Impact of laser-contaminant interaction on the performance of the protective capping layer of 1 ω high-reflection...

High dielectric constant multilayer coatings are commonly used on high-reflection mirrors for high-peak-power laser systems because of their high laser-damage resistance. However, surface contaminants often lead to damage upon laser exposure, thus limiting the mirror's lifetime and performance. One plausible approach to improve the overall mirror resistance against laser damage, including that induced by laser-contaminant coupling, is to coat the multilayers with a thin protective capping (absentee) layer on top of the multilayer coatings. An understanding of the underlying mechanism by which laser-particle interaction leads to capping layer damage is important for the rational design and selection of capping materials of high-reflection multilayer coatings. In this paper, we examine the responses of two candidate capping layer materials, made of SiO₂ and Al₂O₃, over silica-hafnia multilayer coatings. These are exposed to a single oblique shot of a 1053 nm laser beam (fluence ∼10  J/cm², pulse length 14 ns), in the presence of Ti particles on the surface. We find that the two capping layers show markedly different responses to the laser-particle interaction. The Al₂O₃ cap layer exhibits severe damage, with the capping layer becoming completely delaminated at the particle locations. The SiO₂ capping layer, on the other hand, is only mildly modified by a shallow depression. Combining the observations with optical modeling and thermal/mechanical calculations, we argue that a high-temperature thermal field from plasma generated by the laser-particle interaction above a critical fluence is responsible for the surface modification of each capping layer. The great difference in damage behavior is mainly attributed to the large disparity in the thermal expansion coefficient of the two capping materials, with that of Al₂O₃ layer being about 15 times greater than that of SiO₂.