The role of film interfaces in near-ultraviolet absorption and pulsed-laser damage in ion-beam-sputtered coatings based on HfO2/SiO2 thin-film pairs

The role of thin-film interfaces in the near-ultraviolet absorption and pulsed-laser–induced damage was studied for ion-beam–sputtered coatings comprised of HfO2 and SiO2 thin-film pairs. To separate contributions from the bulk of the film and from interfacial areas, absorption and damage threshold were measured for a one-wave (355-nm)–thick HfO2 single-layer film and for a film containing seven narrow HfO2 layers separated by SiO2 layers. The seven-layer film was designed to have a total optical thickness of HfO2 layers equal to one wave at 355 nm and an E-field peak and average intensity similar to a single-layer HfO2 film. Absorption in both types of films was measured using laser calorimetry and photothermal heterodyne imaging. The results showed a small contribution to total absorption from thinfilm interfaces, as compared to HfO2 film material. The relevance of obtained absorption data to coating near-ultraviolet, nanosecond-pulse laser damage was verified by measuring the damage threshold and characterizing damage morphology. The results of this study revealed a higher damage resistance in the seven-layer coating as compared to the single-layer HfO2 film, in agreement with data recently reported for similarly designed electron-beam–deposited coatings. The results are explained through the similarity of interfacial film structure and structure formed during the co-deposition of HfO2 and SiO2 materials.