Impact of particle shape on the laser-contaminant interaction induced damage on the protective capping layer of 1? high reflector mirror coatings

摘要

We report an investigation on the response to laser exposure of a protective capping layer of lco (1053 nm) high-reflector mirror coatings, in the presence of differently shaped Ti particles. We consider two candidate capping layer materials, namely SiO_2 and Al_2O_3. They are coated over multiple silica-hafnia multilayer coatings. Each sample is exposed to a single oblique (45°) shot of a 1053 nm laser beam (p polarization, fluence ～ 10 J/cm~2, pulse length 14 ns), in the presence of spherically or irregularly shaped Ti particles on the surface. We observe that the two capping layers show markedly different responses. For spherically shaped particles, the Al_2O_3 cap layer exhibits severe damage, with the capping layer becoming completely delaminated at the particle locations. In contrast, the SiO_2 capping layer is only mildly modified by a shallow depression, likely due to plasma erosion. For irregularly shaped Ti filings, the Al_2O_3 capping layer displays minimal to no damage while the SiO_2 capping layer is significantly damaged. In the case of the spherical particles, we attribute the different response of the capping layer to the large difference in thermal expansion coefficient of the materials, with that of the Al_2O_3 about 15 times greater than that of the SiO_2 layer. For the irregularly shaped filings, we attribute the difference in damage response to the large difference in mechanical toughness between the two materials, with that of the Al_2O_3 being about 10 times stronger than that of the SiO_2.