Origin for ellipticity of high-order harmonics generated in atomic gases and the sublaser-cycle evolution of harmonic polarization.

We investigate numerically and analytically the polarization properties of high-order harmonics generated by an atom in intense elliptically polarized laser field. The offset angle of the harmonic polarization ellipse can be well described with the semiclassic "simple-man" high-harmonic generation model. The harmonic ellipticity itself, however, can be hardly understood within this model. We show that this ellipticity originates from quantum-mechanical uncertainty of the electron motion. We develop a theoretical approach describing this ellipticity and, more generally, the time evolution of the high-harmonic polarization state within the laser cycle. The analytical results are verified with the exact numerical solution; to make the comparison accurately, we develop a specific technique for separating the contributions of quantum paths in the numerical calculation.