Electro-optic response of chromophores in a viscoelastic polymer matrix to a combined dc and ac poling field

We present a generalized model for the electro-optic response of chromophores in a viscoelastic polymer matrix to a combined dc and ac applied poling field. The model includes a local molecular field of random orientation at each chromophore site to take into account the influence of the polymer matrix on the chromophore reorientation. Hence the model relies on a physically intuitive picture of chromophore dynamics in a viscoelastic polymer matrix. The dynamics is described by the rotational diffusion equation, where the local molecular field is inferred and a solution is presented with a variational approach. We obtain an analytical expression for the electro-optic response both at the modulating frequency and at two times the modulating frequency, having explicit frequency and molecular-field dependence. The model is successfully compared with frequency-resolved ellipsometric measurements in an azo-dye containing polymer guest-host system that is poled in a combined dc and ac electric field. The experimental setup is a modified Teng-Man ellipsometer in a balanced detection scheme, and, as a model system for investigating the electro-optic response, we use the chromophore Disperse Red 1 in a polymer matrix of poly(methyl methacrylate). Finally, the model is compared with independent experiments on a 2,5-dimethyl-4-(p-nitrophenylazo)-anisole containing photorefractive polymer composite, and good qualitative agreement is found. (C) 2003 Optical Society of America.