Optical breakdown of silicate glasses under conditions of two-photon absorption of laser radiation

An investigation was made of the optical breakdown in sodium silicate and lead silicate glasses subjected to neodymium laser radiation at the fundamental frequency and to the second, third, and fourth harmonics of this radiation. It was found that the optical breakdown threshold of both glasses decreased by a factor of SO when the photon energy became greater than half the band gap of each glass. This effect was attributed to two-photon ionization of intrinsic centers in the glass matrix. It was found that the breakdown threshold corresponded to the formation of a critical concentration of charged particles in the focal region.