Mixture and nanolaminate – a pathway to understand coating mechanical loss*

Composite oxides in the form of mixture and nanolaminate are extensively used in optical coatings and as dielectric materials. Tunable properties accessible through mixing and nanolayering have drawn great attention to these composite thin films. TiO2-Ta2O5 mixture coating has been shown to reduce the mechanical loss compared to pure Ta2O5 coating thereby improving the sensitivity of the Advanced LIGO gravitational wave detector. In this work, reactive biased target ion beam deposition was used to grow mixture and nanolaminate coatings of SiO2-Ta2O5 and TiO2-Ta2O5. Different mechanisms of mechanical loss reduction through heat treatment are observed. Ta2O5-SiO2 mixtures annealed at 750°C show diffraction only from the crystallized orthorhombic Ta2O5 phase, indicating an immiscibility of SiO2 in Ta2O5. Additionally, Ta2O5-SiO2 nanolayers remained separate by sharp interface after annealing. On the contrary, mixing identified by blurring of interfaces is observed in Ta2O5-TiO2 nanolaminate after annealing at 650°C. This behavior resembles the mixture coating. This work brings out new understanding of the physical mechanism of coating mechanical loss reduction at room temperature.