Evaporated HfO2/SiO2 Optical Coatings and Modifications for High-Power Laser Applications

Evaporated optical coatings fabricated from hafnium dioxide and silicon dioxide are the standard approach for high-peak-power laser components due to the high laser damage resistance of such coatings and the ability to deposit on largeaperture substrates. The tensile film stresses of such coatings may lead to failure, however, particularly in low-relative-humidity purged or vacuum use environments. Careful control of thin-film stresses is essential to maintain high-quality surface flatness of the coated components and preserve the optical performance of the laser system. This work explores changes in the deposition process for hafnia/silica multilayer coatings in order to preserve or ideally improve the laser damage thresholds while limiting the mechanical stress in the film. Changes in the evaporation process, including deposition rate, chamber pressure, temperature, and optical coating design, were studied to evaluate their influence on the performance of the coating. Process changes were expanded to include the use of energetic plasmaassisted deposition as well as the incorporation of aluminum oxide to further alter the thin-film stress. The state of the thin-film stress, laser damage thresholds, and photometric performance of the optical coating were evaluated throughout this effort. Modifications to the evaporated electron-beam deposition process parameters of hafnium dioxide and silicon dioxide were insufficient to provide a near-neutral film stress. However, the use of plasma-assisted deposition and the inclusion of