Basic mechanisms of optical ordering processes of azo dyes in polymer matrices are now well understood. Photo-Induced Anisotropy (PIA), Photoassisted Electrical Poling (PAEP) and All-Optical Poling (AOP) are produced by an angularly selective trans-cis photoisomerization and by the angular redistribution resulting from this reversible isomerization. They differ by the symmetry of the excitation. We present new theoretical and experimental developments on this subject. Experimentally, the problem is to extend these methods to a panel of NLO molecules wider than azobenzene derivatives. Non-photoisomerizable molecules present very small effects, probably due to a selective photodegradation. A route under investigation is to build structures associating two functions: an efficient NLO function and a diazo group to stir up the polymer. Theoretically, the model for the orientation of rod-like photoisomerisable molecules is well established. We are extending it to 2D and 3D molecules, characterized by their tensorial properties and excited by light of any symmetry (particularly octupolar symmetry). The probability of excitation is formally easy to write, but the angular redistribution process and the successive photoisomerization cycles (saturation of optical pumping) modify the tensorial quantities. In AOP, the competition between centrosymmetric and non-centrosymmetric contributions is often detrimental to the building of a good (chi) .
Michel L. Dumont,
"New developments in optical ordering of NLO dyes in polymers", Proc. SPIE 4461, Linear and Nonlinear Optics of Organic Materials, (7 December 2001); doi: 10.1117/12.449827; https://doi.org/10.1117/12.449827