Parametric resonances on the polarization spectrum of a two-level atom interacting with a strong polychromatic pump field and an arbitrarily intense probe field

We discuss the physical nature of the parametric resonances exhibited on the polarization spectrum of a two-level atom driven by a strong polychromatic field. An analytical expression of the polarization spectrum for the special case of driving fields which are symmetrically positioned with respect to the transition frequency is used for the analysis. It is shown that, for the case of three driving fields, there is amplification without population inversion of the probe field at the line center, even when the amplitude of the probe field is very weak.