To enhance the PETawatt Aquitaine Laser (PETAL) operation, efforts are directed towards increasing the Laser-Induced Damage Threshold (LIDT) of transport mirrors. Three approaches are being considered : i) changing the design of thin film stacks, ii) the materials, and iii) the deposition process.
Monolayers of pure SiO2, HfO2, Sc2O3 and mixtures of HfO2/SiO2 and Sc2O3/SiO2 were elaborated by magnetron sputtering using oxide targets. Laser damage tests, combined with optical and physicochemical characterizations, revealed that the Sc2O3/SiO2 mixture exhibits the highest LIDT. The introduction of a small amount of oxygen into the plasma reduced the refractive index and improved the LIDT.
A Bragg mirror, designed for PETAL's specifications (R > 99% at 1053 nm for s polarization at 45° incidence) is being manufactured using HfO2 (high refractive index) and Sc2O3/SiO2 (low refractive index). The films thicknesses are finely controlled with the quartz crystal microbalance technique.
KEYWORDS: Optical coatings, Laser microstructuring, High power lasers, Chemical composition, Silica, Resistance, Laser induced damage, Oxygen, Matrices, Laser development
To build quarter-wave plate components for a high-power laser application, the Laboratory for Laser Energetics has developed a 21-layer silica coating fabricated by GLancing Angle Deposition. This stack alternates columnar birefringent layers with isotropic layers. We present a study on the SiO2 matrix state, the sub-stoichiometry and presence of oxygen vacancies that affect robustness and a reduced laser damage resistance. The composition throughout the film thickness is investigated thanks to GD-OES and Tof-SIMS combined with photoelectron spectroscopies for the composition. Anisotropic and isotropic layers exhibit differences in composition, between them and throughout the depth. Photoluminescence measurements show a peak that could represent oxygen vacancies that may reduce the damage threshold. Vibrational characterization further supports our findings. This comprehensive overview is discussed in relation to deposition process and resistance to laser-induced damage and will enable us to improve our current coatings.
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