MECHANISM OF OPTICAL-RECORDING IN DOPED LIQUID-CRYSTALS

A new principal mechanism underlying real-time holographic grating rec ording in a thin cell filled with a nematic liquid crystal doped with a dye is proposed. In the experiment a laser light interference patter n is mapped in the form of a spatial modulation of index of refraction . This is achieved by respective modulation of an electric field in th e vicinity of the nematic liquid crystal due to its bulk photoconducti vity and subsequent space charge field formation. In the spatially mod ulated electric field the nematic director is locally reoriented to so me degree, leading to the appearance of a phase Delta n grating due to the birefringent properties of a nematic. The externally applied elec tric field plays an important role in the whole process: it changes th e initial planar alignment of liquid crystal molecules and enhances th e photogeneration quantum yield. The role of electrical edge condition s in the cell is pointed out as they determine the field distribution in the liquid crystal, which is generally different from the space cha rge field distribution described for photorefractive materials.