  We demonstrate that phase-matched frequency upconversion of ultrafast laser
light can be extended to shorter wavelengths by using longer driving laser
wavelengths. Experimentally, we show that the phase-matching cutoff for
harmonic generation in argon increases from 45 to 100 eV when the driving laser
wavelength is increased from 0.8 to 1.3 micrometers. Phase matching is also
obtained at higher pressures using a longer-wavelength driving laser,
mitigating the unfavorable scaling of the single-atom response. Theoretical
calculations suggest that phase-matched high harmonic frequency upconversion
driven by mid-infrared pulses could be extended to extremely high photon
energies.
