Dramatic extension of the high-order harmonic cutoff by using a long-wavelength driving field

We present an experimental demonstration of extending the high-order harmonic cutoff photon energy more than a factor of 2 when the driving-field wavelength is changed from 0.8 to 1.51 mum with an optical parametric amplifier. With argon gas, the cutoff has been extended from 64 to 160 eV. We predict that coherent keV x rays can be generated by exciting helium gas with the long-wavelength driving pulses, Experiments on xenon gas with several pump wavelengths also showed the dramatic cutoff extension, as well as full tunability of the generated XUV wavelengths.