Nonlinear response of a three-level system driven by a polychromatic field and probed by an arbitrarily intense field

A numerical method is developed in which we study the nonlinear absorption and dispersion of a three-level system driven by a polychromatic field and probed by an arbitrarily intense monochromatic field. This method is based on a harmonic expansion of all eight density matrix elements and a transformation of an infinite set of equations for the slowly varying amplitudes into a vector equation. When we substitute a strong probe field for a weak probe field, the absorption and dispersion spectra evolve from the well resolved multiple narrow peaks into powered broadened broad structures. The present method has an advantage over the previous studies in that it does no longer require a single population recurrence relation and it is applicable for the systems with more atomic levels and more field components.