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.
Glancing-angle–deposited thin films are used to fabricate half-wave plates in a 1-D striped geometry, forming alternating regions of linearly polarized light on a single 100-mm-diam substrate. MgO is selected for fabricating the birefringent films for use in vacuum, based on its formation of isolated columns that avoid potential tensile-stress failure of the porous film. While large-area tests have shown high defect densities for fluences <10 J/cm2 , small-spot laser-damage testing has shown resistance to fluences up to 30 J/cm2 (351-nm, 5-ns pulse). An amorphous silica film is investigated to match the optical thickness in the intermediate regions in an effort to fabricate a polarizationcontrol device to reduce focal-point modulation (“beam smoothing”) in high-intensity laser systems. Ongoing efforts to improve the laser-damage threshold and minimize optical losses caused by scatter are essential to realizing a practical device. Scalability of the process to meter-scale substrates is also explored.
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