Thermal Fixation of the Photorefractive Holograms Recorded in Lithium Niobate and Related Crystals

With emphasis on LiNbO 3 the basic principles, the developments over four decades, and recent applications of the thermal fixation of photorefractive holograms are reviewed. Thermal fixing is the most promising method to realize optically stable memories in photorefractive media. The microscopic origin of thermal fixing is mainly caused by screening the electronic space-charges by protons thermally activated at about 120°C. After fixing a diffraction efficiency comparable to that of untreated crystals is achieved and lifetimes of at least 2 years have been observed, whereas for special arrangements scientific sound estimates point to long terms as high as 100 years. A theoretical description is quite well established and will guide further improvements. Thermally fixed holograms have been tested in nonvolatile optical storage systems based on hologram multiplexing and are the basis in narrow-band devices with very high angular selectivity.