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 HfO2/SiO2 and Ta2O5/SiO2 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 HfO2/SiO2 HR coating exhibited stronger laser resistance than the Ta2O5/SiO2 HR coating. Flat-bottom pits, pinpoints, and funnel pits were the three typical damage morphologies for the experimental HR coatings. The damage to the HfO2/SiO2 HR coating was primarily dominated by flat-bottom pits, whereas dense pinpoints were the most significant damage for the Ta2O5/SiO2 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 SiO2 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.
Herein, X-ray photoelectron spectrometer (XPS), angle-resolved XPS (ARXPS), and atomic force microscopy (AFM) are used to study the surface changes of Ta2O5 bombarded by Ar+ ions with different energies. The results reveal that the Ar+ bombardment of Ta2O5 leads to a preferential sputtering of O atoms, which results in an imbalance in the Ta/O ratio on the material surface; and the formation of an “altered layer” composed of Ta2O5, Ta1+, Ta2+, Ta3+, and Ta4+. The Ta/O ratio increases from 0.34 to 0.55 with the sputtering time; however, it does not vary with ion energy. Before reaching a steady-state, the thickness of the altered layer increases with the sputtering time; however, after reaching a steady-state, the thickness of the altered layer does not exceed 3 nm. Concurrently, it increases with increasing sputtering energy. Further, AFM measurements reveal that low-energy Ar+ bombardment leads to a slight increased surface roughness, which does not exceed the initial value (0.41 nm) by 25%.
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