Laser-induced generation of structural defects in vitreous silica and in activated silicate glass

Ruptures of a silica network in a 1.06 μm-laser irradiated vitreous silica and activated silicate glass were observed by infrared and Raman spectroscopy. Irradiation conditions were so chosen that the laser beam should not produce visible traces of damage in the sample bulk or on the surface. The formation of broken chemical bonds was found not to be of threshold character, but the effect increases sharply as the power density in a laser pulse grows. The dependence of the concentration of newly formed ruptures on the number of repeated pulses exhibits a saturation and represents a kinetic curve of chemical bond destruction in a glass. It is assumed that the accumulation process is the cause of macroscopic breakdown of transparent dielectrics under repated laser irradiation.