The degenerate two-level system and high-order harmonic generation

A previously considered two-level system interacting with an external electromagnetic wave is particularised to a degenerate system and utilized as a model for the process of high-order harmonic generation (HOHG). It is shown that this model can be reduced to a linear coupled map and that the plateau and cut-off which are characteristic signatures of HOHG can be obtained from this map. Exploring the dependence of the intensity of the generated harmonics on the amplitude of the external electromagnetic wave (parameterized in terms of the Rabi frequency), it is found that there are significant reductions in intensity for specific orders of the harmonics. It is conjectured that the experimentally observed absence of certain harmonics may not be due to the atomic structure and still may be a single-atom effect.