Laser damage of UV hafnia-based multilayer dielectric coatings at 355  nm wavelength

Megajoule ultraviolet nanosecond pulses were used to demonstrate ignition at the National Ignition Facility in December 2022 [Nature601, 542 (2022)10.1038/s41586-021-04281-w]. The energy scaling of laser fusion drivers relies, in part, on significant improvements in the UV multilayer dielectric (MLD) coatings, which are critical optics in the laser architecture. This paper reports on the laser damage behavior of MLD coatings based on <i>H</i><i>f</i><i>O</i>₂, <i>S</i><i>i</i><i>O</i>₂, and <i>A</i><i>l</i>₂<i>O</i>₃, designed for operation at <i>λ</i>=355<i>n</i><i>m</i>. Two-layer anti-reflection (AR) coatings containing <i>H</i><i>f</i><i>O</i>₂-based mixtures were co-sputtered by reactive biased target deposition (BTD) as the high-index layer. The laser-induced damage threshold was assessed from 1-on-1 and S-on-1 tests and compared to MLD ARs of <i>H</i><i>f</i><i>O</i>₂/<i>S</i><i>i</i><i>O</i>₂ fabricated by ion beam sputtering and electron beam evaporation. It is shown that the 1-on-1 LIDT of BTD ARs, which contain <i>H</i><i>f</i>_1-<i>y</i><i>S</i><i>i</i>_<i>y</i><i>O</i>_<i>x</i> and <i>H</i><i>f</i>_1-<i>y</i><i>A</i><i>l</i>_<i>y</i><i>O</i>_<i>x</i> mixtures, is higher than that of BTD <i>H</i><i>f</i><i>O</i>₂/<i>S</i><i>i</i><i>O</i>₂ ARs. However, this increase is below that predicted by the scaling of LIDT with the optical bandgap, calculated from the LIDT of <i>H</i><i>f</i><i>O</i>₂/<i>S</i><i>i</i><i>O</i>₂ and <i>H</i><i>f</i><i>O</i>₂/<i>A</i><i>l</i>₂<i>O</i>₃. The S-on-1 LIDT of BTD ARs decreases by ∼25<i>%</i> for <i>S</i>=10 and remains unchanged to <i>S</i>=10⁴ laser shots, indicating no accumulation fatigue. Neither UV preconditioning nor etching of the substrate prior to coating deposition caused a major improvement in the 1-on-1 LIDT of BTD ARs.