Polarization-dependent, defect-driven damage in multilayer dielectric coatings under picosecond laser irradiation

In the picosecond (ps) regime, laser-induced damage in multilayer dielectric (MLD) coatings is jointly triggered by electric field intensity (EFI) and pre-existing defects. MLD coatings located in the beam transport section of picosecond–petawatt-class laser systems were subjected to laser irradiation under varying polarization states. However, the effect of laser polarization on defect-driven damage mechanisms remains unclear. This study investigated the role of polarization in the ps laser damage mechanisms of two typical types of defects in MLD coatings, namely nano-absorbing defects and nodular defects. Laser damage experiments were conducted on high-reflectivity HfO₂/SiO₂ coatings using 1053 nm, 8.6 ps laser pulses under both s- and p-polarization conditions. For damage pits induced by nano-absorbing defects, the laser-induced damage threshold (LIDT) was lower under p-polarization than s-polarization conditions. Simulations based on the two-temperature model revealed that, under the same laser fluence, the peak temperature within the defects was higher under p-polarization than s-polarization conditions, which explains the reduced LIDT in the p-polarization case. Under s-polarization conditions, asymmetric melting was observed in the molten cavities, with more severe damage on the incident laser side, which is consistent with the simulated temperature profiles. Regarding the nodular defects, the damage site was strongly correlated with the peak EFI; under s-polarization conditions, damage occurred in the reflected laser direction, whereas under p-polarization conditions, damage manifested in the incident laser direction.