Photorefractive effect due to a photoinduced surface-charge modulation in undoped liquid crystals

We demonstrate that some peculiarities of the surface-induced photorefractive effect (SIPRE) in undoped nematic planar cells can be simply explained considering the electric field induced by a modulated surface-charge distribution. Polarization-dependent forced light scattering and two-beam coupling experiments indicate that the observed anisotropy of the diffraction efficiency and the energy transfer between the pump beams strongly depend on the experimental geometry. The investigation suggests that the unusual dichotomy between local and nonlocal behavior can be ascribed to a modulated longitudinal electric field component, in phase with the interference pattern, which is not accountable by the conventional photorefractivity. In a simple and general approach we demonstrate that the conceived charge distribution model for the SIPRE produces a space-charge field having two orthogonal modulated components, in-phase and pi/2 out of phase, respectively. The electric field configuration within the nematic sample gives reason for the main experimental features.