Optical Control of Orientational Bistability in Photorefractive Liquid Crystals

Materials with light-controlled properties are extremely attractive for information technologies and, among the wide range of approaches, that of photorefractive media is particularly relevant1,2. Photorefractivity is observed in photoconductors, where charge carriers photogenerated by nonuniform illumination redistribute over macroscopic distances, and where the resulting electric field modulates the refractive index. This effect has been used in a variety of devices for applications ranging from image treatment to beamsteering and medical imaging. Compared with the more commonly studied inorganic crystals, photorefractive non-crystalline organic systems3–6 offer a more technologically appealing alternative, as these materials are mechanically flexible, can be easily modified chemically and are inexpensive. Here we show how a photogenerated space–charge field can be used to control the switching between the two stable orientational states of ferroelectric liquid crystals. The resulting refractive-index patterns are stable, easily erasable and rewritable. In contrast to more traditional organic photorefractive materials, where the refractive index is continuously modulated by the applied field, the liquid-crystalline device described here is a ‘binary’ system, in which the refractive index n switches between two values (n∼3×10−2 in this case, but n of the order of 10−1 is, in principle, possible).