Observation of defect size evolution with annealing using optical scattering

High-quality optical coatings with low mechanical and optical losses are essential for the sensitivity of gravitational-wave detectors. While annealing is required to reduce the mechanical loss of amorphous coating materials, it often induces crystallization, which degrades optical performance through increased scattering. Standard characterization techniques, such as X-ray diffraction, are limited in detecting sparse populations of crystallites or characterizing their individual size distributions. In this work, we demonstrate a supplementary method based on angle-resolved Rayleigh–Mie scattering to detect and characterize individual crystallites within an amorphous coating. We apply this technique to a titanium-doped germania (GeO 2 :TiO 2 ) coating, a promising material for future detectors, subjected to stepwise annealing at 625 ∘ C. By comparing measured differential scattering cross-sections to T-matrix simulations, we extract the radius of individual defects and successfully monitor the evolution of the emerging defect population as a function of annealing. This technique allows for the detection of crystallites or other defects with radii down to ≈80nm and provides a complementary tool for studying the early stages of crystallization in low-noise optical coatings.