Characteristics of nodular defect in HfO2/SiO2 multilayer optical coatings

Laser-induced damage is associated with nodular defects in HfO2/SiO2 multilayer films. In order to investigate the damage characteristics of HfO2/SiO2 multilayer mirrors and find the information of improving laser-induced damage threshold, nodular defects are characterized by multiple analytical techniques; the damage morphologies induced by nodular-ejections are presented; the depths of nodular-ejection pits are investigated; the laser-induced damage threshold of zero probability and the stabilities of nodular-ejection pits exposed to repetitive illuminations are studied. Results show that domes in the film surface are nodular defects. Reliable depth information of nodular-ejection pits is obtained by counting layers from the damage edge. The depth statistical result implies nodular defects in these samples are usually originated from deep seeds. Some process optimizations suggestions are given based on the depth information. A simple tractable method is proposed to determine the functional damage threshold of these HfO2/SiO2 multilayer films basing on the damage experiments.