Model calculations of polarization-dependent two-color high-harmonic generation

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Abstract
Emission rates for high-harmonic generation by a zero-range-potential model atom in the superposition of two monochromatic plane-wave fields are calculated. Several polarizations of the driving fields are considered: two linear polarizations enclosing an arbitrary angle, and two circularly polarized fields that co- or counter-rotate in the same plane. Transition amplitudes are obtained in the form of sums of one-dimensional integrals that have to be computed numerically. For commensurate frequencies of the driving fields the results depend critically on the relative phase between the two fields. Parallel driving fields are not always more efficient in harmonic generation than perpendicular fields; also, two circular polarizations can be at least as effective. The odd harmonics of one field are usually weakened by the addition of the other field in favor of the mixed harmonics. If the ratio of the frequencies of the two incident fields equals the ratio of two odd integers, then harmonics with elliptic polarization can be generated by two linearly polarized driving fields. Harmonics with circular polarization can readily be produced with the help of two incident circularly polarized fields whose field vectors co-rotate or counter-rotate in the same plane.