The role of surface charge field in two-beam coupling in liquid crystal cells with photoconducting polymer layers

In liquid crystal cells with photoconductive poly(N-vinlyl carbazole) polymer layers, an external dc field can be completely screened by surface charge layers that develop at the liquid crystal-polymer interface. Under spatially modulated illumination, surface charge layers can be discharged in bright areas and lead to reorientation and spatially modulated Freedericksz transition. As a result, an asymmetric energy exchange in the photorefractive two-beam coupling process can take place. We propose a model to explain the origin of reorientation and phase shift in the two-beam coupling process, based on the profile and tilt of the refractive index grating. We also show that cells with just one photoconducting layer are more efficient than a typical design with two layers. (C) 2004 American Institute of Physics.