Shape dependence of laser–particle interaction-induced damage on the protective capping layer of 1 ω high reflector mirror coatings

Abstract

© The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. The response of a potential candidate protective capping layer (SiO 2 or Al 2 O 3 ) to laser exposure of 1ω (1053 nm) to high-reflector silica-hafnia multilayer coatings in the presence of variously shaped Ti particles is investigated by combining laser damage testing and numerical modeling. Each sample is exposed to a single oblique angle (45 deg) laser shot (p-polarization, ∼10 J/cm 2 , 14 ns) in the presence of spherically or irregularly shaped Ti particles on the surface. The two capping layers show markedly different responses. For the spherical particles, the Al 2 O 3 cap layer exhibits severe damage, with the capping layer becoming completely delaminated at the particle locations. The SiO 2 capping layer is only mildly modified by a shallow depression, likely due to plasma erosion. The different response of the capping layer is attributed to the large difference in the thermal expansion coefficient of the materials, with that of the Al 2 O 3 about 15 times greater than that of the SiO2 layer. For the irregular particles, the Al 2 O 3 capping layer displays minimal to no damage while the SiO2 capping layer is significantly damaged. The difference is due to the disparity in mechanical strength with Al 2 O 3 possessing approximately 10 times higher fracture toughness.