Attosecond pulse trains with unusual nonlinear polarization

The phases of the circularly polarized harmonics with alternating helicity generated by a bichromatic laser field whose two components are circularly polarized in the same plane but rotate in opposite directions are investigated. Only one trajectory contributes to harmonic generation in the plateau region. The dependence of the harmonic phase on the laser field intensity is weak (with the slope ∼0.2U_p/ω). Adjacent harmonics having the same helicity are relatively closely phase locked. As a result, a train of three attosecond pulses per optical cycle of the driving field is generated, each having a width of 80 as. Depending upon whether the two helicity components can be separated the polarization of the pulses is close to circular or close to linear with three different orientations per optical cycle.