High Power Optical Coatings by Atomic Layer Deposition and Signatures of Laser-Induced Damage

Abstract : This report describes an investigation of the properties of optical coating oxides deposited by atomic layer deposition. Specifically, two oxides were chosen as indicative of the group as a whole: aluminum oxide (Al2O3) and hafnium oxide (HfO2). Uniform films and nanolaminates (films with inserted layers of alternate materials) were deposited and tested for thermal conductivity. absorption, and laser damage threshold, among other properties. Significant effort went into the analysis of the thermal conductivities of the films since thermal breakdown is particularly relevant to directed energy applications. The interface thermal resistance in hafnia-alumina nanolaminates is very low and does not dominate the film thermal conductivity. The degree of crystallinity of these films appears to have a much larger effect on thermal conductivity than that of interface density. Cryogenic measurements show partial agreement with both the minimum thermal conductivity model for disordered solids and the diffuse mismatch model of interface resistance. Absorption measurements show results less than or near 1ppm, which matches or exceeds the performance of the best films deposited by other methods.