Dynamic grating features for surface induced photorefractive effect in undoped nematics

We investigate the highly sensitive surface-induced photorefractive gratings, observed in undoped nematic planar cells for low dc voltage (few volts) and laser intensity (few milliwatts per square centimeters). Forced-light-scattering and two-beam coupling measurements on E7 nematic samples, aligned with rubbed polyvinyl alcohol layers, verify the orientational effect of a space-charge field and the crucial role of the polymer-liquid-crystal interface in the photoinduced processes, through a wavelength-dependent photoelectric activation. The experimental behavior supports the surface-induced photorefractive effect model, according to which light with the proper wavelength locally reduces the interfacial, charge density, thus producing a space-charge electric field in the cell.