Fig. 3. Terahertz single pulse energy and optical-to-terahertz energy conversion efficiency versus year for intense terahertz pulses generated by tilted-pulse-front technique. (a) Pulse energy; (b) energy conversion efficiency

Fig. 4. Diagram of generation of terahertz emission pulses with high energy conversion efficiency based on cascading effect

Fig. 5. Typical experimental setup for terahertz emission based on tilted-pulse-front technique

Fig. 6. Large diffraction spot size induced by angular dispersion

Fig. 7. Schematic of two methods for stretching pumping femtosecond laser

Fig. 8. Terahertz emission with central frequency of 100 GHz produced by 4 ps laser pulse. (a) Terahertz emission energy and its energy conversion efficiency versus pumping power density; (b) terahertz temporal waveform and its spectrum (shot dot: experimental; red line: theoretical prediction)

Fig. 9. Decrease of energy conversion efficiency of teraherza pulse due to ultrashort high power density excitation. (a) Optimization of pumping pulse width; (b) optimization of pumping pulse energy

Fig. 10. Mechanism for variations in spot size and position caused by nonlinear distortion effect in tilted-pulse-front technique

Fig. 11. Principle diagram of pulse-front-tilting realized by reflective step mirror

Fig. 12. Typical imaging setups in tilted-pulse-front setup. (a) Plano convex lens; (b) double horizontal cylindrical lens; (c) telescope system with double plano convex lens; (d) double cylindrical lens; (e) composite triple lens

Fig. 13. Schematic of terahertz emission with elliptical beam pumping. (a) Circular beam pumping; (b) elliptical beam pumping; (c) schematic of elliptical beam inside crystal

Fig. 14. Refractive indexes and absorption coefficients of congruent lithium niobite crystals under different crystal temperatures. (a) Refractive indexes of extraordinary light; (b) absorption coefficients of extraordinary light; (c) refractive indexes of ordinary light; (d) absorption coefficients of ordinary light

Fig. 15. Highly efficient terahertz emission due to cooling of lithium niobite crystal. (a) Energy conversion efficiency versus temperature; (b) energy conversion efficiency versus pump energy

Fig. 16. Terahertz temporal waveforms and their corresponding spectra at 100 K and 300 K, respectively. (a) Temporal waveforms; (b) spectra

Fig. 17. Focused spot of terahertz beam with high quality. (a) Measured terahertz beam profile; beam diameters of spot in (b) horizontal and (c) vertical directions

Fig. 18. High-energy terahertz emission realized by Yttrium Lithium Fluoride (YLF) laser. (a) Extracted terahertz energy pumped by YLF laser; (b) corresponding energy conversion efficiency; (c) spectral broadening of residual pumping laser under different pump powers

Fig. 19. Peak field and frequency range for each intense terahertz source