Doping the photorefractive material bismuth germanium oxide Bi12GeO20(BGO)
with chromium significantly affects its optical properties. An absorption s
pectrum taken at 300 K shows that the introduction of chromium produces a s
trong unresolved band which overlaps the absorption cutoff of BGO and a ser
ies 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 nea
r 510 nm. This peak is responsible for the reddish-brown color of BGO:Cr. E
xcitation at room temperature with visible light increases the bands in the
650-1100 nm range. The production of this additional room temperature phot
ochromic absorption is most efficient with excitation near 490 nm but exten
ds throughout the visible. Chromium occupies the tetrahedral germanium site
in BGO; and the observed spectra are consistent with a Cr4+ tetrahedral st
ate. During crystal growth some of the chromium gives up an electron to the
antisite bismuth native defect and becomes Cr5+. Exposing the sample to li
ght returns the electron to the chromium and increases the Cr4+ absorption.
The photoinduced absorption is thermally stable for temperatures up to abo
ut 425 K. This stability suggests that doping BGO with Cr should lead to ro
om temperature persistent photorefractive gratings. (C) 2001 American Insti
tute of Physics.