Reduction of Eu3 + to Eu2 + in Al-Codoped Silica Glasses Fabricated by the Sol-Gel Technique and CO2-Laser Processing

The spectroscopic properties of europium in aluminium codoped silica glasses produced by the sol-gel technique have been studied with respect to the dopant concentrations and the thermal processing applied to the samples. After thermal annealing at temperatures up to 950_°C the bright red fluorescence around 613 nm characteristic for the trivalent europium ions (Eu^{3 +}) has been observed. The lifetime was measured to be 0.1–2.4 ms depending on dopant concentrations and thermal treatment. Subsequent CO₂-laser processing in air (short time remelting) gave rise to a bright blue fluorescence consisting of two broad bands, lying around 450 and 490 nm, with their peak position depending on the ratio between the aluminium and europium concentrations. The fluorescence lifetimes were found to be shorter than 1 s. This blue fluorescence is attributed to the divalent europium ion (Eu^{2 +}), leading to the conclusion that the CO₂-laser processing of europium doped alumina-silica glasses resulted in the reduction of the trivalent to the divalent europium ion. Laser processing could therefore be a valid alternative to conventional thermal annealing for the generation of Eu^{2 +} in alumina-silica glasses.