Investigation on nano-absorbing precursors in the picosecond regime via the comparative study of HfO ₂ /SiO ₂ and Ta ₂ O ₅ /SiO ₂ high-reflectivity coating damage

Laser-induced damage to the final reflective and diffractive optics limits the total output energy of petawatt laser systems with pulse durations ranging from a few hundred femtoseconds (fs) up to a few tens of picoseconds (ps). In this study, the laser damage to HfO ₂ /SiO ₂ and Ta ₂ O ₅ /SiO ₂ multilayer dielectric high-reflectivity (HR) coatings induced by a 1053 nm laser with a pulse width of 8.6 ps was studied to investigate the nano-absorbing precursors in ps regimes. The HfO ₂ /SiO ₂ HR coating exhibited stronger laser resistance than the Ta ₂ O ₅ /SiO ₂ HR coating. Flat-bottom pits, pinpoints, and funnel pits were the three typical damage morphologies for the experimental HR coatings. The damage to the HfO ₂ /SiO ₂ HR coating was primarily dominated by flat-bottom pits, whereas dense pinpoints were the most significant damage for the Ta ₂ O ₅ /SiO ₂ HR coating. The nano-absorbing precursors introduced by the ion-assisted deposition process were proved to be the damage precursors that trigger pinpoints under a strong electric field intensity (EFI). The nano-absorbing precursors located in the second EFI peak of the SiO ₂ top layer induced the funnel pits. The funnel pits were expected to be the previous stage of the flat-bottom pits. After they grew along the upward-sloping crack and separated from the interface, the flat-bottom pits were formed. In addition, poor-binding interfaces promoted the formation of flat-bottom pits.