THE LOW-TEMPERATURE PHOTOCHROMIC RESPONSE OF BISMUTH SILICON-OXIDE

Exposing the photorefractive material bismuth silicon oxide (BSO) at low temperatures to 2.4-3.3 eV light produces photochromic absorption bands. In undoped and Fe-doped BSO these bands appear to consist of a series of overlapping bands ranging from around 1.5 eV in the infrared to near the band edge. The infrared component is always weaker than the visible range contributions. The infrared portion anneals just above 100 K; in some samples this anneal is accompanied by the appearance of additional structure in the visible region. In undoped BSO the major anneal of the photochromic bands takes place above 200 K. If iron is present the photochromic bands are weaker and an anneal stage in the 120-150 K range appears. Bleaching with either 1.51 or 2.28 eV laser light uniformly lowered the photochromic bands in both undoped and Fe-doped BSO. In BSO:Al the aluminum electronically compensates the deep donor centers responsible for the yellow coloration observed in undoped crystals. At low temperatures, photoexcitation using near band-edge light produces the same overlapping bands at 1, 1.38, and 2.45 eV that were observed in BGO:Al. These bands anneal together between 80 and 100 K. The [AlO4]0 center which causes the coloration observed in smoky quartz is a plausible model for the photochromic bands in BSO:Al.