Photoinduce dichroism as a tool for understanding orientational mobility of photoisomerizable dyes in amorphous matrices

In amorphous materials, the excitation of photoisomerizable dye molecules (most often azo dyes) by polarized light induces a more or less permanent anisotropy (dichroism and birefringence). Photoinduced anisotropy (PIA) is the result of the competition of three processes: (1) Angular hole burning (AHB) by polarized light; (2) Angular redistribution (AR) during the photoisomerization, the lifetime of the photo-isomer and the (spontaneous or photoinduced) return to the stable isomeric structure; (3) Thermal diffusion in the ground state. Photoassisted electrical poling (PAEP) and all-optical poling (AOP) proceed from the same mechanisms, but with different symmetries. In order to study optical ordering mechanisms and to characterize materials for photonic applications, a multiple wavelength experimental setup is used for recording the dynamics of the growing and of the relaxation of photoinduced dichroism. Results are interpreted with the help of a simple phenomenological model based on diffusion and pumping rates. Several examples are presented which illustrate the ability of this method for characterizing the orientational behavior of dye molecules.