Interface absorption versus film absorption in HfO2/SiO2 thin-film pairs in the near-ultraviolet and the relation to pulsed-laser damage

Near-ultraviolet absorption in hafnium oxide and silica oxide thin-film pairs in a configuration strongly departing from the regular quarter-wave-thickness approach has been studied with the goal of separating film and interfacial contributions to absorption and pulsed laser damage. For this purpose, we manufactured a model HfO2/SiO2 thin-film coating containing seven HfO2 layers separated by narrow SiO2 layers and a single-layer HfO2 film in one coating run. The two coatings were characterized by a one-wave total optical thickness for the HfO2 material and similar E-field peak intensity inside the film. Absorption in the electron-beam-deposited films was measured using photothermal heterodyne imaging. By comparing absorption for the seven-layer and single-layer films, one can estimate the partial HfO2/SiO2 interface contribution. Relevance of obtained data to the thin-film pulsed-laser damage was verified by conducting 351-nm, nanosecond-laser-damage measurements and damage-morphology characterization using atomic force microscopy.