Development of mirror materials for gravitational wave detectors

This project seeks to develop new mirror materials to enable to construction of future generations of more sensitive gravitational wave detectors. The first detection of gravitational waves in 2015 was a huge achievement in global physics research, opening a new window on the Universe. Gravitational wave observations have already resulted in new observations of black holes, new estimates of the expansion rate of the Universe, and the identification of merging neutron stars as a source of gamma-ray bursts. To enable more gravitational waves to be detected, from a larger range of sources, improvements in the sensitivity of current detectors are required. One key limit to the performance of current detectors is coating thermal noise. This is associated with temperature-induced vibrations of the mirror coatings used in the detectors, and it is essential that new, lower-noise coatings are developed to enable future detectors to meet their target sensitivities.
This project aims to develop new, low noise mirror coatings for use in future detectors. These coatings must meet strict requirements for thermal noise and also for optical absorption, to minimise heating and thermal deformation of the detector mirrors. Specific objectives include:
* experimental verification of proposed new multi-material coating designs, including feasibility study of applying them to allow novel solutions to meeting the Advanced LIGO+ coating thermal noise target e.g. through the use of amorphous silicon, which has excellent thermal noise properties but was previously considered unfeasible due to high optical absorption.
* testing novel coatings consisting of nano-scale layers of material, and the use non-oxide materials such as amorphous silicon and silicon nitride in these designs.
* identifying correlations between thermal noise and the atomic structure of coating layers, in particular how the relative orientation of atomic structural units affects thermal noise.
The project will involve the development of new facilities for cryogenic measurements of the mechanical loss of multi-layer coatings and for cryogenic optical absorption measurements. These measurements will take place in a newly commissioned cryostat.
This project will be carried out in close collaboration with members of the Optics Working Group of the LIGO Scientific Collaboration, in particular with Strathcylde University (studies of coating deposition parameters), Stanford University (studies of optical properties of coatings and mirror substrates) and National Tsing-Hua University in Taiwan (studies of nitride-based coatings, supported by a Royal Society travel grant).
Consolidating the UK's leadership in gravitational waves remains a priority of STFC, and this project will contribute both the shoter-term upgrades to Advanced LIGO and to longer term development of much more sensitive gravitational wave detectors which will dramatically increase the number of detections, and range of sources detected - vastly increasing the astrophysics and cosmology which can be studied.
In addition to being critical for future gravitational wave detectors, this research is likely to have impact on several other fields of precision measurements, where coatings with low thermal noise and excellent optical properties are required. Examples include optical atomic clocks, high resolution spectroscopy and quantum mechanical experiments investigating the nature of interactions between light and matter. Understanding links between coating structure, thermal noise properties and optical properties is highly relevant for developing coatings with a high laser-damage threshold: such coatings are required for many industrial applications involving high-power lasers.