Optical two-beam coupling for a surface-induced photorefractive effect in undoped liquid crystals

We report beam-coupling measurements of undoped nematic planar cells, for which the surface-induced photorefractive effect (SIPRE) has been confirmed to be due to photoelectric interface activation. We show that the energy transfer strongly depends on experimental geometry. Diffraction efficiency and beam-coupling analyses suggest that the unusual local and (or) nonlocal behavior can be ascribed to an additional in-phase-modulated longitudinal electric field component, unaccountable by standard photorefractivity. We consider a simple charge distribution model of the SIPRE, which produces a space-charge field having two orthogonal modulated components, in phase and pi/2 out of phase. Through the liquid crystal's electric response, the exploited geometries verify the presence of the two components.