High-order harmonic generation by atoms in an elliptically polarized laser field: Harmonic polarization properties and laser threshold ellipticity

High-order harmonic generation (HHG) by an atom in an elliptically polarized laser field is studied numerically and theoretically. We derive equations describing the effect of the driving ellipticity on the HHG efficiency, harmonic ellipticity, and rotation angle for the cases of the ground state with the orbital angular momentum $l=0$ and $l\ensuremath{\ne}0$. These equations provide good agreement with the results of our three-dimensional time-dependent Schr\"odinger equation calculations, as well as with the published experimental data. We show that HHG sensitivity to the driving laser ellipticity increases with the drive wavelength. This predicts high efficiency of the polarization gating schemes using midinfrared laser fields. Furthermore, our study shows significant enhancement of the harmonic ellipticity near the Cooper minimum.