Fig. 1. Schematic of semi-classical 3-step model for HHG^[38]

Fig. 2. Relationship between photon flux and medium length (in unit of L_abs) ^[44]

Fig. 3. Typical experimental optical path and HHG output results ^[53]. (a) Typical setup for loose focusing driven HHG using single shot XUV wave front sensor and adaptive optics; (b) HHG from Ar gas; (c) principle of Hartmann wave front sensor

Fig. 4. ELI-ALPS GHHG SYLOS Long beam line setup driven by loose focusing laser^[60]

Fig. 5. Autocorrelation measurements of isolated attosecond pulse using N⁺ ion signal ^[75]

Fig. 6. Principle of coherent light synthesizer^[81]. (a) Coherent light source with different wavelengths; (b) synthesized light field from the coherent sum of light sources with controllable intensity and time delay; (c) results of time-domain measurement of synthesized waveform

Fig. 7. HHG spectra driven by lasers with different wavelengths under phase matching conditions^[86]

Fig. 8. Characterization of the 43 as isolated pulse generated by mid-inferred femtosecond laser^[20]. (a) Measured attosecond streaking spectrogram in Xe gas; (b) retrieved attosecond streaking spectrogram using ML-VTGPA algorithm; (c) temporal amplitude and phase of the reconstructed isolated attosecond pulse using ML-VTGPA algorithm, Fourier transform limit pulse is shown in dashed line; (d) reconstructed mid-inferred pulse using ML-VTGPA algorithm com

Fig. 9. Relationship between HHG phase matching pressure and beam radius^[100]

Fig. 10. Experimental diagram of HHG in resonant enhanced cavity^[113]

Fig. 11. Experimental results^[117]. (a) Intensity of HHG from solid state varies with order; (b) cut off order of HHG from solid state varies with driving laser power

Fig. 12. Measurement result^[120]. (a) Intensity of different orders of HHG varies with time delay between fundamental frequency light and double-frequency perturbing field; (b) accumulated phase retrieved from electron-hole pairs; (c) bandgap dispersion relationship retrieved from phase inversion