Nonlinear polarization in comb-spectroscopy

Two methods were applied to compute the 2-level atom driven by a polychromatic field. The first method is a direct numerical solution of the density matrix equations and the second one is a harmonic basis decomposition resulting in an infinite linear algebraic equation system. It is showed the resonance frequency mostly follows the transition frequency. But some frequency pulling or pushing is observed depending on the transition frequency inside the field component bounds or not.