The role of laser-induced primary defect formation in optical breakdown of NaCl

Rapid and efficient formation of primary defects (
V_{k}-, F
-, 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 ps- 30 ns) laser pulses at the
\lambda = 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 offset 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 was found to be a slight reduction in the damage threshold and a marked decrease of the free carrier densities required for damage to occur.