updated July 2018
Authors: Iain Martin (UoG), Stuart Reid (UoS).
Mechanical loss measurements by Svetoslava Angelova (UoS).
Coatings manufactured at UoS by Svetoslava Angelova and Ross Birney.
Photothermal absorption measurements carried out at UoG by Simon Tait.
Ellipsometry measurements carried out at UoG by Iain Martin and Svetoslava Angelova.
Holding page for results for zirconia-tantala alloy coatings, from the Strathclyde, Glasgow and UWS groups.
Introduction:
Improvements in the mechanical dissipation from iLIGO to aLIGO were associated with mixing alloys of titania and tantala, with correlations observed between the mechanical loss and the the ratio of hybrid “fragments” within the amorphous atomic network. Alternative metallic oxides have been proposed which could further reduce mechanical dissipation. Zirconia-tantala has been proposed for the following reasons:
Ion beam deposition system at Strathclyde
In collaboration with Glasgow and UWS, and colleagues elsewhere in the LSC, a novel ion beam deposition facility has been created at the University of Strathclyde (within the group led by Stuart Reid). Ion beam deposition has (thus far) been the only deposition technique that can satisfy the optical requirements for LIGO, over the areas required. The Strathclyde lab has therefore developed an ECR (electron cyclotron resonance) ion beam deposition system with increased flexibility as compared to commercial systems. This will enable the investigation of a wide range of deposition conditions for research towards the A+ coatings (and beyond). Alongside this, the facility has demonstrated the first elevated temperature ion beam deposition, with a recent paper published by Caltech, Strathclyde and LSC colleagues (http://iopscience.iop.org/article/10.1088/1361-6382/aaad7c/meta).
Mechanical loss measurements
The following plots below show examples of the mechanical loss measured in zirconia-tantala, showing that losses below ~2e-4 may be achievable in these thin film coatings. Mechanical losses were measured using the ringdown method on fused silica cantilevers (approx 150um thickness, 40mm length). The Zr content was measured to be 8-9% by WDX.
It is important to note the following:
Optical absorption measurements
Preliminary absorption measurements have been carried out. The plot below compares the zirconia-doped tantala to an as-deposited undoped tantala IBS coating from CSIRO, at 1064nm. One would expect the absorption of all coatings to reduce with heat-treatment, and the CSIRO coating would be expected to achieve the absorption required for use in aLIGO. That is, the typical reduction in optical absorption observed in tantala films through heat heat treatment would reduce from 10-30ppm measured absorption to ppm or sub-ppm.
Further details on deposition parameters
There exists a large parameter space to be investigated regarding optimum deposition conditions. At Strathclyde, the following parameters can be investigated:
Future plans
We are commissioning a new (larger) ECR-IBD system which will enable larger area coatings to be deposited (4 x 4” wafers), which will allow disk measurements on a nodal support to be conducted too. The new system had rotational staging installed in June 2018, to improve coating uniformity. Planetary motion will be installed over the coming months. We plan to repeat Zr:Ta2O5 coatings in the new system and verify the repeatability of the above results.
Image of the new set up: