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

Near-resonant optical pumping is applied for encoding the anisotropy of thi rd-order susceptibility chi ((3)) in a Disperse Red 1 poly (methyl methacry late) polymer thin film. The encoding process involves one- or two-photon e xcitation that results in trans-cis isomerization of azo molecules, Third-h armonic generation from the sample is used to probe the chi ((3)) anisotrop y. We demonstrate large reversible variations of the third-harmonic generat ion (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 present ed. The model is in good qualitative agreement with the experimental result s. (C) 2001 Optical Society of America.