Atomic high-order harmonic generation from a circularly polarized laser pulse

We demonstrate theoretically that the high-order harmonic of atom can be generated by a circularly polarized laser pulse. The harmonic spectrum shows a clear cutoff with an energy I_p+2U_p. Especially, the high-order harmonic generation comes from the multiple recombination of the ionized electron with non-zero initial velocity. These results are verified by the classical model theory and the time-frequency analysis of harmonic spectrum.