PHASE OF HARMONICS FROM STRONGLY DRIVEN 2-LEVEL ATOMS

Harmonic generation is studied in an idealized model medium in which t he laser field essentially interacts only with a single atomic transit ion and where ionization can be neglected. Such two-level models are k nown to generate high harmonics with a plateau and a cutoff. We put fo rward a quantitative rule for the cutoff frequency in the harmonic spe ctrum in this model which we relate to the ''I-p + 3 U-p rule'' in rea l systems with ionization. We examine the phase properties of the emit ted harmonics and show that their coherence and thus phase matching is more favorable in the cutoff regime than in the plateau of the harmon ic spectrum. This behavior is known to occur in realistic models of ne on, and that this occurs also for the simple idealized two-level model strongly suggests that the sudden change of phase coherence from the cutoff to the plateau regime is as intrinsic a property of nonlinear s ystems as are the plateau and the cutoff.