Amorphous oxide interference coatings for multi-gigashot lifetime for inertial fusion energy laser drivers

192 ultraviolet (UV) laser beams delivering an energy of 2.05 MJ onto a hydrogen fuel target made possible the demonstration of ignition at the National Ignition Facility at Lawrence Livermore National Laboratory in December 2022. In the inertial fusion energy (IFE) laser driver that made possible this remarkable achievement there are multiple optical components that damage prematurely due intense UV radiation exposure. For IFE laser drivers to reach the performance envisioned for the generation of fusion energy, finding solutions to mitigate UV damage and to extend the lifetime of coatings in critical optics is a priority. XUV Lasers proposes to develop and commercialize key interference coating technology for 355 nm which will be resistant to UV laser damage at irradiances above 10 J/cm2 and will be capable to operate over long periods, i.e., multi-giga-shots exposure in a vacuum environment without degradation associated with the fusion environment. To achieve the performance metric described above, the Phase II research plan will: a) develop oxide mixtures for the engineering of 355 nm multilayer dielectric coatings (MLD) to replace the more prone damage high index layers in the stacks; b) identify strategies to condition substratesĺ surfaces prior to deposition of the coating stack; and c) identify modifications to the materials and the optics that occur due to UV exposure at irradiances below the threshold for damage. The Phase II research is supported by the results of Phase I which demonstrated: a) the use of HfO2:SiO2 mixtures in high transmission coatings is most promising in terms of damage threshold and lifetime to failure predicted to be ~5x1012 shots; b) conditioning of the substrate by dry etching increases the laser induced damage fluence by >4; and c) IBS high reflection HfO2/SiO2 reached a damage fluence of 8 J/cm2 with projected lifetime to failure 5x107 shots. The proposed research will contribute to accelerating the development of IFE laser drivers by providing solutions that overcome the premature UV laser damage of MLD coating technologies. The know-how developed through the proposed research has also the potential to impact MLD coatings of high energy lasers operating at near infrared and visible wavelengths. Besides IFE drivers, these MLD coatings that are resistant to high UV irradiances UV could also enhance performance of excimer lasers - the workhorses in medical and lithography applications.