Electronic versus vibrational optical nonlinearities of push-pull polymers

The electronic and nuclear contributions to the second-order polarizabilities of donor/acceptor substituted polymers are calculated in the intermediate frequency regime relevant for optical material applications (laser frequency high compared to nuclear and low compared to electronic transitions). We find that the purely electronic contribution is positive and vibrations only make a ~5-10% negative correction. This is in marked contrast with the static (zero frequency) response where both contributions are positive and have comparable magnitudes. Despite recent suggestions to the contrary, purely electronic structure/polarizability relations provide a useful guide for the synthesis of new materials.