Photoassisted poling of spiropyrans in polymer films-experiments and theoretical model

Three years ago, we demonstrated Photo-Assisted Poling1,2 (PAP), a new optical technique for breaking the centrosymmetry of some dye-doped polymer films, in order to induce Second Harmonic Generation (SHG) and Electro-Optic (EO) properties. The first experiments have been performed with azobenzene derivatives, particularly Dispersed Red 1 (DR1), in which the photoassisted poling is due to the Trans-to-Cis photoisomerization (N=N double bond). When molecules relax back from the bent Cis shape to the linear Trans state, their orientation is modified: this process induces an anisotropic angular distribution of molecules, because of the anisotropy of the pumping light and a noncentrosymmetric distribution, if a DC electric field is applied during the optical pumping. In PMMA-DR1 copolymer films, PAP has produced large and stable χ(2).