Semiconductor CdF₂:Ga and CdF₂:In Crystals as Media for Real-Time Holography

Monocrystalline cadmium fluoride is a dielectric solid that can be converted into a semiconductor by doping with donor impurities and subsequent heating in the reduction atmosphere. For two donor elements, Ga and In, the donor ("shallow") state is a metastable one separated from the ground ("deep") state by a barrier. Photoinduced deep-to-shallow state transition underlies the photochromism of CdF₂:Ga and CdF₂:In. Real-time phase holograms are recorded in these crystals capable of following up optical processes in a wide frequency range. The features of photochromic transformations in CdF₂:Ga and CdF₂:In crystals as well as holographic characteristics of these media are discussed. Exemplary applications of CdF₂-based holographic elements are given.