Damage threshold prediction of hafnia-silica multilayer coatings by nondestructive evaluation of fluence-limiting defects

摘要

A variety of microscopic techniques were employed to characterize fluence-limiting defects in hafnia-ailica multilayer coatings manufactured for the National Ignition Facility, a fusion laser with a wavelength of 1.053 μm and a pulse width of 3 ns. Photothermal microscopy, with the surface thermal lens effect, was used to map the absorption and thermal characteristics of 3 mm×3 mm areas of the coatings. High-resolution subaperture scans, with a 1-μm step size and a 3-μm pump-beam diameter, were conducted on the defects to characterize their photothermal properties. Optical and atomic force microscopy were used to identify defects and characterize their topography. The defects were then irradiated by a damage testing laser (1.06 μm and 3 ns) in single-shot mode until damage occurred. The results were analyzed to determine the role of nodular and nonnodular defects in limiting the damage thresholds of the multilayer coatings. It was found that, although different types of defect were present in these coatings, the fluence-limiting ones had the highest photothermal signals (up to 126× over the host coating). The implication of this study is that coating process improvements for hafnia-silica multilayer coatings should have a broader focus than just elimination of source ejection, since high photothermal signals frequently occur at nodule-free regions. The study also demonstrates that, for optics subject to absorption-induced thermal damage, photothermal microscopy is an appropriate tool for nondestructive identification of fluence-limiting defects.