Investigation on hafnium oxide mixtures for UV coatings for fusion energy applications

Abstract

Metal oxide thin films play a crucial role in optical coatings for high power lasers. High laser damage resistance optical coatings find use in vacuum windows, mirrors, laser crystals, and harmonic generation crystals in high average and peak power lasers. The laser damage of multi-layer dielectric optical coatings is limited by that of the high index of refraction material, which is typically hafnium dioxide. Therefore, this work is focused on improving the laser damage performance of hafnium oxide by modifying the material's optical and structural properties through doping or mixing with other metal cations for applications in ultraviolet interference coatings for λ=355 nm wavelength. UV coatings capable of enduring billions of laser shots unscathed are pivotal to drivers for inertial confined fusion energy (IFE). The laser-induced damage threshold (LIDT) of metal oxide thin films is constrained by various intrinsic and extrinsic factors, including crystallinity, fabrication method, and defect incorporation during processing. The implementation of high LIDT optical coatings is primarily limited by the ability to engineer amorphous oxide layers with controlled structural and optical properties. For UV lasers, amorphous dielectric coatings are multilayer stacks of alternating layers of HfO2 and SiO2. HfO2 has a high index of refraction and a high band gap at λ=355 nn while SiO2 has a low index of refraction, offering sufficient index contrast to engineer high reflector and anti-reflection coatings. To optimize LIDT, mixed amorphous oxide alloys offer a promising research direction for UV high energy lasers for IFE. This thesis investigates a variety of metal oxide mixture thin films as the high index material in anti-reflection coatings for λ=355 nm. Primarily, the optical properties, laser damage performance, amorphous morphology, and electronic state analysis of hafnium oxide mixtures with SiO2 and Al2O3 are investigated. These mixtures are fabricated with the biased target deposition (BTD) technique which is entirely unexplored in the context of high LIDT interference coatings. Two-layer anti-reflection (AR) coatings were designed and fabricated by reactive biased target deposition (BTD) using mixtures of HfO2 and SiO2, HfO2 and Al2O3, as the high index layer and SiO2 as the low index layer. The laser damage response was assessed from 1-on-1 and S-on-1 tests from which the laser induced damage threshold (LIDT) fluence was determined. It is shown that in BTD 2-layer ARs using mixtures of Hf1-ySiyOx and Hf1-yAlyOx the 1-on-1 LIDT increases with respect to the HfO2/SiO2 AR. The BTD ARs have a density higher than electron beam evaporated (EBE) HfO2/SiO2 ARs but lower than ion beam sputtered (IBS) HfO2/SiO2 ARs. The 1-on-1 LIDT of the BTD ARs is slightly lower than that of electron beam evaporated (EBE) HfO2/SiO2 ARs and ion beam sputtered (IBS) HfO2/SiO2 ARs. While the 104-on-1 LIDT of EBE ARs was higher than that of either sputtering method. Substrate etching prior to deposition and UV conditioning increase the 1-on-1 LIDT of sputtered ARs. The S-on-1 LIDT of BTD ARs decreases by ~25% for S=10 and remains unchanged to S=104 laser shots, indicating no accumulation fatigue.