Photorefractive holographic contrast enhancement via increased birefringence in polymer composites containing electro-optic chromophores with different alkyl substituents

The birefringence of a photorefractive polymer composite in response to an applied electric field is used to predict the holographic index contrast attainable. Predictions have been obtained for three different polymer composites based on the charge transfer complex matrix poly(N-vinyl carbazole)/2,4,7-trinitro-9-fluorenone, doped with electro-optic azo chromophores differing only in their alkyl substitutions. A comparison with experimental data indicates that unexpected large variations in the holographic contrast obtainable from different chromophores are, in most cases, accounted for by a corresponding variation in electric field-induced birefringence. This variation may be due to a combination of the chromophore number density and the composite viscosity. Where the birefringence does not correlate with holographic index contrast, a prediction based on a trap density limited space-charge field fits the data well. (C) 2000 American Institute of Physics. [S0021-8979(00)01202-0].