Establishing the design and development of novel crystalline-amorphous hybrid optical coatings for precision measurements and frequency standards

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Optical coatings - typically consisting of multiple thin layers of alternating amorphous (glass-like) materials - are widely used to form highly reflective mirrors, efficient anti-reflective coatings or optical filters which allow particular wavelengths to be transmitted. Highly-reflective coatings are a key part of gravitational wave detectors. These instruments measure tiny changes in length caused by gravitational waves passing through the Earth, and have so far detected almost 100 astrophysical gravitational wave signals from merging black holes and neutron stars. Future, more sensitive detectors are planned to expand the capabilities of gravitational wave astronomy, and improving the performance of optical mirror coatings is a critical challenge for these new detectors. This project aims to combine a traditional multilayer optical coating with a thin crystalline cap-layer, creating a crystalline-amorphous hybrid coating, enabling optimised coating designs with a number of advantages for both scientific and industrial applications.Using an appropriate crystalline top-layer can allow a significant fraction of the incoming light to be reflected by the top layer along, reducing the laser power present in the multi-layer stack below. This reduced power can in turn change the requirements for the coating materials which can be used in the stack, for example allowing the use of materials with a high optical absorption which would be intolerant to higher light power. This has direct applications in gravitational wave detectors, where minimising the absorption of light in the coatings is essential to allow future mirrors to operated at cryogenic temperatures. There are also many potential applications in industry, where coatings which are tolerant of high light intensity are required in situations where high laser powers, or very small laser beams, are required.