Abstract

In low T_g photorefractive doped polymers, large modulations of the refractive index result from the orientation of the nonlinear optical chromophores by the internal space charge field. This property, known as "orientational enhancement",⁽¹⁾ requires a high rotational mobility of the molecules, i.e. low viscosity, generally obtained for temperatures near or above T_g. We demonstrate here that chromophores mobility, governing the dynamics of orientation and consequently the dynamics of electro-optic effects, depends on the mechanical properties of the material only. For this purpose, and for the first time, we present joint results on dielectric spectroscopy, frequency resolved ellipsometry, and frequency dependent stress-strain measurements obtained for temperatures above T_g. These latter measurements lead to the complete viscoelastic behavior of the material, whereas usual T_g measurements only give relative information. These characterizations have been performed on plasticized poly (N-vinyl carbazole) doped with different chromophores. We demonstrate that the orientational mobility of the chromophores is coupled with the mechanical properties of the material. This work opens up new possibilities based on viscoelastic considerations for the optimization of photorefractive properties in doped low-T_g polymers.

© 2001 Optical Society of America