Induced density changes in 193-nm excimer-laser-damaged silica glass: a kinetic model

Silica glass exposed to pulsed UV excimer laser irradiation undergoes optical changes that can include either an optical path increase or a decrease. During a given exposure the sign of the induced optical path change can reverse as a function of pulse count. The reduced optical path and sign reversal are only observed in H-2-containing glasses, and at high exposure fluence only optical path increase is observed. In past work(1,2,3) we proposed an induced density change model invoking a dynamic equilibrium density to explain the high fluence experiments. Here we present a model that extends the density model to the low fluence regime by allowing the equilibrium density to be a function of the time-dependent break-up of the silica network during exposure. The network break-up is tracked by calculation of the induced SiH concentration in the glass. The agreement of optical path change obtained from experimental data with that deduced from the kinetic approach covers a wide range of exposure fluence and molecular hydrogen concentration. Using the model one can predict the change in optical path that arises from the excimer laser exposure.