A theory model on multi-shot laser-induced damage of multilayer mirrors in nanosecond

The accumulation effects in high-reflectivity (HR) HfO2/SiO2 coatings under laser irradiation at different laser wavelengths are investigated respectively. It was found that the multi-shot laser induced damage thresholds (LIDTs) were always lower than that under single-shot at 1064 nm and 532 nm because of "fatigue effect". And the evolution of LIDTs versus shot number, namely the optic lifetime was acquired. The previous analysis verified the accumulation damage mechanism was mainly due to the influence of newly created defects, namely, the laser-induced defects or intrinsic defects with irreversible changes under multiple pulse irradiations. Thus, a correlative theory model based on critical conduction band electron density is constructed to elucidate the experimental phenomena in nanosecond at different wavelengths with a 5Hz repetition rate. In particular, shallow trap (defect state) of varying absorption cross-section with irradiated shot numbers is used to simulate the material modification process under multiple pulse irradiations. It's found that the absorption cross-section of the defect state at 532 nm is about one order of magnitude higher than that at 1064 nm, and defects at 532 nm need much less shot numbers to reach saturation with a higher growth factor.