Laser induced dielectric breakdown and mechanical damage in silicate glasses.

Dielectric breakdown and mechanical damage in silicate glasses under high intensity laser radiation is investigated in detail. A Q-switched ruby laser is used to induce photoconductivity in soda glass., fused quartz and quartz crystal. 27 The number of charge carriers produced per laser pulse of 10 -2 -1 photons cm s is accounted for by multiphoton ionization of nonbridging oxygens in the silicon oxygen network. The relative magnitude of the effect proves that the photoconductivity does not result from ionization of sodium in glass as extensively quoted in literature. The lifetime of the charge carriers produced is estimated to be 10 s. Prom 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 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 electronic instability in few nanoseconds. At this electron density the absorption in the focal volume is 10 cm" , 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 - iii - 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. The study of the mechanical damage caused by a laser pulse., leads to the estimation of the surface energy of the (-material, which in the case of soda glass is found to be 10 ergs/cm The enhancement of the photoconductivity signal is obtained when -5 -4 a second laser pulse comes within the time 10 -5 x 10 sec. of the first pulse. The additional number of electrons produced by the second pulse is accounted for by the ionization of the color centers,, caused by trapping the electrons produced by the first pulse. - iv -