The Role of Laser-Induced Primary Defect Formation in Optical Breakdown of NaCl

Abstract

Rapid and efficient formation of primary defects (Vk-, F-1, and H-centers, self-trapped excitons) influences the kinetic processes involved in laser-induced optical breakdown in alkali halides. These processes have been studied for the case of NaCl exposed to short (30 psec to 30 nsec) laser pulses at λ = 532 nm wavelength. Rapid defect formation acts as a drain on the free electrons and holes, produced by multiphoton absorption in the laser photon field. As a consequence, the rate of temperature increase of the lattice by free or “dressed” carrier absorption is reduced and, thus the damage threshold is increased. This effect is compensated by additional channels of energy deposition to the lattice which are provided by formation of and photon absorption by the generated primary defects. The net effect of the photochemical processes of primary defect formation is a slight reduction in the damage threshold and a marked decrease of the free carrier densities required for damage to occur. Beam deformation, caused by free-carrier induced changes of the dielectric function, should therefore be less pronounced than that predicted by model calculations of laser damage in the absence of primary defect formation.