The development of picosecond (ps) petawatt laser systems proposes higher requirements on the laser damage resistance of multilayer dielectric (MLD) high-reflectivity (HR) coatings. As a post-processing technique, laser conditioning (LC) has demonstrated great potential in enhancing the laser damage resistance of optical components. However, there are no studies to report the LC-induced improvements in the ps laser-induced damage thresholds (LIDTs) of MLD HR coatings. Considering the ps short-pulse damage mechanism linked to the electronic non-linear dissociation process, this work extends the sub-ns LC from crystals to MLD coatings. Focusing on HfO₂/SiO₂ HR coatings deposited by the electron beam evaporation technique, this study investigates the effects of 500 ps LC on their 1, 10, and 20 ps laser-induced damage resistance. Based on the ionization rate equation and the two-temperature model (TTM), the mechanism of the LIDT improvement under 10 ps and 20 ps laser irradiation is analyzed. This study provides new insights for enhancing the ps laser damage resistance of MLD HR coatings.

A sub-ns LC setup operating at 1064 nm with a pulse duration of 500 ps is used to condition the HfO₂/SiO₂ MLD HR coatings. The 1-on-1 LIDTs of the coatings before and after LC are compared under 1, 10, and 20 ps laser irradiation at 1053 nm. The damage morphologies are preliminarily observed using optical microscope (OM), followed with the high-resolution characterization by focused ion beam scanning electron microscope (FIB-SEM). The LIDT is defined as the maximum fluence when no nanoscale defect-induced damage is observed. To analyze the defect parameters, the Krol defect density statistical model is applied to fit the damage probability curves, and then the defect density, mean damage threshold, and threshold standard deviation are extracted. Additionally, finite element simulations are conducted to study the mechanism of improvement in LIDT by combining the ionization rate equation and the TTM.

Focusing on HfO₂/SiO₂ HR coatings deposited by electron beam evaporation, this study investigates the effects of 500 ps LC on the 1, 10, and 20 ps laser-induced damage resistance. The results demonstrate that the conditioned samples exhibit a negligible LIDT variation under 1 ps laser irradiation, while achieving 8% and 9% improvements in LIDTs under 10 ps and 20 ps laser irradiation, respectively. Meanwhile, the damage morphologies and defect density remain statistically unchanged before and after LC. Under 1 ps to 20 ps laser irradiation, the damage initiation process 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 LIDTs in the case of 1 ps laser irradiation is not significant, which is attributed to the higher defect density and the coupled non-linear ionization processes induced by the 1 ps laser. Based on the ionization rate equation and the TTM, 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 electrons under 10 ps and 20 ps laser irradiation, and thereby improves the laser-induced damage resistance of coatings.

This study investigates the optimization effects of 500 ps LC on the LIDTs of HfO₂/SiO₂ HR coatings under 1, 10, and 20 ps laser irradiation. By integrating the ionization rate equation with the TTM, the influencing mechanism of 500 ps LC on the damage resistance of HR coatings under 10 ps and 20 ps laser irradiation is revealed. The main conclusions are as follows:

1) By using 500 ps LC, the LIDTs of HR coatings under 10 ps and 20 ps laser irradiation are improved by 7% and 9%, respectively. However, 500 ps LC shows a negligible improvement in the LIDTs of HR coatings under 1 ps laser irradiation.

2) Under 10 ps and 20 ps laser irradiation, the average LIDT of defects in the conditioned HR coatings increases, while the defect density and the damage types remain unchanged. This indicates that LC does not eliminate defects but enhances their damage resistance.

3) LC optimizes the localized defect energy level distribution in the material. This optimization reduces the migration probability of defect-state electrons under 10 ps and 20 ps laser irradiation, thereby improving the damage resistance of the HR coatings at these pulse durations.

4) Under 1 ps laser irradiation, during the damage initiation process, a significant increase in defect density is responsible for the enhanced coupling of nonlinear ionization processes among defects. Consequently, the “passivation” effect of LC on defects fails to influence the 1 ps LIDTs of the HR coatings.