Exploring Fabrication of Ultra-low Absorption, Multilayer, Hafnium High Reflectors
Hoang Nguyen | 21-FS-040
Ultra-low absorption, high damage threshold coatings are critical to advancing high-power continuous-wave (CW) laser technologies, and in most cases, are a limiting factor on the output power. Since high-power CW laser systems enable extremely high irradiances, when the light is incident on a highly reflective (HR) mirror, part of the energy is absorbed and converted into heat, which can have many detrimental side effects. The worst-case scenarios are optical distortion and laser-induced damage. These optics must operate reliably with irradiances of hundreds of kilowatts to megawatts per square centimeter (kW/cm^2 to MW/cm^2). Although pristine optics can now routinely survive such exposure, the presence of surface contamination and defects can drastically reduce their performance, leading to far lower reliability when operating in real-world conditions. Since thermal absorption is the dominant mechanism in a continuous-wave regime, absorption of the optical component must be reduced to a minimum level.
To date, the vast majority of high-power CW HR optics are composed of niobia-silica, titanium oxide-silica, and tantala-silica. However, hafnia-silica reflectors hold enormous potential as these higher bandgap reflectors can be an order of magnitude more resilient with contamination, although they also exhibit high absorption and scatter properties, factors of more than one hundred times higher than tantala-silica. The goals and objectives of this study were to determine whether readily available hafnium or hafnia target materials could be used to produce hafnia high reflector coatings with >99.9% reflectance and having absorption less than 10 parts per million. These materials would represent some of the best hafnia coatings available today.
This feasibility study met and exceeded all the proposed goals and objectives, demonstrating (1) the capability to coat and post-process hafnia-silica-based high reflectors with very low absorption and (2) that use of either "composite" hafnium-silica or "doped" hafnia-silica materials could produce high reflector coatings with absorption values on par with some of the lowest absorption coatings from amorphous target materials commercially available today. Additionally, this study showed a strong effect of annealing on the material structure and optical properties, the correlation between absorption value and nanosecond and femtosecond laser damage, and that the optical distortion due to coating stress is greatly reduced during annealing.
Optical coatings that handle hundred kW/cm^2 to MW/cm^2 laser power levels without optically distorting and damaging are critical technology for the future of high-power lasers, a core competency of Lawrence Livermore National Laboratory. The development of this technology aligns with the Laboratory's 2021 Investment Strategy for Science and Technology that includes sustaining its world leadership in high-energy, high-power laser systems to accelerate stockpile-stewardship-related high-energy-density science and to support Department of Defense initiatives. A successful demonstration of a low absorption hafnia high reflector coating would pave a path to higher power, more robust, and more compact laser systems.