Subsurface structure and laser damage threshold

The laser-induced surface damage threshold of fused silica is discussed with respect to the near but below the surface zone of microcracks, strain, and dislocations. This subsurface zone was varied in thickness throughout a set of matched samples by means of interrupted controlled grinding followed by optical polishing. Laser damage testing was performed using a TEM
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beam having a wavelength of 1.06 μm, a pulsewidth of 40.5 ns, and a spot size of 147 μm. The experimentally measured breakdown electric fields were corrected using the technique of roughness normalization and plotted versus the thickness of the subsurface zone. Analysis of the data leads to the conclusion that laser-induced surface damage is dominated by the superficial polish layer and that the effect of the disturbed subsurface zone is essentially negligible.