Polymeric Thin Films for Electro-Optic Modulator and High Density Optical Memory Applications
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THIN
FILMS
FOR
ELECTRO-OPTIC
The electro-optic or Pockel's effect requires that chromophores characterized by finite first molecular hyperpolarizability be arranged in a noncentrosymmetric manner in the macroscopic material lattice.
Three general approaches have been used to obtain
such macroscopic
noncentrosymmetric order [1,2]: (1) Molecular self-assembly methods including crystal growth, exploitation of inclusion complexes, utilization of ordered phases including liquid crystalline materials and layered materials, synthesis of asymmetric dendritic materials, and exploitation of asymmetric block copolymer assembly. (2) Sequential synthesis methods exploiting Van der Waals, covalent, and ionic interactions have been used effectively to obtain highly ordered submicron films but the sensitivity of this approach to defects has inhibited the realization of films of thickness greater than a micron. Sequential synthesis methods including Langmuir-Blodgett film fabrication methods [3], molecular beam epitaxy methods [4,5], Merrified-type covalent coupling reactions [6], and inorganic coupling reactions [7]. (3) External force techniques such as electric field poling and laser assisted poling have provided the most general approach to the preparation of second order nonlinear optical materials. This latter approach will be the primary focus of this article. The most commonly employed external force method of achieving chromophore alignment is that of electric field poling of chromophore containing polymers near the glass transition temperature of the polymer matrix. Nonlinear optical chromophores have been incorporated into polymers as composites, as covalently coupled pendants to polymer main chains, as covalently incorporated components of polymer backbones, and as crosslinking elements coupling two polymer chains. While the use of polymer composites has been favored by some because commercially available chromophores and polymers can simply be mixed together, a number of problems are encountered with the use of polymer composites including (1) poor solubility of the chromophore in the host polymer leading to poor chromophore number density, (2) poor coupling of chromophore dynamics to that of the host polymer, (3) phase separation and chromophore aggregation with processing and aging, (4) sublimation of chromophores with high temperature processing, and (5) 43
Mat. Res. Soc. Symp. Proc. Vol. 392 © 1995 Materials Research Society
plasticization of polymer with increased chromophore loading. Because of these problems, there has been an increasing trend to utilize covalent incorporation of chromophores [2,8]. For nonlinear optical materials to be useful for device applications, they must exhibit both long and short term thermal stability. A bench mark of appropriate long term thermal stability can be defined as stability of optical nonlinearity for 1000 hours at 100'C while a typical short term stability benchmark would be full retention of optical nonlinearity for 5 minutes at 200'C. To achieve these benchmarks
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