Photorefractive properties of a thiapyrylium-dye-sensitized polymer composite: the dynamics of holographic grating formation and erasure

We present the photorefractive properties of a new organic composite operating at near IR wavelengths. The composite contains an ionic thiapyrylium dye, 4-(4-dimethylaminophenyl)- 2,6-diphenyl thiapyrylium 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 which also serves as a charge transporting agent. Both degenerate and nondegenerate four-wave mixing experiments have been carried out and holographic diffraction efficiencies as high as 2% have been achieved. Asymmetric energy transfer has been observed with a gain of 7 cm-1 in a two-beam coupling experiment. This paper focuses on the dynamics of the formation and erasure processes of the holographic gratings. Both the writing and erasure times exhibit sublinear dependencies on the illumination power intensity, which is a result of the sublinear photoconductivity of the material. The dependence of the 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.