Effects of laser conditioning with different pulse widths on picosecond laser-induced damage resistance in HfO2/SiO2 high-reflectivity coatings

Current optimizations in the fabrication processes of multilayer dielectric (MLD) high-reflectivity (HR) coatings struggle to meet the demand for picosecond (ps) laser damage resistance in high-energy petawatt laser systems. This study investigates laser conditioning (LC) effects of 250 ps, 500 ps, 800 ps, and 10 nanosecond (ns) laser pulses on the ps laser-induced damage resistance of electron beam (e-beam) HfO 2 /SiO 2 HR coatings. Results demonstrate a 12% enhancement in the laser-induced damage threshold (LIDT) under ∼10 ps laser irradiation for 500 ps conditioned samples compared to unconditioned ones, while negligible LIDT variations are observed for other pulse widths. The defect density and damage types in unconditioned and conditioned samples exhibit no statistical alteration. This indicates that LC does not eliminate nanoscale defects but only increases their LIDTs for 500 ps conditioned samples. To reveal the improvement mechanism of 500 ps LC on the ps laser-induced damage resistance, a simulated model of the interaction between sub-ns laser pulses and localized defect states is proposed, combining ionization rate equations with the two-temperature model (TTM). For 500 ps LC, although the initial electron number density of the defect energy level is increased, the defect energy level gap is also widened due to the appropriate lattice temperature. The effect of this widening gap surpasses that of the aforementioned increased initial electron density, making electron ionization more difficult under ∼10 ps laser irradiation. This work provides new strategies and approaches for enhancing the ps laser-induced damage resistance of MLD HR coatings.