It was shown that the bistability and metastability of In and Ga impurities in the fluoride crystals could be used for volume holographic gratings. The photo-transformation of these impurities, from the localized to the hydrogenic state, led to a large change in the local polarizability of the host, and thus to a significant change in the refractive index. The magnitude of the refractive index changes were related to photo-induced metastable deep-shallow transitions.

B.Koziarska, J.M.Langer, A.I.Ryskin, A.S.Shcheulin, A.Suchocki: Materials Science Forum, 1995, 196-201, 1103-8