Observation and morphology of small-scale laser induced damage

Laser-induced damage of lambda/4 ThF(4)/ZnS coatings for 10.6 microm is studied. Extended areas of the coated surfaces are recorded on high resolution film both before and after irradiation using Twyman-Green and knife-edge techniques, the latter method proving to be superior and quite useful. Comparisons are made to establish a possible correlation between the location of damage sites and previous defects. The morphology of the damage is also studied using Nomarski microscopy to determine possible causes of damage. Damage on coated silicon substrates resembles melts and burns with cracking and/or annealing around some burned areas, while damage to the coated silica substrates resembles bubbles that are arranged along scratches or in clusters. The high correlation between damage sites and previous defects suggests that clean defect-free coatings have greatly increased damage thresholds.