Introduction: Optimizing the properties of amorphous optical coatings through thermal annealing is crucial particularly in the field of gravitational wave detection (GWD)^1,2. Traditionally, the effects of annealing protocols on interferometry mirror coatings have been studied post-process, leaving the real-time dynamics during annealing largely unexplored. This study introduces a novel technique for real-time monitoring of optical properties during annealing, applicable to all transparent thin-film materials. We applied this technique to Titania–Tantala, Hafnia–Tantala and Titania–Germania thin films to investigate the impact of annealing parameters on their optical properties.

Methods: We employed real-time, in situ spectroscopic ellipsometry (SE) to track changes in the refractive index and thickness of transparent thin-film coatings during a controlled annealing process. The standard annealing protocol for current GWD mirrors was utilized, involving a heating ramp from room temperature to 500°C, a 10-hour plateau at 500°C, followed by cooling. Continuous SE measurements were recorded throughout the cycle. Additionally, various annealing protocols were tested on multiple samples to fine-tune the annealing parameters.

Results: For Titania–Tantala coatings, significant changes in thickness and refractive index were observed during the heating ramp, beginning at approximately 200°C and accelerating between 250°C and 350°C. A smaller, steady evolution was noted during the 10-hour high-temperature plateau. Similar trends were observed for other materials, each exhibiting interesting characteristic behavior.

Conclusion: Our results suggest potential improvements to the current annealing protocols for Titania–Tantala and other candidate materials for GWD mirrors, such as Titania–Germania and Hafnia–Tantala. Traditional ex post analysis fails to capture these real-time dynamics, highlighting the necessity of our approach for systematic, real-time investigations. This method not only enhances our understanding of how annealing parameters influence optical coating but also facilitates the optimization of protocols for new GWD mirror materials.