Scaling of keV HHG photon yield with drive wavelength.

We study semi-analytically and numerically the photon yield of high harmonic generation (HHG) on the level of the single atom response under ideal conditions: no initial depletion of the ground state prior to the main peak of the pump pulse. We show that the yield decreases exponentially as function of the cutoff energy starting at about 0.5keV in the case of a Ti:sapphire source and a helium target. We show that the yield in helium beyond the 1keV energy range can be increased by orders of magnitude when long wavelength driver sources in the range from 1.5 mum to 3 mum are used. This finding leads to the conclusion that significant HHG beyond 1keV is possible through long wavelength driver pulses.