Using monodisperse SiO2 microspheres to study laser-induced damage of nodules in HfO2/SiO2 high reflectors

Nodules have been proved to play an important role in the activation of laser damage in 1.053 mu m HfO2/SiO2 high reflectors. However, some damage test results revealed that the ejection fluences of some big nodules with height around 1 mu m were abnormally high. To find the correlation between the surface dimensions of nodules and their susceptibility to nano-second pulsed laser radiation, monodisperse SiO2 microspheres with five different sizes were used to create engineered nodules in 1.053 mu m HfO2/SiO2 high reflectors. The defect density of nodules that were created from SiO2 microspheres was purposely controlled to be around 20-40 mm(2) and special care was taken to minimize clusters of SiO2 microspheres as less as possible. This enabled us to take a raster scan test and to get the statistical value of ejection fluences of these engineered nodules. The height and width dimensions of the engineered nodules, especially the discontinuity of nodular boundary, were measured by cross-sectioning of nodular defects using a focused ion-beam milling instrument. Based on the above information, the damage test results were interpreted from the aspects of electric field enhancement model and mechanical stability of nodular structures.