Controlling polarization of high-order harmonic generation by molecular alignment in a bicircular laser field

Abstract

The polarization of high-order harmonic generation in ${\mathrm{H}}_{2}{}^{+}$ driven by a counterrotating bichromatic $\ensuremath{\omega}/3\ensuremath{\omega}$ circularly polarized laser field is investigated theoretically. The elliptically polarized harmonics are characterized by ellipticity and rotation angle, where the latter is expanded to $[\ensuremath{-}\ensuremath{\pi}/2,\ensuremath{\pi}/2]$ by an optimized definition. We find a critical alignment angle where the ellipticity of high-energy harmonics is almost the same, so an attosecond pulse train with a high degree of polarization can be synthesized. In addition, we find that the rotation angle is always different from the alignment angle by ${90}^{\ensuremath{\circ}}$ with a small deflection. Thus, we propose a scheme to control the harmonic polarization state by the molecular alignment angle.