Generation of circularly polarized high-order harmonics by two-color coplanar field mixing - art. no. 063403

Pin efficient method is investigated for the generation of,circularly polar ized high-order harmonics by a bichromatic laser field whose two components with frequencies omega and 2 omega are circularly polarized in the same pl ane, but rotate in opposite directions. The generation of intense harmonics by such a driving-field configuration was already confirmed by a previous experiment. With the help of both a semiclassical three-step model as well as a saddle-point analysis, the mechanism of harmonic generation in this ca se is elucidated and the plateau structure of the harmonic response and the ir cutoffs are established. The sensitivity of the harmonic yield and the p olarization of the harmonics to imperfect circular polarization of the driv ing fields are investigated. Optimization of both the cutoff frequency and the harmonic efficiency with respect to the intensity ratio of the two comp onents of the driving field is discussed. The electron trajectories respons ible for the emission of particular harmonics are identified. Unlike the ca se of a linearly polarized driving field, they have a nonzero start velocit y. By comparison with the driving-field configuration where the two compone nts rotate in the same direction, the mechanism of the intense harmonic emi ssion is further clarified. Depending on the (unknown) saturation intensity for the bichromatic field with counter-rotating polarizations, this scheme might be of practical interest not only because of the circular polarizati on of the produced harmonics, but also because of their production efficien cy.