On the microscopic origin of the photochromic and photorefractive behavior of doped Bi4Ge3O12 single crystals

Doping Bi4Ge3O12 single crystals with Co impurities enables the presence of a photochromic behavior which is induced at room temperature (rt) with ultraviolet (uv) light. As a consequence of the uv illumination a broad optical absorption from 1.0 to 4.1 eV is formed. This absorption is attributed to the creation and trapping of holes. The spectral efficiency of the damage and its thermal and optical stability have been characterized. Optically induced holographic gratings have been observed. They have been attributed mainly to a photorefractive effect. Some of those gratings exhibit a transient behavior that has been related to the simultaneous diffusion of electrons and holes. A microscopic model to account for the photochromic and photorefractive effects is discussed.