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 [ Nature 601 , 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 HfO 2 , SiO 2 , and Al 2 O 3 , designed for operation at λ =355nm. Two-layer anti-reflection (AR) coatings containing HfO 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 HfO 2 /SiO 2 fabricated by ion beam sputtering and electron beam evaporation. It is shown that the 1-on-1 LIDT of BTD ARs, which contain Hf 1−y Si y O x and Hf 1−y Al y O x mixtures, is higher than that of BTD HfO 2 /SiO 2 ARs. However, this increase is below that predicted by the scaling of LIDT with the optical bandgap, calculated from the LIDT of HfO 2 /SiO 2 and HfO 2 /Al 2 O 3 . The S-on-1 LIDT of BTD ARs decreases by ∼25% for S=10 and remains unchanged to S=10 4 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.