Exploring in situ the thermal annealing of amorphous TiO2-GeO2 coatings

Thermal annealing is often exploited to enhance the properties of optical coatings, such as those forming the dielectric mirrors in gravitational wave detectors (GWD). An amorphous mixture of TiO2 and GeO2 has recently been proposed as a key element for the next-generation GWD mirrors, however the effects of thermal annealing on this mixture are poorly understood and hardly predictable because GeO2 – unlike TiO2 and many other transition metal oxides commonly used in optical coatings – is a strong glass former. In this work, we track by means of in situ spectroscopic ellipsometry the evolution of thickness and refractive index of amorphous TiO2-GeO2 coatings during several annealing cycles with different heating rates, maximum temperatures and time durations. We find that the thickness of TiO2-GeO2 coatings increases during the heating ramp but rapidly decreases when 600 °C are reached. Because of this remarkable trend, and unlike all the other oxide coatings considered so far for the GWD mirrors, the thickness of the TiO2-GeO2 coatings after the end of the annealing at 600 °C is almost the same as measured in the initial state. Physicochemical mechanisms that might explain the variations observed in the TiO2-GeO2 properties during the annealing, including the remarkable thickness reduction at 600 °C, are discussed.