Elliptically polarized high-order harmonic generation by controlling quantum paths with two-dimensional laser fields

Circularly and elliptically polarized high-order harmonics are powerful tools for probing ultrafast dynamics in chiral and magnetic materials. However, previous methods for generating elliptically polarized harmonics impose stringent requirements on the driving laser or the target medium. Here, we present a new method for generating harmonics with tunable ellipticity by driving both atoms and randomly aligned molecules with a two-dimensional two-color field. This field consists of an elliptically polarized fundamental component and a linearly polarized second-harmonic component, which together control the two-dimensional electron motion and hence the intra-cycle interference of harmonic radiation, resulting in elliptically polarized harmonics. Molecular structure effects can further modulate the harmonic polarization, enabling the generation of harmonics with consistently positive or negative helicity across a broad spectral range as well as the synthesis of attosecond pulses with high ellipticity. Our method provides a broadly applicable and flexible approach to generating elliptically polarized high-order harmonics and attosecond pulses.