Coherence-population interdependence in nonlinear optics

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Abstract
Two-level systems are widely used in nonlinear optics; some authors have considered larger systems, but in most of these descriptions the additional levels participate in the dynamics of the two main levels through kinetic rate constants only. Here, we adopt a different point of view. Knowing that two-level models treated in a perturbational approach show peculiar relations between populations and coherences, we analyze the effect of a third perturbating level on these relations. An estimate of the extra contribution to the third-order coherence is made. The role of the dephasing processes on this contribution is analyzed.