Towards understanding the difference in ultraviolet, ns-laser damage resistance between hafnia films produced by electron beam evaporation and ion beam sputtering methods

It is well known that dielectric coatings used in high energy laser systems for beam steering are susceptible to laser damage. The laser damage ensued in high refractive index materials, such as hafnia, is responsible for limiting the laser operation fluence and lifetime. Although hafnia is an ideal high refractive index material used in dielectric coatings for a broad range of laser wavelengths, defects developed during the deposition process leads to laser-induced damage. In order to increase the resistance to laser damage and improve laser performance, it is imperative to understand the underlying physics of laser damage in high index coating materials. Earlier work observed a substantial difference in laser damage thresholds for hafnia coatings produced by different deposition methods, yet the underlying mechanisms for the observed difference remains elusive. In this work we investigated the responses of single layer hafnia films produced by two deposition processes, electron beam (e-beam) evaporation and ion beam sputtering (IBS) methods upon UV ns-laser exposure. The films underwent laser damage testing using a 1-on-1 laser damage testing protocol with a beam size of 650 µm (1/e2) at 355 nm and 8 ns pulse duration. Both S and P polarizations were tested at a 45° angle of incidence. Chemical, structural and morphological characterizations of the films both pre- and post-laser damage were performed using Rutherford backscattering spectroscopy, glancing incidence X-Ray diffraction, and optical and scanning/transmission electron microscopy. We found that films deposited from the e-beam process had a higher damage onset threshold (4.4 +/- 0.1 J/cm2) than those deposited by IBS method (2.1 +/- 0.2 J/cm2). Furthermore, a polarization-dependent damage threshold onset was observed for the e-beam evaporated coatings but was not observed in IBS films. Although the typical size of the damage in general is larger for the e-beam produced films, the morphology shows similar foamy appearance in both films. The density of the damage sites, on the other hand, was much greater in the IBS produced films than that by the e-beam method. The observed difference can be attributed to their resulting structural/textural differences inherited in each method: porous in the e-beam films and dense with isolated nanobubbles in the IBS films, which can lead to a large difference in laser-defect coupling. The underlying physical mechanism will be discussed in detail. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. IM Release# LLNL-ABS-809117