Mechanisms responsible for the ultraviolet photosensitivity of SnO2-doped silica

Tin-doped silica samples were prepared by the sol-gel method. Optical absorption and electron paramagnetic resonance measurements were carded out before and after exposure to 266-nm pulsed radiation from the fourth harmonic of the Nd-YAG (yttrium-aluminum-garnet) laser. The laser-induced change of refractive index was analyzed at different power densities (from 1 to 6×107 W/cm2, 6-ns pulse duration) and exposure times. Positive changes of the refractive index up to 4×10-4 were observed. The presence and the consequent laser-induced bleaching of the 5-eV absorption band due to oxygen deficient centers, does not appear relevant, since good positive photosensitivity was observed in samples without detectable absorption at this energy.