Excimer laser-induced expansion in hydrogen-loaded silica

High-purity silica that contains a high concentration of dissolved molecular hydrogen is found to undergo expansion, rather than densification, as a consequence of exposure to deep ultraviolet irradiation. Concurrent with the decreased density, we suggest that a positive photorefractive effect occurs, which increases the index. The contributions of the density change and the photorefractive effect can be observed by comparison of the birefringence and the wave front change obtained through interferometry. Expansion and the photorefractive effect are interpreted in terms of the photolytic formation of SiOH.