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 $E_{\mathrm{ph}}=0.5\mathrm{}\mathrm{keV}$ ( $\lambda <2.5\mathrm{}\mathrm{nm}$). The high-energy end of the continuum in the range of $E_{\mathrm{ph}}\ge 0.2\mathrm{keV}$ ( $\lambda \le 6\mathrm{nm}$) 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 ( $<0.5\mathrm{}\mathrm{mJ}$) 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.