Compaction versus expansion behavior related to the OH-content of synthetic fused silica under prolonged UV-laser irradiation

The behavior of different types of synthetic fused silica under prolonged 193 nm excimer laser irradiation at very low energy densities was investigated. Irradiation experiments at energy densities below 50 μJ/cm2 for tens of billions of laser pulses were performed. Effects of changes in the density in the irradiated region of the samples were measured at 633 nm, both with an interferometer as an optical path difference and as the distribution of stress induced birefringence. Samples of synthetic fused silica of type III and IIIa with different levels of hydroxyl-content and different levels of molecular hydrogen content were manufactured. Depending on the composition of the fused silica samples, compaction as well as rarefaction was observed. The OH-content in the sample is found to be the dominating factor in the expansion-versus-compaction behavior. A qualitative model is postulated in order to describe the dependence on the OH-content. The induced optical path difference as a function of energy density and number of laser pulses is described with a phenomenological model function.