INTENSITY DEPENDENCE OF THE PHASE OF THE FIELD-INDUCED ATOMIC DIPOLE IN HIGH-HARMONIC GENERATION

The Fourier components of the field-induced atomic dipole moment are n ot, in general, in phase with the corresponding power of the driving f ield. The phase lags depend on the intensity of the driving field, and , as a consequence, they vary throughout the focal volume of the irrad iated gas sample. For a zero-range binding potential, we present plots of this phase lag as a function of the intensity for various binding energies and harmonic numbers, along with the corresponding harmonic i ntensities. The intensity dependence of the phase lags is very complex and displays characteristic patterns which are different in the three distinct regimes of the harmonic response: below, within, and beyond the plateau. We suggest and discuss various mechanisms that are respon sible for the behavior in the various regimes.