Abstract
Hafnium dioxide films deposited using electron-beam evaporation tend to exhibit high tensile stresses, particularly when deposited on low-thermal-expansion substrates for use in a low-relative-humidity environment. Hafnia has been shown to be a critical material, however, for use in high-peak-power laser coatings, providing exceptional deposition control and laser-damage resistance. To correct for tensile thin-film stresses in hafnia/silica multilayer coatings, alumina compensation layers were incorporated in the multilayer design. Determination of the stresses resulting from alumina layers in different coating designs has led to the realization of the influence of water diffusion and the diffusion-barrier properties of alumina that must be considered. The inclusion of alumina layers in a hafnia/silica multilayer provides the ability to produce low-compressive-stress, high-laser-damage-threshold coatings.
Highlights
Optical coatings are a critical technology for the successful construction and operation of high-peak-power laser systems
The goal of this work is to alter the tensile stress in the hafnia/silica multilayer coatings, shifting it to a low-magnitude compressive stress to eliminate the risk of cracking the coating while minimizing substrate deformation
Composite film stresses in hafnia/silica multilayers, were tensile, but alumina/silica multilayers remained quite compressive
Summary
Optical coatings are a critical technology for the successful construction and operation of high-peak-power laser systems. The determination to date that such coatings produce the highest laser-damage thresholds has led to the use of electron-beam evaporation as the primary deposition process for large, high-peak-power laser components for systems such as OMEGA, OMEGA EP, the National Ignition Facility, Laser Megajoule, and others. 6_1 - ositfR where v is the film stress, R is the radius of curvature of the surface, Es is Young’s modulus of the substrate, os is Poisson’s ratio for the substrate, and tf and ts are the thickness of the film and substrate, respectively This describes the impact of the stress on the radius of curvature of the optic surface, leading to changes in the flatness and corresponding optical performance of the component. This relationship is given by Hutchinson and Suo:[14] hc
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