We present theoretical calculations for polarization and ellipticity of high-order harmonics from aligned ${\mathrm{N}}_2$, CO${}_2$, and ${\mathrm{O}}_2$ molecules generated by linearly polarized lasers. Within the rescattering model, the two polarization amplitudes of the harmonics are determined by the photo-recombination amplitudes for photons emitted with polarization parallel or perpendicular to the direction of the same returning electron wave packet. Our results show clear species-dependent polarization states, in excellent agreement with experiments. We further note that the measured polarization ellipse of the harmonic furnishes the needed parameters for a “complete” experiment in molecules.

• Received 17 February 2010

DOI:https://doi.org/10.1103/PhysRevA.82.023814