Dynamical study of photoinduced anisotropy and orientational relaxation of azo dyes in polymeric films: poling at room temperature

The reversible photoisomerization of azobenzene derivatives is known for its anisotropic properties. We study the dichroism and the anisotropy induced by a linearly polarized pump in DR1 doped PMMA films and in a copolymer of PMMA and DR1. We demonstrate the competition of three processes, the angular hole burning (AHB), the orientational redistribution, and the orientational relaxation. The three processes are included in a theoretical model which can be simplified by using symmetry properties (isotropic polymer matrix): the time behavior of anisotropy is well explained by this theory. In the last part, we demonstrate a novel method for poling azodye-containing films, at room temperature, by optical pumping in the presence of a dc electric field: large (chi) (2) are obtained. In the guest-host system, when the pump is switched off, the greatest part of the photoinduced anisotropy and of the orientation is destroyed by the orientational relaxation (non exponential relaxation with a rate from seconds to hours). On the contrary, in the copolymer, there is almost no relaxation and it is possible to write and erase information in the form of molecular alignment. Similarly photoassisted poling produces a quasi permanent (chi) (2), in the copolymer (a r33 of 5.7 pm/V have been measured, 2 days after poling).