Real-time damage event imaging reveals the absorber inducing laser damage with low density in solgel antireflective coatings

Abstract

A reliable method, combining raster scan process and real-time damage event imaging, has been developed to accurately determine the initiator inducing low-density laser damage at 1064 nm in large aperture solgel coatings. It is revealed that there were two distinct initiators, the visible surface defect and the invisible defect. Online detection and offline morphology analysis demonstrated that ∼96% of low-density damage sites were initiated by surface defects. All types of surface defects were investigated to find their reaction during exposure to laser irradiation. A kind of surface defect consisting of a central spot and a circle of microvariation was admitted as the main initiator. Its size and hidden depth revealed the characteristics of a laser damage initiator. The morphology and depth profile of the laser damage site consisted of a central point and surrounding delamination area, reconfirming our judgment of initiator property and disclosed laser damage mechanism. A high temperature, generated by initiator absorption, induced a change of phase and produced a huge local pressure. This pressure could hump the surrounding solgel coating, break off the humped coating with a shearing stress, and finally remove it from the sample surface.