Fig. 1. Characteristic of HHG spectrum

Fig. 2. Three step model of HHG

Fig. 3. Reported photon energies of HHGs in X ray band

Fig. 4. HHGs with pump lasers with different wavelengths^[28]

Fig. 5. Microscopic mechanisms for solid-state HHG^[58]

Fig. 6. Worldwide development of isolated attosecond pulse bandwidth

Fig. 7. The characteristic properties of the HHG pulses in spectral and temporal domain

Fig. 8. The contributions of different half cycles to the generated high harmonic spectra with amplitude gating^[76]

Fig. 9. The scheme of polarization gating^[86]

Fig. 10. The time-dependent ellipticity of the laser pulse formed in the polarization gating^[89]

Fig. 11. Control of the generation of HHG in gases based on two color gating^[90]

Fig. 12. Simulated HHG spectrum based on two color gating^[95]

Fig. 13. 13 Scheme of GDOG^[101]

Fig. 14. Comparison of DOG and GDOG laser fields^[101]

Fig. 15. Theory of attosecond lighthouse and the corresponding high harmonic spectrum^[76]

Fig. 16. Principle and experimental setup of noncollinear gating^[104]

Fig. 17. Transmission spectra and dispersion property of different materials^[115]

Fig. 18. Si and SiC reflection efficiencies in XUV band at the pump pulse's (800 nn) Brewster angle^[116]

Fig. 19. Change of harmonic photon energy with the recombination time^[115]

Fig. 20. Principle of chirped multilayer mirror^[123]

Fig. 21. The quantum paths contributing to the photoelectrons in RABITT^[132]

Fig. 22. The experimental setup of RABITT^[132]

Fig. 23. Effect of a strong laser field on the photoelectrons ionized by attosecond pulses^[135], dashed circle-velocity distribution without the laser field, solid circle-distribution with the laser field

Fig. 24. Principle of the all-optical FROG measurement of an isolated attosecond pulse^[68]

Fig. 25. Experimental layout of the all-optical FROG for isolated attosecond pulse reconstruction and the retrieved result^[68]

Fig. 26. Reported results of soft X-ray attosecond pulses with energy up to nJ level^[148]