Nonlinear optical properties of photorefractive polymers

This paper describes the recent progress of our group in the field of photorefractive polymers. Three separate systems are discussed. The first two consist of polymers with optically nonlinear molecules attached in a pendant side-chain configuration, which are made photoconductive through the addition of a molecular charge-transport agent. The photorefractive properties are investigated using degenerate four-wave mixing (FWM) as well as two-beam coupling (2BC) techniques. The latter technique is essential for confirming the photorefractive origin of the FWM signals through the observation of asymmetric 2BC. The performance of various sensitizing agents used to generate mobile charges at long wavelengths is also compared for such systems. The third system consists of a charge-transporting polymer, which is made nonlinear through the addition of a optically nonlinear guest molecule. The photorefractive properties of this system are found to be superior of those of other polymeric systems, with diffraction efficiencies of 1% in a 125 μm thick sample for gratings written with red light (647 and 676 nm), and a 2BC gain coefficient Γ which exceeds the absorption coefficient by 7.2 cm^-1 at 753 nm. These properties are comparable to those of inorganic crystals, and should generate interest in organic photorefractive materials for device applications.