Study of micron-scale defects in optical coatings for high-power lasers deposited by ion beam sputtering

Optical coatings prepared by ion beam deposition are dense, with good mechanical properties and environmental stability, which have a wide range of applications in high-power lasers such as ultrafast lasers and space lasers. In the field of nanosecond-laser damage, the size, composition, density and distribution of defects in materials are closely related to laser damage. In this work, the types of defects in ion beam sputtered coatings and their effect on laser damage are investigated. Micron-scale defects and damage morphology on the surface of ion beam sputtered coatings are observed using focused ion beam scanning electron microscopy. The defects on the coatings surface are found to be mainly due to structural defects from the substrate surface and nodules formed during the coating process. Micron-scale defects have a greater impact on laser damage in coatings with stronger standing wave fields. Experimentally pits defects copied from substrates can reduce the damage threshold of high-reflective coatings by nearly a factor of two, and nodules can cause a greater laser damage threshold drop. This study helps to improve the laser damage threshold of ion beam sputtered coatings.