The low-temperature photochromic response of bismuth germanium oxide

Abstract

Exposing the photorefractive bismuth germanium oxide (BGO) at low temperatures to 2.5–3.3 eV light produces photochromic absorption bands. In both undoped and Fe-doped material the absorption consists of overlapping bands at 1.5, 2.2, 2.7, and 3.1 eV. The 1.5- and 2.2-eV components are relatively weaker in Fe-doped BGO. In undoped BGO the photochromic bands anneal together above 200 K. Doping with iron adds an anneal stage in the 110–200 K range that matches the recovery of Fe3+. Optical bleaching with 1.5-eV light was much less efficient than with visible band light. Thus, the 1.5-eV photochromic band may be an internal transition of the center responsible for the 2.2-eV band. In BGO:Al the aluminum electronically compensates the deep donor center responsible for the yellow coloration observed in undoped crystals. Photoexcitation at 9 K produces overlapping absorption bands at 1, 1.38, and 2.45 eV. All three bands have major anneal stage in the 80–100 K range. They also bleach out together when the sample is exposed to either infrared or visible range light. The [AlO4]0 center which causes the coloration observed in smoky quartz is a plausible model for the visible range photochromic center in BGO:Al.