Enhancement of non-resonant non-linear refractive index with reduction of absorption in push-pull molecules by reduction of their donor group strength

We show that reducing the strength of the amine donor group in push-pull mo lecules increases their non-resonant non-linear refractive index in the inf rared although the blue-shift of their visible absorption peak, associated with the donor strength reduction, can decrease their linear and non-linear absorption at the excitation wavelength. This effect is explained phenomen ologically using a two-level model, and we present experimental results in good agreement with semi-empirical calculations. (C) 2000 Elsevier Science B.V. All rights reserved.