PHOTOREFRACTIVE POLYMERS - A STATUS-REPORT

A photorefractive system is one which is simultaneously photoconductiv e and electrooptic. Diffraction gratings or holograms can be produced in a photorefractive material by the photogeneration, drift or diffusi on and subsequent trapping of mobile charges. The grating is produced by the internal space charge field set-up by these charges which, via the electrooptic effect, produces an index of refraction grating. Unti l 1990 all photorefractive systems were inorganic crystals such as LiN bO3, BaTiO3, B12SiO20, InP:Fe, GaAs, or multiple quantum well material s. In 1990, the first observation of the photorefractive effect in an organic material, a carefully grown, doped molecular crystal was descr ibed. This was followed in short time by the discovery of photorefract ive polymers. The early photorefractive systems, polymeric and crystal line, were inefficient compared to inorganic systems but in recent yea rs the efficiency and sensitivity of photorefractive polymers have bec ome equal to inorganic crystals. This paper describes the origins of t he photorefractive effect and the design, synthesis and characterizati on of photorefractive polymers. In addition an orientational enhanceme nt of the photorefractive diffraction efficiency is discussed. This or ientational enhancement does not occur in crystalline systems, since i t relies on the ability of the optically nonlinear chromophores to be aligned not only by an externally applied field, but also by the sinus oidally varying space charge field produced during photorefractive gra ting formation.