Influence of composition and seed dimension on the structure and laser damage of nodular defects in HfO_2/SiO_2 high reflectors

摘要

Seeds are the sources for forming nodular defects that largely limit the improvement of laser-induced damage threshold of 1ω laser mirrors in the nanosecond pulse regime. To shed more light of the composition and sizes of seeds on the associated structure of nodular defects and laser damage sensitivity, nodular defects were generated in 1064 nm HfO_2/SiO_2 high reflectors with different sizes of absorbing Au and nonabsorbing SiO_2 nanoparticles located on the surfaces of substrates. The width dimensions, inner structures, and damage morphologies of nodular defects were characterized by an atomic force microscope, a field emission scanning electron microscope, and a focused ion beam. It was found that the composition and size both influenced the structure and the laser damage of nodular defects. The width of nodules from SiO_2 seeds were larger than that formed by the same size of Au seed. A nodule grown from a small seed generally tends to have a continuous and stable boundary. The ejection fluences of nodules generated from different size absorbing Au and nonabsorbing SiO_2 seeds were totally different. The results were interpreted from the aspects of absorption cross sections of seeds and mechanical stability of nodular structures.