Surface-mediated director reorientation in a pure nematic liquid crystal (LC) cell in the presence of both a dc and low-power optical field has been observed. A surface-dependent lowering of the Friedericksz transition voltage in a planar cell upon light irradiation was found. It is believed that this effect is due to a light-induced change in the near-surface ion concentration in the presence of a dc field and a photosensitive surface. Enrichment in the ion concentration near the surface causes a redistribution of the electric field in the cell and its localization near the surface. As a consequence, the energy of interaction between the LC and the dc field near the surface overcomes the anchoring energy and results in the director reorientation.

This work concerns. the experimental study of potential possibility to use the ITOI5CB/Si/Al structures based on a low resistive silicon as liquid crystal spatial light modulator (LC SLM). It is shown, that in such type of structures there are at least two charge processes causes the opposite influence on electrooptic reaction of structure. First process is connected with ion charge accumulation at silicon surface that induces the formation of depleted surface layer in silicon by means of a field effect. Second one is due to leakage current through Si/liquid crystal interface that decreases a photosensitivity of structure. Because of sufficient large ion currents achieved in such a type of structure as well as charge accumulation the possibility exists to form the local variable area of high ion concentration in liquid crystal layer that can play a role of a "quasi"-electrode which enables to modulate a surface conductivity of silicon. Ion control of photosensitivity as well as dynamic selection of input image by the structure has been demonstrated.

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We propose a simple method for the qualitative study of charge processes in a liquid crystal layer at the silicon surface. The method is based on the field effect that has been induced by charges that are localized near the silicon surface. The method is sensitive to the sign of accumulating charge and does allow monitoring a change of the surface charge.