ANOMALOUS 2ND-ORDER NONLINEAR-OPTICAL RESPONSE OF INPLANE POLED GLASSY-POLYMERS - SPECTROSCOPIC AND THEORETICAL SUPPORT FOR THE IMPORTANCE OF CHARGED CHROMOPHORE AGGREGATES

We report here measurements of the orientational and frequency depende nce of the anomalously large second-order nonlinear optical (NLO) susc eptibilities observed in guest-host DANS [4-(N,N-dimethylamino)-4'-nit rostilbene]/PMMA [poly(methyl methacrylate)] films poled in an ''in-pl ane'' 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 anomalo us 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 experimenta l EPR evidence for nitrogen-centered cation radicals, hence the creati on of charged chromophores trapped in the polymeric matrix by charge i njection from the poling electrodes. These observations are in qualita tive agreement with the very large NLO response magnitudes and distinc t spectral features predicted by sum-over-states perturbative calculat ions performed for positively charged centrosymmetric chromophore dime rs and provide strong computational evidence that the observed anomalo us NLO response is due to charged chromophore aggregates.