Photochromic response of bismuth germanium oxide doped with chromium

Doping the photorefractive material bismuth germanium oxide Bi12GeO20(BGO) with chromium significantly affects its optical properties. An absorption spectrum taken at 300 K shows that the introduction of chromium produces a strong unresolved band which overlaps the absorption cutoff of BGO and a series of smaller overlapping bands between 650 and 1100 nm. Cooling BGO:Cr to 10 K sharpens the small bands and resolves the strong band into a peak near 510 nm. This peak is responsible for the reddish-brown color of BGO:Cr. Excitation at room temperature with visible light increases the bands in the 650-1100 nm range. The production of this additional room temperature photochromic absorption is most efficient with excitation near 490 nm but extends throughout the visible. Chromium occupies the tetrahedral germanium site in BGO; and the observed spectra are consistent with a Cr4+ tetrahedral state. During crystal growth some of the chromium gives up an electron to the antisite bismuth native defect and becomes Cr5+. Exposing the sample to light returns the electron to the chromium and increases the Cr4+ absorption. The photoinduced absorption is thermally stable for temperatures up to about 425 K. This stability suggests that doping BGO with Cr should lead to room temperature persistent photorefractive gratings. (C) 2001 American Institute of Physics.