Wavelength dependence of high-order harmonic generation with the ionization and ponderomotive energy controlled by an extreme ultraviolet pulse

We theoretically study the scaling with the driving wavelength of the high-order harmonic generation (HHG) under the simultaneous irradiation of an extreme ultraviolet (XUV) pulse. Surprisingly, when the cutoff energy and ionization yield are fixed, the harmonic yield is nearly independent of the driving wavelength. We identify its origin as the combination of the initial spatial width of the states excited by the XUV pulse, making the wavepacket spreading less prominent, and the shallowing of the ionization potential, which suggests complex nature of the wavelength dependence of HHG.