Coatings for Next Generation Gravitational Wave Interferometers

This award supports research in relativity and relativistic astrophysics and it addresses the priority areas of NSF's "Windows on the Universe" Big Idea. The first detection of gravitational waves from the collision of two massive black holes by the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors in Livingston, Louisiana, and Hanford, Washington in 2015 not only confirmed Einstein's predictions of the existence of gravitational waves but also ratified years of efforts in the development of gravitational wave detectors (GWD).

Gravitational wave detectors are interferometers in which an intense laser beam bounces between two sets of mirrors in orthogonal arms. Gravitational waves cause differences in the path length of the laser beam between the two interferometer arms, giving rise to a distinct interference pattern that when analyzed identifies the event that created them.

The detection sensitivity of GWD is determined by various noise sources, among which thermal noise of the coatings in the mirrors of the interferometer is a main component. Thermal noise in the high reflectance amorphous oxide coatings in the interferometer's mirrors causes path length differences that can mask the signals from gravitational waves. To increase the sensitivity of present GWD and meet the demands of future GWD, a concerted effort to understand and control the mechanisms that give rise to thermal noise in amorphous oxide coatings is imperative.

The projects will offer a diverse group of graduate students opportunities to gain in-depth understanding of the physical mechanisms that affect internal friction in amorphous materials and at the same time gain valuable expertise in optical sciences. This interdisciplinary research project will train students with valuable skills to contribute to advance science and technology in academic, national laboratory and industrial settings.

The PI's team has recently achieved a breakthrough result. They have identified mixtures of titanium dioxide and germanium dioxide that show internal dissipations at a level of 0.0001. Such a low level of mechanical loss can provide for an almost a factor of two improvement on the level of Brownian noise with respect to the state-of-the-art materials.

These results will make it possible to produce the mirrors that will meet the thermal noise requirements for the planned upgrades of the Advanced LIGO detectors. The team will investigate the fundamental mechanisms in TiO2 doped GeO2 that lead to such low mechanical loss and implement strategies to further reduce it. It will also investigate the optical properties of thin films to meet the stringent absorption loss requirements of the coatings for Advanced LIGO detectors.

Multilayer dielectric coatings based on these optimized materials will be deposited by ion beam sputtering and characterized for their Brownian noise, which reduction greatly impacts the sensitivity of gravitational wave detectors.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.