Photorefractive effects in organic glass

In most organic photorefractive systems discrete components are assigned for each function, i.e., a photochemical charge generating species, a transport media, a nonlinear chromophore and, in some cases, even trapping species. All these components are mixed with, attached to, or otherwise constitute some kind of polymer binder that may also contain plasticizers to control the glass transition temperature. These systems are obviously complex mixtures that may sometimes suffer from heterogeneity, phase separation, and resulting poor linear optical properties due to scattering and other fluctuations. A general goal then is to design compositions that contain a minimum number of discrete components and this can be accomplished by designing molecules which are multifunctional and perform more than one of the requisite tasks. It is even possible to eliminate the polymer altogether and form high optical quality monomeric molecular glasses from specially modified chromophores. It turns out that glass formation in organic monomers is really not that uncommon. However, the optimization of monomer glasses to enhance the temperature range and stability of the glass phase entails understanding and control of the appropriate structure features such as hydrogen bonding and conformational flexibility. For photorefractive applications highly birefringent chromophores in the pyridone and methylenedihydropyridine families were identified and by appropriate structure modifications were converted to highly stable glasses. The characterization of the electronic and structural properties of these of these materials and other related chromophores to provide useful photorefractive media is being actively pursued.