Towards Ultrastable Amorphous Coatings for LIGO

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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. This work will form part of that effort. This award supports research aimed at demonstrating amorphous oxide coatings with significantly reduced thermal noise and superior optical properties that will enable an approximate 10x increase in sensitivity of state-of-the-art GWD. The research effort has two specific aims: 1) to realize amorphous metal oxide thin films by ion beam sputtering with reduced thermal noise through a systematic parametric study of the deposition conditions used to grow single films; and 2) to engineer multilayer dielectric coatings using these materials with reduced mechanical loss compared to state-of-the-art. A synergistic characterization effort will support the materials' growth. In particular the CSU researchers will collaborate with Caltech researchers to assess mechanical loss in single and multilayer dielectric coatings. This research will support the LIGO program in different ways. Through fundamental studies in amorphous oxides it will enable investigations of the physical mechanisms that come into play in affecting thermal noise. It will also offer the opportunity to seek for new coating designs that will increase sensitivity of advanced LIGO and of the next generation GWD. Synergistic collaborations with the LIGO Scientific Collaboration (LSC) Coatings Group are planned to congruently advance the fundamental material science and coatings engineering efforts.