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.