Anisotropic materials, polarized light effects

The most useful of the known photorefractives are LiNbC>3 and BaTiC>3. Both are ferroelectric materials. Light absorption, presumably by impurities, creates electron/hole pairs within the material which migrate anisotropically in the internal field of the polar crystal, to be trapped eventually with the creation of new, internal space charge fields which alter the local index of refraction of the material via the Pockels effect. If this mechanism is correct (and it appears established for the materials known to date), then only polar, photoconductive materials will be effective photorefractives. However, if more effective materials are to be discovered, a new mechanism will probably have to be discovered in order to increase the speed, now limited by the mobility of carriers in the materials, and sensitivity of the process. 

The material presented so far was based on the creation of anisotropic distribution of molecular angular momenta under the direct effect of light absorption. We are now going to discuss briefly some ideas and examples of experimental realization of other methods leading to the production of polarized molecules, including those which are not directly connected with light effects, such as polarization caused by collisions and external electric or magnetic fields. 

The aligned-DNA, transparent, self-standing, and flexible film is of interest as a new naturally-occurring functional material, as well as an anisotropic conductive film. For example, the DNA-lipid film is effective as an adsorption filter of carcinogens such as acridine orange and ethidium bromide. The aligned-DNA film also shows polarization of light. 

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