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, ii) change in molecular interaction and iii) local recrystallisation. The domain director rotation is considered as a result of the conservation of the angular momentum for the whole system, which includes the substrate. A new method, based on double illumination of photo-oriented films, is proposed to observe the light-induced optical axis rotation. Experimental results concerning photo-induced rotation of the optical axis in photo-oriented Langmuir-Blodgett films are presented and explained by the model. The angular dependence of the photo-induced angular torque is confirmed. The existence of an angular threshold for out-for-plane rotation (θ-rotation) is demonstrated and qualitatively explained by anchoring of molecular layers with the substrate.