Effect of residual impurities on the behavior and laser-induced damage of oxide coatings exposed to deep space radiation

Atomic layer deposition (ALD) is an emerging coating deposition technique filled with potential that is expected to deposit coatings with high laser-induced damage threshold (LIDT). As lasers play an important role in space exploration which are gradually moving to higher orbits and longer lifetimes, a higher demand for space laser coatings has been raising. At the current state of the art, the reliability of ALD-deposited laser coatings for space applications is unclear. This work presents investigations on the stability of three ALD-deposited oxide coatings under space proton and UV radiation at values typical for space environment. Changes in the coatings containing low and high amounts of impurities under space radiation were analyzed through optical parse and composition analysis respectively. Both SiO2 and Al2O3 coatings with few impurities generated color centers after irradiation, which resulted in increased absorption loss and reduced transmittance and optical band gap, and increased probability of damage under laser irradiation. However, the changes in the ZrO2 coating were surprising, when the ZrO2 coating with a large amount of impurities was exposed to radiation, the impurity atoms bound to Zr were displaced first, and the released Zr combined with the oxygen vacancies in the coating, which reduced the number of defects, reduced the absorption loss of the coating, and led to a higher transmittance and an increased optical band-gap. This work provided a novel insight on the impact of the impurities on the stability of the ALD-manufactured oxide coatings against laser-induced damage in effect with space proton and UV radiation. Additionally, we show that high initial LIDT in ALD-prepared oxides may be compromised by the space radiation, while fine tuning of the impurities may result a mitigation approach leading to enhanced laser coatings for space applications.