Generating attosecond pulses with high ellipticity by use of He atoms

To control the polarization of attosecond pulse trains (APTs), the most widely used way is to adjust the intensity ratio between the two components in a bicircular laser field. Here we demonstrate another effective means to obtain highly elliptically polarized APTs. For this purpose, we propose a circular-elliptical laser field, which consists of a circularly polarized field with its coplanar counter-rotating elliptical harmonic pulse, in order to generate high-order harmonics by He atom. We show that the polarization of APTs depends evidently on the ellipticity xi of harmonic laser field. Most importantly, nearly circular APTs can be obtained with a proper xi for driving laser field. Our results contribute to the generation of high elliptically polarized APTs with s-state atom.