Optically induced anisotropy of third-order susceptibility in azo-dye polymers

Near-resonant optical pumping is applied for encoding the anisotropy of third-order susceptibility χ(3) in a Disperse Red 1 poly (methyl methacrylate) polymer thin film. The encoding process involves one- or two-photon excitation that results in trans–cis isomerization of azo molecules. Third-harmonic generation from the sample is used to probe the χ(3) anisotropy. We demonstrate large reversible variations of the third-harmonic generation (several times in intensity) in a sample pumped by the second harmonic of a Nd:YAG laser or a cw He–Cd laser. A microscopic model, which includes mechanisms for angular hole burning and molecular reorientation, is presented. The model is in good qualitative agreement with the experimental results.