Correlation of damage threshold and surface geometry of nodular defects on HR coatings as determined by in-situ atomic force microscopy

摘要

Atomic force microscopy (AFM) was used to determine in-situ the correlation between the surface dimensions of defects in dielectric multilayer optical coatings and their susceptibility to damage by pulsed laser illumination. The primary surface defects studied were $mu@m-scale domes associated with the classic nodule defect. The optical film studied was a highly reflective dielectric multilayer consisting of pairs of alternating HfO$-2$/ and SiO$-2$/ layers of quarter wave thickness at 1.06 $mu@m. Nodule defect height and width dimensions were measured prior to laser illumination on two different samples. Correlation between these dimensions supported a simple model for the defect geometry. Defects with high nodule heights ($GRT 0.6 $mu@m) were found to be most susceptible to laser damage over a range of fluences between 0 - 35 J/cm$+2$/ (1.06 $mu@m, 10 ns, and 1/e$+2$/ diam. of 1.3 mm). Crater defects, formed by nodules ejected from the coating prior to illumination, were also studied. None of the crater defects damaged when illuminated over the same range of fluences that the nodule defects were subjected to.