Electronic and nuclear dynamics play essential roles in the properties and functions of matter. Such dynamics span timescales from picoseconds down to attoseconds[2–7]. In many cases, ultrashort pulses are required to initiate and probe the selected dynamics. Such pulses can be provided by both optical-frequency lasers and free-electron lasers (FELs)[8–10]. Optical-wavelength pulses can reach pulse durations down to a few femtoseconds and, high-order harmonic generation, can in turn be used to produce pulses in the extreme ultraviolet/soft X-ray spectral regime with pulse durations below 100 attoseconds. In the past decades, these pulses have been successfully applied to many studies of ultrafast dynamics in gases and solids (see Chang and Nisoli et al. and references therein). One of the major limitations of these high-harmonic sources is low photon flux at photon energies above 200 eV.
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