Symmetries in harmonic generation by a two-color field: an application to a simple model atom

We show that the parity of the order of the harmonics generated by a bichro matic field with commensurate frequencies may be related to the invariance properties of the time-dependent Hamiltonian with respect to some suitable transformations. To simplify the mathematical treatment, we have used a sim ple model atom to perform calculations that show the effects of the relativ e phase of the driving fields. Harmonic generation has been described as a two-step process that involves replicas of the Stark levels. In agreement w ith other previous treatments in which other model atoms were used, we have found that a small contamination of the fundamental harmonic can produce s ignificant modifications of the emission spectra. (C) 1999 Optical Society of America [S0740-3224(99)00206-4] OCIS codes: 190.4160, 190.4180.