Using engineered defects to study laser-induced damage in optical thin films with nanosecond pulses

By creating nodules from artificial seeds, the damage behaviors of engineered nodules were systematically studied from experimental approaches. The seed diameters, seed absorption and film absorption were varied independently to uncover a single factor's influence on the damage behavior of nodules. First, non-absorbing monodisperse SiO₂ microspheres with five different sizes were used to create engineered nodules in 1.053 μm HfO₂/SiO₂ high reflectors that were prepared by EB process. Laser damage test results showed that the ejection fluences of nodules monotonically decreased with the increase of silica microsphere diameters. And to our surprise, nodules initiating from 0.3 and 0.6 μm silica seeds survived the maximum fluence of 170 J/cm² (10 ns). Film absorption also has big influence on the damage behaviors of nodules. Compared to the nodules in low absorbing reflectors that were prepared by EB process, the nodules in high absorbing reflectors that were prepared by IAD process exhibited a much lower ejection fluences, although the seed diameters for the comprising nodules were same. Moreover, aluminum seeds were also used to create engineered nodules. Laser damage test results showed that the ejection fluences of nodules initiating from aluminum seeds were around 2 J/cm² (10 ns), which is more than an order of magnitude less than the ejection fluences of nodules created from nonabsorbing silica seeds. This result revealed that the seed absorption played a very important role in the laser damage of nodules.