Role of HfO_2/SiO_2 thin-film interfaces in near-ultraviolet absorption and pulsed laser damage

摘要

The role of thin-film interfaces in the near-ultraviolet (near-UV) absorption and pulsed laser-induced damage was studied for ion-beam-sputtered and electron-beam-evaporated coatings comprised from HfO_2 and SiO_2 thin-film pairs. To separate contributions from the bulk of the film and from interfacial areas, absorption and damage threshold measurements were performed for a one-wave (355-nm wavelength) thick, HfO_2 single-layer film and for a film containing seven narrow HfO_2 layers separated by SiO_2 layers. The seven-layer film was designed to have a total optical thickness of HfO_2 layers, equal to one wave at 355 nm and an E-field peak and average intensity similar to a single-layer HfO_2 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 thin-film interfaces as compared to HfO_2 film material. The relevance of obtained absorption data to coating near-UV, 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 HfO_2 film in both sputtered and evaporated coatings. The results are explained through the similarity of interfacial film structure with structure formed during the codeposition of HfO_2 and SiO_2 materials.