Effect asymmetry of diffraction efficiency in LC cells with different command surfaces

We report the new results on the measurements of diffraction efficiency in liquid crystal (LC) cells with different orienting surfaces in the presence of a DC electric field. Diffraction gratings were recorded in combined cells with E7 liquid crystal. We used LC cells with thickness of similar to 10 mu m. Gold nanoparticles, chalcogenide and ITO nano-surfaces were used as command surfaces. Diffraction gratings were formed during irradiation of LC cells by 2-beam spatially modulated laser light (lambda = 532 nm) with linear p-polarization in experiments of two-wave mixing. As a result, we found an asymmetry in the diffraction efficiency of the recorded gratings in different cells, which depends on the type of command surface and the polarity of applied voltage. The obtained results are appropriately described in the terms of surface-induced photorefractive mechanism, when a diffraction grating arises due to the formation of an unstable charge in pure liquid crystal. We believe that unstable charge depends on a surface on which different types of charge carriers are excited by light.