Amplification of high-order harmonics using weak perturbative high-frequency radiation

The mechanism underlying the substantial amplification of the high-order harmonics q +/- 2K (K integer) upon the addition of a weak seed extreme uv (XUV) field of harmonic frequency q omega to a strong IR field of frequency omega is analyzed in the framework of the quantum-mechanical Floquet formalism and the semiclassical recollision model. According to the Floquet analysis, the high-frequency field induces transitions between several Floquet states and leads to the appearance of new dipole cross terms. The semiclassical recollision model suggests that the origin of the enhancement lies in the time-dependent modulation of the ground electronic state induced by the XUV field.