LASER-INDUCED PHOTOCONDUCTIVITY IN SILICATE GLASSES BY MULTIPHOTON EXCITATION, A PRECURSOR OF DIELECTRIC BREAKDOWN AND MECHANICAL DAMAGE

Abstract

A Q-switched ruby laser is used to induce photoconductivity in soda glass, fused quartz, and quartz crystal. The number of charge carriers produced per laser pulse of 10²⁷ photons cm⁻² s⁻¹ is accounted for by multiphoton ionization of nonbridging oxygens in the silicon oxygen network. The relative magnitude of the effect proves that photoconductivity does not result from the ionization of sodium in glass as extensively quoted in the existing literature. The lifetime of the charge carriers produced is estimated to be 10⁻⁵ s. Arguments to the effect that this photoconductivity is the primary cause of dielectric breakdown and damage in glasses under excitation by a Q-switched laser are presented.