Coherent 0.5-keV X-ray emission from helium driven by a sub-10-fs laser

Helium atoms ionized by intense few-cycle light pulses in the barrier suppression regime emit spatially coherent extreme ultraviolet continuum extending to photon energies greater than E-ph = 0.5 keV (lambda < 2.5 nm). The high-energy end of the continuum in the range of E-ph greater than or equal to 0.2 keV (lambda less than or equal to 6 nm) was characterized spectrally over a considerable dynamic range using energy-dispersive detection. The sub-10-fs laser pulse duration was found to be crucial for generating radiation with the highest photon energies at the low (<0.5 mJ) pump energy levels used in the experiments. The single-atom quantum theory of high-order harmonic generation combined with Maxwell's wave equation provides a satisfactory account for the experimental observations.