Effects of Low-Temperature Annealing Time on Microstructural Evolution and Optical Loss in Electron-Beam-Evaporated HfO2/SiO2 Highly Reflective Coatings

Electron-beam evaporation (EBE) is a multi-energy conversion process in which electrons are accelerated by an electric field, transfer kinetic energy to the evaporation source, and generate localized thermal energy for material evaporation and film growth. Post-deposition annealing provides an additional thermal field that promotes structural relaxation and defect evolution. In this study, 1064 nm HfO2/SiO2 highly reflective coatings deposited by EBE were annealed at 200 °C for 1–8 h to clarify how the duration of the post-deposition thermal field affects microstructure and optical-loss evolution. Atomic force microscopy combined with power spectral density analysis, spatial photoluminescence mapping, cavity ring-down measurements, and thermal absorption testing were employed. The coating response was distinctly non-monotonic: moderate annealing suppressed multiscale surface fluctuations and defect-related luminescence, whereas prolonged treatment partially reversed these changes. The total optical loss decreased from 1191.05 ppm to 837.88 ppm after 2 h, a reduction of 29.7%. Loss separation showed that absorption remained nearly stable after its initial decrease, while scattering recovered markedly at longer durations and dominated the subsequent increase in total loss. Scattering correlated strongly with surface roughness (r = 0.92), whereas absorption showed a positive correlation with photoluminescence intensity (r = 0.85). These results are consistent with distinct morphology-related and defect-related contributions to scattering and absorption, respectively, in EBE HfO2/SiO2 coatings.