Optimal control over high-order-harmonic ellipticity in two-color cross-linearly-polarized laser fields

We investigate the generation of high-order harmonics with arbitrary ellipticity using synthesized two-color (800-nm + 400-nm) cross-linearly-polarized laser fields. It is shown that one can realize control over harmonic ellipticity by finely adjusting the relative phase and the crossing angle of such two-color fields. Through analyzing the time-frequency distributions and the quantum orbits of harmonic radiations, we show that the harmonic helicity is originated from the unsymmetrical contribution between different quantum orbits released in one 800-nm cycle. And, we further reveal that the controllability of harmonic ellipticity in such two-color fields is closely related to the modulation of crossing angle and relative phase on the driving light's spin angular momentum. Finally, we show the probability of producing and modulating isolated helical attosecond pulses with few-cycle two-color cross-linearly-polarized laser fields.