coatings grown by reactive sputtering for dielectric mirrors used for gravitational wave detection*

The ability of LIGO and others in the gravitational wave community to detect astronomical events relies on the quality of optical mirrors used in interferometers. A lot of effort is devoted to reduce optical absorption and mechanical loss in the layers that make up the mirror coatings. Amorphous tantala is of interest as the high index of refraction layer but contributes the most to overall mechanical loss. The mechanisms that lead to mechanical loss must be understood in order to minimize the losses.

Amorphous tantala 500nm films are deposited using reactive sputtering of a tantalum target where growth temperatures are varied from room temperature to 600C. Thermally activated and tunneling mechanisms both contribute to the overall mechanical loss which can be measured through internal friction techniques. The thermally activated are measured at room temperature using Gentle Nodal Suspension and the tunneling are measured at temperatures below 10K using Double Paddle Oscillators. These results provide a deeper understanding of the energy dissipation in amorphous tantala due to both tunneling and thermally activated loss mechanisms.