Laser-induced dielectric breakdown and mechanical damage in silicate glasses

Abstract

Dielectric breakdown and mechanical damage in silicate glasses under high-intensity laser radiation are investigated in detail. From thermoelastic considerations a criterion for the validity of possible damage mechanisms is established. It is shown that stimulated Brillouin scattering cannot give rise to an effective absorption of 50 cm−1 in the focal volume as required by the thermoelastic considerations. It is proposed and established that the mechanical damage is caused by the acceleration of primary electrons produced by multiphoton ionization, leading to a fully developed instability in a few nanoseconds. At this electron density, the absorption in the focal volume is 105 cm−1, and is responsible for the complete absorption of the laser pulse at intensities above the threshold for breakdown. The diffusion and recombination of electrons are found to be negligible, the only rate-limiting process being the loss of electron energy to the lattice. The variation in the threshold intensity for breakdown in different glasses is due to the variations in the elastic scattering cross section.