How Defects Make Holographic Storage Media Tick

Abstract

Materials for holographic storage media were originally investigated with the perspective of the realization of high-density storage of information [1]. In recent years some other applications emerged for the same class of materials, namely optical phase conjugation devices [2], novelty filters [3], ultrasound detection [4], dense wavelength multiplexing decoders [5], and holographic optical elements for neutrons [6]. The main interest in material research was thereby focusing on photorefractive materials based on electro-optical crystals, the most widely studied representative being LiNbO3. Changes of the optical permittivity (or of the refractive index) are induced in these crystals by photoinduced space-charge fields via the so-called electro-optical effect. The most widely accepted theory for the formation of space-charge fields from defects of the crystals upon irradiation became the one formulated by Kukhtarev et al. [7]. Even today most papers on holographic storage media make reference to this standard model as ``Kukhtarev̊qs equations” [8]. In our paper we will review this famous theory [7], specify and clarify some presumptions made therein, correct some errors and include the effect of screening of the photovoltaic contribution to the space-charge field. Essentially, we rewrite the theory of Kukhtarev et al. for the generation of space-charge fields by a periodically modulated intensity pattern for the limiting case of a static situation approached as the time of holographic recording goes to infinity. In order to ease a comparison with the original version of the theory, we use - as long as it is adequate - the notation of the original work [7].