Correlations between the mechanical loss and atomic structure of amorphous TiO2-doped Ta2O5 coatings

Highly reflective dielectric mirror coatings are critical components in a range of precision optics applications including frequency combs, optical atomic clocks, precision interferometry and ring laser gyroscopes. A key limitation to the performance in these applications is thermal noise, arising from the mechanical loss of the coatings. The origins of the mechanical loss from these coatings is not well understood. Recent work suggests that the mechanical loss of amorphous Ta2O5 coatings can drop by as much as 40% when it is doped with TiO2. We use a combination of electron diffraction data and atomic modelling using molecular dynamics to probe the atomic structure of these coatings, and examine the correlations between changes in the atomic structure and changes in the mechanical loss of these coatings. Our results show the first correlation between changes in the mechanical loss and experimentally measured changes in the atomic structure resulting from variations in the level of TiO2 doping in TiO2-doped Ta2O5 coatings, in that increased homogeneity at the nearest-neighbour level appears to correlate with reduced mechanical loss. It is demonstrated that subtle but measurable changes in the nearest-neighbour homogeneity in an amorphous material can correlate with significant changes in macroscopic properties. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.