Optical data storage in liquid-crystalline azobenzene side-chain polymers

Summary form only given, as follows. Among the potential materials for reversible optical data storage the liquid crystalline azobenzene side-chain polymers are especially promising due to their unique optical properties. Holograms of very high diffraction efficiency can be written (and erased) by laser illumination in this material and, equally important, the holograms are exceptionally stable. In order to clarify the mechanism responsible for these remarkable properties we have constructed a mean-field theory of the storage process. It is shown that the inter-molecular forces are essential for an understanding of the way in which holograms are gradually stored during illumination. In addition, the mean-field theory provides an explanation for the stability of the holograms. The storage process involves the creation of a birefringence grating as well as a surface grating, an example of which is shown in the figure. Experimentally, it has been shown that the surface relief forms even when the material is illuminated by two orthogonally polarized laser beams so that the intensity is completely uniform. Hence, the formation of surface reliefs cannot be due to thermal effects (a thermal driving force would require the presence of a spatially varying intensity). An alternative explanation, based essentially on the mean-field theory, is presented.