Exploring in situ the thermal annealing of amorphous TiO 2 -GeO 2 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 TiO 2 and GeO 2 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 GeO 2 – unlike TiO 2 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 TiO 2 -GeO 2 coatings during several annealing cycles with different heating rates, maximum temperatures and time durations. We find that the thickness of TiO 2 -GeO 2 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 TiO 2 -GeO 2 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 TiO 2 -GeO 2 properties during the annealing, including the remarkable thickness reduction at 600 °C, are discussed.