Absorption and damage thresholds of low-defect-density hafnia deposited with activated oxygen

University of CaliforniaLawrence Livermore National Laborathry, Livermore, CA 94551ABSTRACTMotivation for this work included observations at Lawrence Livermore National Laboratory and elsewhere of acorrelation between increasing laser damage thresholds (DT) and both decreasing nodular-defect density andabsorption of coatings. We reduced the nodular-defect densities by a factor of over 4x in hafnia (Hf02) coatingsdeposited by reactive e-beam evaporation from a Hftarget source. In order to increase the metal oxidationkinetics at the coated surface, Hf was e-beam deposited reactively with 02 activated by a p.-wave discharge.The effect of using activated 02 during the evaporation of a Hf02 target source was also evaluated. A series ofHf02 layers were made with various conditions; we alternated between two p-wave configurations, Hf andHf02 targets and two reactive 02 pressures. Laser DTs (1064 nm -10 ns pulses), absorption (at 511 nm) andnodular-defect densities from these coatings are reported. The DT correlated inversely with the coatingabsorption.1. INTRODUCTIONOptical coatings can fail catastrophically at the high fluence levels that can be achieved in high peak-powerlaser systems. The DTs of the coatings more often than not determine the maximum output level of the lasersystem. These optical coatings are multilayers of alternately high- and low-refractive index materials that are,in less severe applications, considered to be non-absorbing media. However, the coatings are failing by somesort of absorption phenomena which is not fully understood. A major obstacle in verifying any damagemechanisms caused by high-laser fluences is that the sites where damage initiates are difficult to isolate andstudy. However, the laser-damage community has accumulated various properties that coatings must have inorder to survive high-laser fluences. In this paper, we report on our efforts to reduce two types of defectdensities that appear to affect the laser DTs in optical coatings: atomic non-stoichiometry and microscopicnodules.Furthermore, we studied these two defect types in a reactive electron-beam (e-beam) evaporation process. Thee-beam evaporation process has and continues to produce the vast majority ofhigh-damage-threshold optical-multilayer coatings. Any positive results in process understanding and procedural improvements could thenbe easily transferred to current equipment and staff. An understanding ofthe properties leading to higher DTsin a reactive e-beam process may also help the development of other energetic physical vapor depositionprocesses of optical coatings. In short, to enhance the DT of optical coatings, our objectives were to minimizenodular- and atomic-defect densities generated in a reactive e-beam process.There were several reasons for concentrating on hafnia (Hf02) as the coating material. Of the high-refractiveindex (high n) materials, Hf02 has the highest UP. Bettis1 argued that the n is a measure ofhow easily anelectron moves away from its host atom in the presence of an electric field (i.e., the polarizability of thematerial). The higher n materials ionize at lower laser fluences and are thus more susceptible to damage viaplasma absorption. The DTs of single-layer oxide coatings in general supported this argument.2 We assumedthen that the DTs of multilayer oxide coatings are limited by the high n material. Next, Hf02 is the high nmaterial currently specified with silica (Si02) in fabricating high reflectors and polarizers for the high-peak-power laser system, Nova, at Lawrence Livermore National Laboratory (LLNL). Hafnia is also known as a'spitter' because during e-beam evaporation, one can actually see molten particles spewing out from the Hf02target. These particles are a likely source of the nodular-coating defects. Silica layers, on the other hand, have