Low-Refractive Index Ta ₂ O ₅ versus Hybrid TiO ₂ –SiO ₂ for Laser-Durable Broadband Antireflection Coatings

Achieving high optical transmission over a broad spectral bandwidth while maintaining a high laser-induced damage threshold (LIDT) remains a central challenge in laser optics. Conventional high–low refractive index (n) antireflection (AR) designs struggle to satisfy these requirements simultaneously, partly due to the limited availability of intermediate n materials. A simple, low-cost, and scalable sol–gel approach was employed to develop low-density Ta2O5 with intermediate n and high LIDT, and its performance was evaluated alongside an analogous TiO2–SiO2 hybrid as a buffer layer in TiO2/SiO2 AR coatings. Both the buffer layer materials exhibit refractive indices of ∼1.67 at 1000 nm, enabling systematic assessment of nanostructure–optical property relationships. A peak transmission of 99.4% with broadband response over 328 nm (>98%) and an LIDT of 18 J·cm–2 were obtained with Ta2O5/TiO2/SiO2 trilayer structure. On the other hand, the (TiO2–SiO2)/TiO2/SiO2 trilayer showed higher peak transmission but narrower bandwidth and lower LIDT. Synchrotron small-angle X-ray scattering revealed the nanoscale morphology of the materials. X-ray reflectivity quantified film density and refractive index while characterizing interface roughness across the multilayer stack, demonstrating exceptionally smooth and well-defined interfaces critical for multilayer optical performance. These structural features modulate scattering pathways that govern the trade-off between transmission, bandwidth, and laser-damage resistance. These findings establish a direct link between nanoscale structure, optical function, and laser-damage behavior, highlighting sol–gel-derived low-n Ta2O5 as a versatile material platform for broadband AR coatings relevant to high-energy laser, photonic, and optoelectronic devices.