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 at tainable. Predictions have been obtained for three different polymer compos ites based on the charge transfer complex matrix poly(N-vinyl carbazole)/2, 4,7-trinitro-9-fluorenone, doped with electro-optic azo chromophores differ ing only in their alkyl substitutions. A comparison with experimental data indicates that unexpected large variations in the holographic contrast obta inable from different chromophores are, in most cases, accounted for by a c orresponding variation in electric field-induced birefringence. This variat ion may be due to a combination of the chromophore number density and the c omposite viscosity. Where the birefringence does not correlate with hologra phic index contrast, a prediction based on a trap density limited space-cha rge field fits the data well. (C) 2000 American Institute of Physics. [S002 1-8979(00)01202-0].