Near-circularly-polarized attosecond pulse generation from carbon monoxide molecules with a combination of linearly and circularly polarized fields

We investigate the polarization properties of high harmonics generated from oriented carbon monoxide molecules driven by a linearly polarized laser field combined with a weaker circularly polarized high-frequency assistant laser field (LPCP). The results show that the harmonics with higher ellipticities are obtained close to the cutoff region. Further analysis shows that the results are due to the manipulation of the electron trajectory by the LPCP field and the molecular asymmetric structure. By taking advantage of the properties above, we propose and theoretically demonstrate a method to generate attosecond extreme ultraviolet pulses with large ellipticity. Because of the difference of harmonic ellipticities contributed from the long and short trajectories, an even higher harmonic ellipticity can be obtained by filtering the long trajectory. In addition, by adjusting the relative phase between linearly polarized and circularly polarized fields, the attosecond XUV pulse varies from being left-elliptically polarized to right-elliptically polarized.