Fig. 1. Typical high-order harmonic spectrum^[16]

Fig. 2. Diagram of three-step model of HHG in gas^[17]

Fig. 3. Dependence of intensity of high-order harmonics on ellipticity of driving laser^[19]

Fig. 4. High-order harmonics of ZnO^[10]. (a) Harmonic spectra under different intensities of driving laser ; (b) dependence of cutoff frequency on electric field intensity of driving laser

Fig. 5. Dependence of solid high-order harmonic yield on ellipticity of fundamental frequency light field. (a) Solid Ar^[20]; (b) MgO crystal^[21]

Fig. 6. Diagram of three-step model in momentum space of solid high-order harmonics^[12]

Fig. 7.

Fig. 8. Effect of two-color laser field on high-order harmonics^[26]. (a) High-order harmonic spectra generated under different driving laser field; (b) tunneling ionization rate and traveling time of electron in fundamental frequency laser field and orthogonal two-color laser field

Fig. 9. Enhance high-order harmonics by nanostructures^[35].(a) SEM image of antenna array on ZnO; (b) high-order harmonics of ZnO with (curve ①) and without (curve ②) antenna

Fig. 10. High-order harmonic response of silicon for different driving laser ellipticity and sample rotation angle^[43]. (a) 5^th order; (b) 7^th order; (c) 9^th order

Fig. 11. HHG of hard X-ray^[45]. (a) Diagram of HHG of hard X-ray; (b) harmonic spectrum generated under driving laser of different wavelengths

Fig. 12. Two gating schemes for generating isolated attosecond pulses^[46]. （a） Amplitude gating； （b） temporal gating

Fig. 13. Development of duration of isolated attosecond laser pulse

Fig. 14. Attosecond pulse from polycrystal SiO₂^[54]. (a) Temporal profile; (b) spectral profile

Fig. 15. Transient absorption spectra of CF₄ at carbon K-edge^[57]. (a) Ionization and dissociation of CF₄ induced by 800 nm infrared laser； （b） relationship between absorption spectra of CF₄ at carbon K-edge and delay of two lasers; （c） diagram of elected transition orbits

Fig. 16. Trajectory resolved high-order harmonic spectra^[58]. (a) High-order harmonic spectrum of D₂ molecule; (b) high-order harmonic spectrum of H₂ molecule; (c) integrated signal of intensity of high-order harmonic spectrum

Fig. 17. Ultrafast vacuum ultraviolet laser spectrometer based on gaseous high-order harmonics. (a) Connected with velocity map imaging spectrometer， for studying atomic and molecular physics；（b）connected with photoemission electron microscope，for studying photoelectron dynamics of solid material and micro-nano structure

Fig. 18. High-order harmonic spectrum generated by driving laser with different wavelengths

Fig. 19. High-order harmonic spectrum of ZnO crystal