Enhancement of non-resonant nonlinear 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 molecules increases their non-resonant non-linear refractive index in the infrared 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 phenomenologically using a two-level model, and we present experimental results in good agreement with semi-empirical calculations.