Densification of synthetic fused silica under ultraviolet irradiation

Fused silica undergoes densification upon prolonged exposure to high energy radiation and explanations in terms of changes in the silica tetrahedral ring structure have been offered. Under laser irradiation, the densification is often less than 10 parts per million, necessitating sensitive optical techniques for its measurement. This paper reviews some of the key literature, with emphasis on recent studies involving 193 nm excimer laser irradiation, and presents recent evidence that the UV laser-induced densification involves optically-induced weakening of bonds, via a two-photon absorption process, and subsequent structural relaxation of the glass. (C) 1997 Elsevier Science B.V.