Fig. 1. HHG with highest average power generated by fiber laser with 1 MHz repetition rate^[26]. (a) Experimental device of high power HHG; (b) high harmonic spectrum generated using krypton gas nozzle and corresponding average power of each order harmonic

Fig. 2. Water window HHG generated by high-repetition-rate thulium-doped fiber laser^[47]. (a) HHG experimental setup; (b) experimentally collected HHG spectrum

Fig. 3. Distribution of repetition rate, monopulse energy, photon energy, and average power of HHG generated by most advanced fiber laser driving ^{[23-24, 26-27,29-30,32-37,39-40,42,44,47,50,54]}

Fig. 4. Phase matching regions. (a) Minimum ionization degree (dash line) and maximum ionization degree (solid line) satisfying phase matching in Kr, Ar, and Ne gases versus photon energy driven by pump light with different wavelengths (λ) and pulse widths (τ); (b) theoretically predicted EPMmax in Ar, Ne, and He gases versus pump laser wavelength; (c) EPMmax in Ar, Ne, and He gases versus pump laser pulse width driven by 1030 nm pump light

Fig. 5. Influence of self-absorption. (a) Variation of absorption length of HHG with gas pressure (pump laser wavelength of 1030 nm); (b) Sq/Sqmax versus Lmed/Labs under different Lcoh/Labs

Fig. 6. Sq/Sqmax and Lcoh versus gas pressure. (a) H25; (b) H33

Fig. 7. Lcoh (dash line) and Sq/Sqmax (solid line) of HHG with wavelength of 13.5 nm generated in Ne medium versus gas pressure after key parameters is adjusted according to scaling law. (a) w₀=37.5 μm，L_med=150 μm; (b) w₀=375.0 μm，L_med=15 μm

Fig. 8. Different modes of coherent diffraction imaging. (a) Traditional CDI; (b) FTH; (c) ptychographic CDI

Fig. 9. Schematic of phase retrieval algorithm

Fig. 10. Imaging examples using HHG ptychography. (a) Sample reconstruction image obtained by ptychography using HHG for the first time^[104]; (b) imaging result of mouse hippocampal neuron based on ptychography^[107]

Fig. 11. Three-dimensional structural profiles measured by OCT^[114]. (a) Depth and lateral information; (b) depth information

Table 1. Main parameters of generating high-repetition-rate HHG experiment generated by fiber laser driving

Table 2. Scaling laws of important parameters between loose and tight focusing regimes^[80-82]