Abstract
Laser-induced damage in coating materials with a high index of refraction, such as hafnia, limits the performance of high power and high energy laser systems. Understanding the underlying physics responsible for laser damage holds the key for developing damage-resistant optical films. Previous studies have reported a substantial difference in laser damage onset for hafnia films produced by different deposition methods, yet the underlying mechanisms for the observed difference remain elusive. We combined laser damage testing with analytical characterizations and theoretical simulations to investigate the response of hafnia films produced by electron (e-) beam evaporation vs ion beam sputtering (IBS) methods upon UV ns-laser exposure. We found that e-beam produced hafnia films were overall more damage resistant; in addition, we observed a polarization anisotropy associated with the onset of damage in the e-beam films, while this effect was absent in the latter films. The observed differences can be attributed to the stark contrast in the pressure inside the pores inherent in both films. The high pressure inside the IBS-induced nanobubbles has been shown to reduce the threshold for laser-induced plasma breakdown leading to film damage. The polarization effects in the e-beam coatings can be related to the asymmetric electric field intensification induced by the columnar void structure. Our findings provide a fundamental basis for developing strategies to produce laser damage-resistant coatings for UV pulsed laser applications.
Highlights
The functionality, durability, and lifetime of high power and high energy laser systems rely on the robustness of various optical components including beam steering dielectric coatings
The field intensity is recorded and post-analyzed to determine the peak value and its location at the different media. For both polarization test configurations, we found the laser damage onset for hafnia films produced by e-beam evaporation to be much higher than that of ion beam sputtering (IBS) films
The hafnia films produced using e-beam evaporation has a higher 355-nm damage resistance compared to IBS produced films
Summary
The functionality, durability, and lifetime of high power and high energy laser systems rely on the robustness of various optical components including beam steering dielectric coatings. Hafnia is one of the most important high index materials used for the fabrication of interference coatings for a broad range of laser wavelengths, ranging from ultraviolet to infrared, due to its relatively high onset to laserinduced damage and high energy bandgap.[1,2,3] In recent decades, researchers have been exploring different production methods to manufacture multilayer dielectric coatings utilizing hafnia to meet the demanding laser applications with complex spectral requirements and harsh operational environments Among these deposition methods, electron beam (e-beam) evaporation and ion beam sputtering (IBS) are two commonly used techniques chosen for their ability to scale up and to make environmentally stable coatings, respectively. In order to develop strategies for producing and fielding high laser damage resistance dielectric coatings using established deposition methods for laser applications including UV wavelengths, it is imperative to understand the underlying mechanisms leading to the observed contrast in damage resistance
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