Damage Characteristics of 500 ps Laser-conditioned Multilayer Dielectric High-Reflectivity Coatings under Picosecond Laser Irradiation

The development of picosecond (ps) petawatt laser systems proposes higher requirements on the laser damage resistance of multilayer dielectric (MLD) high-reflectivity (HR) coatings. Focusing on electron beam-deposited HfO2/SiO2 HR coatings, this study investigates the effects of 500 ps laser conditioning (LC) on the 1 ps, 10 ps, and 20 ps laser-induced damage resistance. Results demonstrate that conditioned samples exhibit negligible laser-induced damage threshold (LIDT) variations under 1 ps laser irradiation, while achieving 8% and 9% enhancements under 10 ps and 20 ps laser irradiation, respectively. The damage morphologies and defect density remain statistically unchanged before and after LC. Under 1 ps to 20 ps laser irradiation, the damage initiation processes in HR coatings involves the combined effects of electric field intensity (EFI) and defects. Although LC does not eliminate defects, it “passivates” defects-induced damage capability under 10 ps and 20 ps laser irradiation. The effect of LC on the 1 ps LIDTs of the sample is not significant, which should be related to the higher defect density induced by the 1 ps laser damage and the coupled non-linear ionization processes resulting from it. Based on the ionization rate equation and the two-temperature model, the mechanism of the LIDT improvement under 10 ps and 20 ps laser irradiation is analyzed: LC optimizes the distribution of localized defect energy levels, reducing the transition probability of the electron under 10 ps and 20 ps laser irradiation, thereby improving the coatings’ damage resistance. This study provides new insights for enhancing the ps laser damage resistance of MLD HR coatings.