Extremely sensitive light-induced reorientation in nondoped nematic liquid crystal cells due to photoelectric activation of the interface

We report an investigation of the extremely sensitive molecular reorientation in pure nematic liquid crystal film induced by the combined application of low dc electric field (less than 0.1 V/mum) and very low intensity optical irradiation (few mW/cm(2)). The effect is observed in planar cells of well-known commercial nematic mixture (E7) aligned with rubbed polyvinyl alcohol layers, which exhibit photorefractive-like effect. We analyze the dependence of the photoinduced changes in birefringence upon the applied dc voltage and the light intensity. According to our results we believe that the effect is due to photoinduced recombination of the opposite charged carriers accumulated near the interface. In the low dc voltage regime (a few volts) the voltage mainly drops on the electric double layers at the interfaces as a consequence of dc field collected charge carriers from liquid crystalline and polymeric films to the border surfaces. Irradiation with appropriate wavelength reduces the interfacial charges density, because of photoinduced carrier injection and recombination processes, and consequently, induces a relocation of the electric field from the surface to the liquid crystal bulk. The light-induced additional electric field component in the nematic film results in a lowering of the Freedericksz threshold or an enhanced molecular reorientation.