Quasi-in-situ characterization and laser damage investigation of flaws in silica antireflection coatings

Silica sol-gel antireflection coatings are used in high-power lasers due to their excellent laser damage resistance. These coatings are prepared using silica nanoparticles, and flaws generated during the coating preparation process are considered one of the factors that can lead to laser damage. Ring-shape flaws of micrometer-size usually appear on the surface of silica sol-gel antireflection coating. The morphologies of these flaws were investigated through optical microscopy, contact-type surface profilometer and scanning electron microscope. The diameters of the ring-shape flaws are from several micrometers to tens of micrometers. It has been demonstrated that the ring-shape flaws are nodule-like structures containing closely packed silica nanoparticles. The quasi-in-situ laser damage tests of the coated samples show the ring-shape flaws have a low damage probability. The formation of ring-shape flaws on the silica sol-gel coating is related to the polydispersity of the colloidal silica nanoparticles. Through the analysis of the size distribution of silica nanoparticles, a mechanism for flaws formation is proposed.