Negativë absorption and Negativë emission in nonlinear optics

Abstract The nonlinear susceptibilities X 3 ( ω 2 = ω 1 − ω 1 + ω 2 ) calculated for stimulated Raman scattering (SRS) and two- photon absorption (TPA) under the condition of one-photon resonance in the case of a multilevel system with broad overlapping absorption bands (corresponding to polyatomic molecules) are discussed. Spectra of Im X 3 with changed sign in some spectral regions are shown. The result is connected with a disbalance of the system, due to the large relaxation constants.