Anomalous intensity dependence of optical reorientation in azo-dye-doped nematic liquid crystals

The intensity dependence of optical nonlinearity in a nematic liquid crystal doped with an azo-dye is investigated. The reorientational part of the nonlinearity changes from self-defocusing to self-focusing character while the intensity passes through the saturation value of trans-cis photoisomerization. This observation, in accordance with previous models, indicates that the optical torque generated by the trans-isomers is of opposite sign than the torque generated by the cis-isomers. At very low intensities a further reorientational nonlinearity was found, which is attributed to light-induced orientational redistribution of adsorbed molecules at the surface.