Long-term aging of SiO₂/HfO₂ PIAD-deposited Laser Mégajoule polarizer coatings

The 176 beams of the Laser Mégajoule (LMJ) are equipped with meter-scale polarizers that filter parasitic light generated before amplification. These multilayer dielectric coatings present instabilities over the years that can lead to a significant transmission loss. To evaluate the causes, HfO₂ and SiO₂ coatings were deposited by evaporation with and without plasma-assistance. Our studies show that a spectral redshift of the polarizers over the years is mainly generated by the SiO₂ layers. A correlation between aging and coatings' moisture sensitivity is evidenced and interpreted as a consequence of water diffusion and the hydrolysis of SiO₂. Hydrothermal treatments successfully accelerated aging, offering a promising method to stabilize the coating before use.