title: Ellipticity of near-threshold harmonics from stretched molecules.
abstract: We study the ellipticity of near-threshold harmonics (NTH) from aligned molecules with large internuclear distances numerically and analytically. The calculated harmonic spectra show a broad plateau for NTH which is several orders of magnitude higher than that for high-order harmonics. In particular, the NTH plateau shows high ellipticity at small and intermediate orientation angles. Our analyses reveal that the main contributions to the NTH plateau come from the transition of the electron from continuum states to these two lowest bound states of the system, which are strongly coupled together by the laser field. Besides continuum states, higher excited states also play a role in the NTH plateau, resulting in a large phase difference between parallel and perpendicular harmonics and accordingly high ellipticity of the NTH plateau. The NTH plateau with high intensity and large ellipticity provides a promising manner for generating strong elliptically-polarized extreme-ultraviolet (EUV) pulses.
authors.forename: Weiyan
authors.initials: W
authors.lastname: Li
authors.forename: Fulong
authors.initials: F
authors.lastname: Dong
authors.forename: Shujuan
authors.initials: S
authors.lastname: Yu
authors.forename: Shang
authors.initials: S
authors.lastname: Wang
authors.forename: Shiping
authors.initials: S
authors.lastname: Yang
authors.forename: Yanjun
authors.initials: Y
authors.lastname: Chen