The LIGO Scientific Collaboration (LSC) Center for Coatings Research (CCR), funded by the NSF and with planned co-funding by the Gordon and Betty Moore Foundation, seeks to extend the reach of the next generation of gravitational wave detectors by addressing the dominant noise source limiting their performance, thermal noise in the interferometer mirrors. This noise reduces the number of observable gravitational wave signals from astronomical sources. It arises from thermal excitation of the vibrational modes of the mirrors in the LIGO detector optics. The effect of these excitations is reduced as the mechanical quality factor (Q) of the mirrors is increased. Since the Q of the mirrors is limited by the reflective coatings deposited on their surfaces, lower noise requires development of better coatings. The CCR combines groups from 10 institutions in the US working on computational modeling of amorphous materials, deposition of coatings, and characterization of their atomic structure and macroscopic properties. These components are often performed by three diverse communities that work in relative isolation from each other. The strength of the CCR and its promise of accelerating discoveries arises from close integration of these three communities focused on a unified research goal. Coating thermal noise is also a limiting factor in the fields of precision timing, quantum information, low noise interferometry, and precision measurements like the search for deviations in the gravitational inverse-square law. Coatings improving on the state of the art in mechanical and optical properties developed under this program would be applicable to these communities as well. On broader impacts in education, the mixture of undergraduate institutions with elite graduate programs provides research opportunities at all education levels and establishes clear avenues of advancement for students who wish to pursue a career within the physical sciences. In addition, the CCR has a strong commitment to the advancement of women and underrepresented minorities; and will also continue its participants' activities in issues of education and public outreach through all major channels of public and social media.
The A+ LIGO detector, planned for 2021, will reduce quantum noise with squeezed light injection, leaving thermal noise dominant in the mid-band of the detector. Reducing this noise source requires reducing the mechanical dissipation in the mirror coatings on the test masses. The goal of this project is to develop mirror coatings consistent with A+ LIGO's mechanical and optical requirements. Meeting this goal requires solution of a longstanding problem in the physics of amorphous materials: the nature and control of the low-energy excitations in amorphous oxides. On a longer time scale, developing mirror coatings for the cryogenic LIGO Voyager detector broadens the possibilities to include amorphous or crystalline semiconductors. The primary research focus is to find conditions under which amorphous metal-oxide coatings can be deposited as "ultrastable glasses", which have a low density of the structural motifs that form two-level systems, the source of elastic dissipation. A second research track seeks methods to stabilize coatings against crystallization during high temperature annealing, another method known to reduce room temperature elastic losses. For the longer term research towards LIGO Voyager applications, the group will investigate non-oxide coatings such as single-crystal semiconductors AlGaAs and AlGaP, which have shown low mechanical loss for small geometries but whose scale up to the size of LIGO optics is challenging; and also amorphous silicon and silicon nitride, which have attractive mechanical properties but whose optical properties must be improved.
