15.1 IntroductionThe idea of using holograms for storage of information was first suggested by van Heerden in 1963, who proposed to store data by recording the information carrying a light interference pattern in a holographic medium. He also predicted that the minimum volume necessary to record a bit of information is ~λ3, where λ is the wavelength used in the holographic recording. This involves an impressive density of data on the order of 10 Tbits∕cm3 for λ ~ 400 nm. In addition to high-density storage, holography permits short access times to the data since the direction of propagation of a light beam changes rapidly without inertia, unlike magnetic disk heads. Furthermore, a high data transfer speed is achieved since the complete sheet of information is recorded or read at the same time. Nevertheless, despite these advantages, after more than 40 years of research and development there are still not holographic drives in our personal computers. This is due principally to the lack of an adequate recording material.15.2 General Requirements fora Holographic Recording MediumUntil the present time, many holographic materials such as photopolymers, inorganic and organic photorefractive materials, dichromatic gelatin, silver halides, photoresists, sol-gel glasses, and thermoplastic, photochromic and photodichroic materials have been developed. Nevertheless, few of them potentially have the characteristics required for data storage applications. Among these, the most important are a sufficient thickness, high refractive index modulation, high sensitivity, excellent optical quality with low levels of scattering and absorption loss, dimensional stability during the recording of the hologram and its functioning in the memory, good thermal and chemical stability, and a moderate price.Optical thickness of the material. A sufficient thickness of the material is needed, typically on the order of several hundreds of micrometers or greater, in order to ensure the diffraction in the Bragg regimen with high angular or spectral selectivity required for the multiplexing techniques. The volume holograms are very sensitive to deviations of the Bragg resonance, and even small changes in the refractive index or the thickness of material may destroy the latter.