Coherent soft-x-ray radiation up to photon energies of 700 eV is obtained by focusing several-mJ, 10-fs near infrared laser pulses into a He gas jet. The observed nearly constant photon yield over several hundred eVs may be attributed to nonadiabatic self-phase matching, originating from a substantial ionization within a fraction of the optical cycle of the driving laser pulse.

By irradiating He and Ne atoms with $3\mathrm{mJ}$ , $12\mathrm{fs}$ , near infrared laser pulses from a tabletop laser system, the authors generated spatially and temporally coherent x rays up to a photon energy of $3.5\mathrm{keV}$ . With this source it is possible to use high-harmonic radiation for x-ray absorption spectroscopy in the keV range. They were able to clearly resolve the $L$ absorption edges of titanium and copper and the $K$ edges of aluminum and silicon. From the fine structure of the x-ray absorption they estimated the interatomic distances.

We report on the realization of an intracavity high harmonic source with a cutoff above 30 eV. The EUV source is based on a high power, hard-aperture, Kerr-lens mode-locked Ti:sapphire oscillator with a repetition rate of 9.4 MHz. The laser is operated in the net negative dispersion regime resulting in intracavity pulses as short as 17 fs with 1 µJ pulse energy. In a second intracavity focus, intensity more than 10¹⁴ W/cm² has been achieved, which is sufficient for high harmonic generation in a Xenon gas jet.