Towards Stable Materials for Electro-Optic Modulation and Photorefractive Applications

A large effort have been devoted to the preparation of organic polymeric materials for electro-optic modulation [1, 2, 3]. These materials contain push-pull chromophores (i.e. combining electron-releasing and electron-withdrawing groups interacting through a conjugated linker) either incorporated as guest in the polymeric matrix (doped polymers) or grafted onto (or into) the polymeric matrix (functionalized polymers) [45]. By heating the polymer above its glass transition temperature (T g ), orientation of the dipolar chromophores can be achieved via application of a strong external electric field. After cooling to room temperature, noncentrosymmetrical poled-polymeric materials are obtained. Such materials are interesting in view of both processability and chemical flexibility that allows for “engineering” of the nonlinear responses. When optimized chromophores are incorporated and poled in polymers, materials with large electrooptical coefficient can in principle be obtained. However, besides the optimization of chromophores aiming at obtaining large molecular figure of merit (EOM), a number of additional issues are to be considered if materials with both large and permanent nonlinear responses are to be achieved. For instance, an interesting way to improve the orientational stability consists in using polymers with very high T, (such as polyimides for instance) [6]. However, this demands organic chromophores with high thermal stability. Also, increasing the chromophore concentration in order to have larger χ (2) (and electro-optic coefficient) values can be detrimental since dipolar interactions between push-pull chromophores can favor antiparallel association thus impeding the poling process, as pointed out by Dalton and coworkers [7].