PHOTOREFRACTIVE PROPERTIES OF A THIAPYRYLIUM-DYE-SENSITIZED POLYMER COMPOSITE

The detailed photorefractive properties of a thiapyrylium-dye-sensitized photorefractive polymeric composite are presented. The composite contains an ionic thiapyrylium dye, 4-(4-dimethylaminophenyl)-2,6-diphenylthiapyrylium Perchlorate, as a photocharge generation sensitizer, 4-(N,N-diethylamino)-(beta)-nitrostyrene as a second-order nonlinear optical chromophore, and poly(9-vinylcarbazole) as a host polymer as well as a charge transporting agent. The electrooptic and photoconductive properties of the composite have been studied and an apparent sublinear dependence of the photoconductivity on the illumination power density has been observed. Both degenerate and nondegenerate four-wave-mixing experiments have been carried out and a holographic diffraction efficiency of 1.1% has been achieved in a 160-mum-thick film at an external field of 62.5 V/mum. A nonlocal character of the photorefractivity has been established through the asymmetric energy transfer with a gain of 7 cm-1 in a two-beam coupling experiment. The dynamics of the formation and erasure of the holographic gratings has been studied. The rise time and erasure time for the space-charge field have been measured as a function of the writing and erasure beam intensities, respectively. The sublinear character of these dependencies can be explained by the apparent sublinear photoconductivity of the composite. The dependence of the grating rise time on the applied electric field is attributed to the field dependence of both the photocharge generation quantum efficiency and the charge mobility in the polymer matrix. The application of this material to information storage and image processing has been explored with demonstrations of holographic image recording and retrieval.