355-nm absorption in HfO2 and SiO2 monolayers with embedded Hf nanoclusters studied using photothermal heterodyne imaging

The role of the Hf nanoclusters as near-UV, nanosecond-pulse laser-damage initiators in HfO2/SiO2-pair-based multilayer coatings remains speculative. In this work we use photothermal heterodyne imaging (PHI) to investigate absorption in HfO2 and SiO2 monolayers containing embedded nanometer-sized Hf clusters produced by backside-through-thin-film ablation. Hf cluster size and areal-density distributions were characterized using transmission electron microscopy. PHI measurements were taken for cluster-containing samples and for similarly prepared HfO2 and SiO2 film samples of the same thickness without clusters. These data allow us to evaluate a possible role in the damage-process initiation of two hypothetical sources of the localized absorption-Hf clusters and high-density areas of electronic defects.