Measurements have been made on intrinsic optical bulk breakdown in ten alkali halides at 1.06 microm and in one at 0.69 microm. By comparing the results to previously reported experiments conducted at 10.6 microm and at direct current, it has been possible to identify the damage mechanism as electron avalanche breakdown. Self-focusing has been controlled by restricting the probe powers to well below the critical powers for catastrophic self-focusing, and damage from inclusions has been distinguished from intrinsic damage. Implications of this work for surface damage studies are explored.

Recent experimental investigations into laser−induced damage in the alkali halides have indicated that the damage is due to avalanche breakdown in its dc limit. Numerical computations for NaCl have been carried out that show that the ionization rate can be accurately computed by a classical diffusion or Fokker−Planck approximation to the Boltzmann equation for ionization rates on the order of 1012 sec−1. For hot electrons it is necessary to add a contribution to the collision frequency that is due to the deformation potential in order to explain the dc character of breakdown by 1.06−μm radiation.

Field strengths at which optical damage is initiated in NaCl have been measured with a mode-locked Nd: YAG laser with pulse durations of 15 and 300 psec. Comparison with previously reported data with a Q-switched laser shows that the field strength required for intrinsic optical damage increases by almost one order of magnitude from 106 V/cm at 10−8 sec to over 107 V/cm at 1.5 × 10−11 sec. This is in qualitative agreement with published estimates based on the electron avalanche breakdown mechanism.

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