Control of attosecond-pulse helicity and ellipticity by the ionization-gating technique

We theoretically study controlling the generation of an attosecond pulse with specific helicity and arbitrary ellipticity from an oriented CO molecule using the ionization-gating technique. The results show that driven by a one-color linearly polarized laser field, the high-order harmonics generated in adjacent half optical cycles possess opposite helicities, resulting in the helicities of the attosecond pulses alternating from pulse to pulse. The introduction of a weak second harmonic field to create the ionization gating can effectively control high-order harmonic generation to filter out attosecond pulses with a given helicity by suppressing the ionization of the electrons every other half optical cycle. Then attosecond pulses with the same helicity are obtained, and the desired helicity can be selectively controlled by adjusting the relative phase of the laser field. Moreover, we show that controlling the ellipticity and helicity of a 155-as pulse is feasible by tuning the molecular orientation in the ionization-gating laser field.