Field Aided Optically Induced Nonlinear Optical Effects in Liquid Crystals Photorefractivity

DC FIELD AIDED OPTICALLY INDUCED NONLINEAR OPTICAL EFFECTS IN LIQUID CRYSTALS PHOTOREFRACTIVITY 

As one can see from the preceding discussions on optical field induced director axis reorientation in hquid crystals, the torque exerted by the optical field on the director axis is basically quadratic in the field amplitude. Except for its dispersion influence on the optical dielectric constant 8(co), the frequency of the electric field is basically not involved. Furthermore, if two or more fields are acting on the director axis, the resulting torque exerted on the director axis is simply proportional to the square amplitude of the total fields. Accordingly, it is possible to enhance the optical field induced effect by application of a low-frequency ac or dc electric field, much as the optically addressed liquid crystal spatial light modulator discussed in Chapter 6. In the latter, the responsible mechanism is the photoconduction generated by the incident optical field in the semicondnctor layer adjacent to the liquid crystals. 

In this section, we discuss a mechanism for enhanced optical nonlinearity in which the photoinduced charges and fields occur within the liquid crystal. It is well known that dc field induced cmient flow in nematic liquid crystals, which possess anisotropic conductivities, could lead to nematic flows and director axis reorientation, and to the creation of a space-charge field. The charged carriers responsible for the electrical conduction come from impurities present in the otherwise purely dielectric nematic liquid crystal. If these impurities are photoionizable, an incident optical intensity [e g., an intensity grating created by the interference of two coherent optical beams (see Fig. 8.13)], it is possible, therefore, to create a space-charge 

The sign and magnitude of the effective electro-optic coefficient Yeff depends on the symmetry class of the ciystal and the direction of the electric field. 

Nematic liquid crystals, on the other hand, possess centrosymmetiy. The field-induced refractive index change is quadratically related to the electric field, that 

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