Anomalous second-order nonlinear optical response of in-plane poled glassy polymers. Spectroscopic and theoretical support for the importance of charged chromophore aggregates

We report here measurements of the orientational and frequency dependence of the anomalously large second-order nonlinear optical (NLO) susceptibilities observed in guest-host DANS [4-(N,N-dimethylamino)-4'-nitrostilbene]/PMMA [poly(methyl methacrylate)] films poled in an ''in-plane'' electrode configuration. The resonant response is sharply peaked (fwhm decreases by a factor of 4) in comparison to identical samples poled by standard corona field techniques. Additionally, these anomalous susceptibilities, which are oriented normal to the electric poling field, exhibit unusually strong features at long wavelengths where the linear absorption is low. We also report the first direct experimental EPR evidence for nitrogen-centered cation radicals, hence the creation of charged chromophores trapped in the polymeric matrix by charge injection from the poling electrodes. These observations are in qualitative agreement with the very large NLO response magnitudes and distinct spectral features predicted by sum-over-states perturbative calculations performed for positively charged centrosymmetric chromophore dimers and provide strong computational evidence that the observed anomalous NLO response is due to charged chromophore aggregates.