  The mechanism underlying the substantial amplification of the high-order
harmonics q \pm 2K (K integer) upon the addition of a weak seed 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 re-collision 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
re-collision model suggests that the origin of the enhancement lies in the
time-dependent modulation of the ground electronic state induced by the XUV
field.
