Friction model of Photo-induced Reorientation of Optical Axis in Photo-oriented Langmuir-Blodgett Films

A model of the Photo-induced Optical Anisotropy (POA) is proposed for molecular films consisting of ordered domains. The model describes the molecular motion in an anisotropic molecular field at different rates of collisions (friction) between, respectively, excited and ground state molecules and a substrate under polarized light illumination into a molecular absorption band. It is shown that if an angular distribution function of domain molecules has a non-zero width, the different friction coefficients would result in domain director rotation. At low light intensity, these different frictions could be due to three processes: i) change in molecular conformation, it) change in molecular interaction and iii) local recrystallisation. The domain director rotation is considered as a result of the conservation of the angular momentum