Sol-gel dip coating of Ta2O5/SiO2 anti-reflection films on fused silica for developing laser optics with high laser induced damage threshold

Abstract

Ta₂O₅/SiO₂ bi-layer anti-reflection (AR) films have been deposited for various wavelengths on ultra-smooth, large fused silica (FS) substrates using the sol-gel dip-coating technique. This method facilitates the development of transmission windows with high laser-induced damage thresholds (LIDT). Super-polished, 50 mm diameter flat FS substrates were AR-coated on both sides using this simple and cost-effective approach. The nearly stoichiometric thin films of Ta₂O₅ and SiO₂ were synthesized using halide-free precursors—tantalum ethoxide and tetraethyl orthosilicate, respectively. The morphology, composition, and optical properties of these thin films were characterized before applying them as bi-layer AR coatings on FS substrates. The refractive indices of the Ta₂O₅ and SiO₂ thin films were determined to be 1.75 and 1.44, at 1000 nm, respectively. AR coatings were deposited for four target wavelengths—850 nm, 1053 nm, 1300 nm, and 1550 nm—by varying the film thickness with precise control over the withdrawal speed during the dip-coating process. Deposition parameters were optimized to minimize optical absorption, achieving a maximum transmission of >99% and reflectivity of <1% for all AR coatings. All sol-gel AR-coated FS transmission windows demonstrated a high LIDT of ~20 J/cm², making them highly suitable for high-energy laser applications.

Graphical Abstract