Birefringence measurements in low-Tg photorefractive materials by means of electro-optic relaxation spectroscopy

The maximum achievable diffraction efficiency of organic photorefractive materials is strongly influenced by the properties of the nonlinear optical chromophores. Here we report on a novel evaluation method of frequency dependent ellipsometric measurements. We fit our theory to the phase shift between the alternating electric field and the detected signal as a function of the applied frequency. The presented method is based on an improved theoretical description of electro-optic relaxation spectroscopy when compared to earlier publications. It allows for the determination of the birefringence contribution CBR to the refractive index change in materials with low glass transition temperature Tg. We apply our method to data obtained on highly efficient photorefractive guest–host systems based on polysiloxane. The nonlinear contributions were also determined and the results are briefly discussed. An error estimate for the value of CBR yields an accuracy of about ±10%. A comparison of electro-optic relaxation spectroscopy with photorefractive methods for determining the ratio CBR/CEO is presented. We conclude that this method allows for an estimate of the orientational enhancement with a relatively simple setup which is independent of the photorefractive experiment.