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

A variety of microscopic techniques were employed to characterize fluence-l imiting defects in hafnia-silica multilayer coatings manufactured for the N ational Ignition Facility, a fusion laser with a wavelength of 1.053 mum an d 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 x 3 mm areas of the coatings. High-resolution subaperture scans, with a 1-mum step size and a 3-mum pump-beam diameter, were conducted on the def ects to characterize their photothermal properties. Optical and atomic forc e microscopy were used to identify defects and characterize their topograph y. The defects were then irradiated by a damage testing laser (1.06 mum 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 da mage thresholds of the multilayer coatings. It was found that, although dif ferent types of defect were present in these coatings, the fluence-limiting ones had the highest photothermal signals (up to 126x over the host coatin g). The implication of this study is that coating process improvements for hafnia-silica multilayer coatings should have a broader focus than just eli mination of source ejection, since high photothermal signals frequently occ ur at nodule-free regions. The study also demonstrates that, for optics sub ject to absorption-induced thermal damage, photothermal microscopy is an ap propriate tool for nondestructive identification of fluence-limiting defect s.