High-order harmonic generation via bound-bound transitions in an elliptically polarized laser field

We use a simplified five-level system to investigate the high-order harmonic generation (HHG) spectrum emitted by an atom driven by a linearly or elliptically polarized laser field. For this model, the Schrödinger equation is exactly analytically reduced to the system of ordinary differential equations, which is solved numerically. Studying the intensity and polarization of the emitted radiation, we find that under high laser ellipticity the harmonic emission is suppressed. However, the harmonic intensity typically depends nonmonotonously on the laser ellipticity. Such anomalous behavior is very pronounced for the resonant harmonic. We offer an explanation of this behavior based on the increasing number of levels effectively involved in resonant HHG when the field ellipticity becomes nonzero. Finally, we find that the ellipticity of the resonant harmonic is, in general, higher than that of the other harmonics.