Generation of high-order harmonics with controllable elliptical polarization.

We propose and numerically demonstrate a method for obtaining high-harmonic radiation with desirable elliptical polarization. Atoms are shined by a combination of a strong linearly-polarized laser field and an additional weak field, which is elliptically polarized in a plane perpendicular to the polarization direction of the strong field. The strong driver ionizes and recollides electrons with their parent ion, while the weak field perturbatively drives the electrons away from "head-on" collision. Upon recombination, new elliptically polarized harmonics with same ellipticity as the weak driver are emitted at efficiency which linearly depends on the intensity of the weak beam, but is independent of its elilipticity.