Photorefraction electro-optic effect

The electro-optic effect leads to the modification of the refractive index of a suitable material when an electric field is applied (Figure 8). The electro-optic effect must be present in a photorefractive material, so that the space charge electric field pattern due the relocated charges will lead to a patterned refractive index in the material this is a hologram. 

Figure 9. Conventional model of photorefraction in crystals iron impurity forms defect states of variable valence within the forbidden band gap of a lithium niobate crystal. Optical excitation of the divalent state leads to creation of a mobile electron in the conduction band. This is able to move and recombines with a trivalent iron impurity at another location which becomes divalent. The displacement of charge leads to an electric field and the Pockels electro-optic effect leads to local modification of the refractive index.

We have prepared multi-component photorefractive polymers for optical data storage applications (8). This photorefractivity is a combination of two functionalities electro-optic effect and photoconductivity. We have achieved holographic diffraction efficiencies up to >30% in our composites. 

As mentioned above, photorefractivity is a multifunctional property which is produced by the combined action of photoconductivity and nonlinear optical properties of a material (24). It is a result of the functions of photogeneration, transport, charge trapping, and the electro-optic effect. 

The material has to simultaneously possess photoconductivity and electro-optical effect to have photorefractive properties. Typical candidate materials have low glass transition temperature, frequently reduced by the plasticizer. Diffraction efficiency is improved by addition of the plasticizer because chromophore groups have higher rotational mobility and increase their contribution of birefringence to the total refractive index modulation. ... 

Poly(N-vinyl carbazole), PVK, is a glassy polymer which easily fails by a cracking fracture. Plasticizers are needed to improve mechanical properties, reduce viscosity, and lower processing temperatnre. PVK is employed in photorefractive systems and as such it must possess photoconductivity and electro-optic effects. These ate related to the crystallization and the glass transition temperature. ... 

Note 3 A polymer that exhibits a nonlinear optical effect due to anisotropic electric susceptibilities when subjected to electric field together with light irradiation is called an electro-optical polymer. A polymer that exhibits electro-optical behavior combined with photoconductivity is called a photorefractive polymer. 

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