Second harmonic generation induced by charge-transfer complexes in a centrosymmetric medium

We consider the second harmonic generation (SHG) caused by impurities embedded in a nonlinear (χ(3)) optical medium which itself does not display χ(2) optical response. In the case when the impurity molecules (charge-transfer (CT) complexes) possess an optical transition accompanied by charge transfer, we demonstrate that the excitation of impurities with a pump field may result in a contribution of the host medium to SHG. Depending on the presence or absence of the angular alignment of the excited static CT dipoles, there may be coherent SHG or weak incoherent SH scattering.