Next generation optical coatings to fully open the window on the gravitational Universe

More than a hundred years ago, Einstein predicted the existence of gravitational waves (GWs) - ripples in space-time caused by violent events such as the merger of two black holes. These GWs curve space and therefore change the distance between objects, however even the largest and most violent astrophysical events will only cause km-scale distances to change by a fraction of an atomic radius. Observing GWs requires an unprecedented level of measurement accuracy, that pushes the very limits of technology across numerous fields of research including quantum dynamics and materials engineering. After decades of detector development, the first detection of GWs (GW150914) was confirmed by Advanced LIGO on the 14th September 2015, associated with the collision of two black holes. The LIGO instrument relies on measuring the position of large suspended mirrors, held under vacuum and separated by 4 km, by laser interferometry. Similar to the advancement in optical telescopes, there is a need to extend the astrophysical of GW detectors in order to observe the entire gravitational Universe. This would open up an entirely new era of multimessager astronomy, in addition to observing events that are otherwise invisible to optical telescopes. The impact that this can have on our understanding of the Universe was first seen in the detection of GW170817 - the first observation of two neutron stars colliding. The collective information from the GW signal and electromagnetic (optical) counterpart and afterglow helped answer key questions including the origin of elements.

The sensitivity of current and future GW observatories is expected to be limited, at the most sensitive frequency band, by thermally-driven motion of the mirrors associated with the coatings required for high-reflectivity. In addition to this, experience of operating the second generation of GW detectors has shown that optical imperfections in these coatings (point defects and nanobubbles that induce absorption and scatter of the laser light) is also creating a fundamental barrier to extending their astrophysical reach. There is therefore an immediate need to push beyond the state-of-the-art in laser mirror coating technology, to allow the gravitational Universe to be fully opened, and understand the extremes of the Universe.

The mirrors are designed to have high-reflectivity by use of thin layers of dielectric materials stacked in a specific way. Research over recent years has identified several materials that display lower levels of thermal noise and could potentially improve current detectors. However, as mentioned, the international community are faced by several major challenge that need to be solved particularly the presence of nanometer sized bubbles in the coatings that cause increased laser absorption/scatter. Within this project, these challenges will be addressed through leveraging world-leading expertise and the state-of-the-art STFC-funded facilities in the UK. I will employ next-generation instrumentation to characterise the coatings and optimise the deposition process to mitigate unwanted features. Data science techniques will be employed to aid the research, potentially providing faster turnaround of results that significantly advance the field. Over recent years, I have researched extensively these materials and demonstrated significant success in providing solutions and pathways for improvements.

As we continue to extend our reach into the mysteries of the Universe, now with tools such as GW detectors, it is a priority for the UK to continue to drive research innovations for these instruments, allowing us to fully open the window on the gravitational Universe.