Charge emission and related precursor events associated with laser damage

Charge emission and surface photoconductivity were used to investigate possible precursors to laser damage, including surface cleaning and hardening. Experiments were conducted with a 1.06 micron ND:YAG laser with 5 nsec pulses. A wide range of samples were investigated, including diamond-turned copper mirrors, silicon wafers, and half-wave dielectric films of ThF/sub 4/, Al/sub 2/O/sub 3/, and Ta/sub 2/O/sub 5/ on fused silica. Charge emission at 1/10 of the single pulse damage threshold was observed for the copper mirrors. Declining emission for successive pulses was characteristic of surface cleaning. Emitted charge quantities increased monotonically with fluence prior to damage, but the charge emission was noisy with site-to-site variations. For silicon, damage was simultaneous with charge emission for both 1-on-1 and N-on-1 tests. The damage morphologies for the two cases were distinctly different. Nonlinear absorption measurements were conducted on silicon for fluences below the single pulse damage threshold. The two oxide dielectric films had gold electrodes evaporated on them leaving gaps of 0.75 to 2.0 mm for measuring surface conduction. Charge was collected from this configuration at 1/20 of the single pulse threshold. The charge quantity was very noisy and apparently uncorrelated with fluence. Surface hardening was observed for the ThF/sub 4/ film and it did not emit until damage was initiated