The four-level model is used to study the nonlinear media with coincident absorption and emission bands in both principal (S_0-S_1) and excited (T_1-T_2) channels. The excitation of molecules occurred by two light fields: radiations with intensity I_{12} at frequency ω_0 tuned at the centre of absorption band of principal singlet-singlet channel and radiations with intensity I_{34} at frequency ω tuned at the centre of absorption band of excited triplet-triplet channel. The monotonic dependence of phase response of media in both principal and excited channels and the saturation processes are studied theoretically and numerically for four-level model.
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