Collaborative Research: LSC Center for Coatings Research

The detections of gravitational waves from coalescing black holes in 2015 launched the field of gravitational wave astronomy. The NSF-funded “A+” upgrade to Advanced LIGO is designed to achieve an order of magnitude increase in detection rate for black hole coalescences, and enable detection of fainter objects like binary neutron stars, greatly increasing their value for multi-messenger astronomy. The A+ upgrade and all 3rd generation detector designs depend on the development of mirrors with low coating thermal noise. The coating thermal noise is reduced, primarily, by lowering the mechanical (elastic) loss of the mirror materials. The core research focus of the LIGO Scientific Collaboration (LSC) Center for Coatings Research (CCR) is the development of mirror coatings with low mechanical and optical losses for use in A+ and 3rd generation detectors. The research mission of the CCR includes: understanding and reducing mechanical loss in amorphous metal-oxides, the most widely-used materials in mirror coatings; and developing and testing crystalline (AlGaAs) coatings, which have demonstrated low losses for small mirrors. On a longer time-scale, the CCR is developing mirrors compatible with the proposed 3G detectors’ cryogenic operation. The residual noise visible in the time-domain gravitational waveforms of black hole mergers first recorded by Advanced LIGO is mostly due to quantum noise of the light and thermal noise due to the mirror coatings. Since that first discovery much progress has been made in reducing quantum noise and the coupling from seismic, scatter and jitter noise, leaving coating thermal noise as the dominant barrier limiting gravitational-wave astronomy in the most sensitive observation band. Reducing this noise source for future generations of detectors requires reducing the mechanical dissipation in the mirror coatings on the test masses, and forms the main goal of the CCR. The CCR combines groups working on computational modeling, coating deposition, and characterization of atomic structure and macroscopic material properties. These components are often performed by four 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 communities focused on a unified research goal. In its first two years of operation, research in the CCR has identified different structural motifs associated with room-temperature vs cryogenic mechanical losses, which led to synthesis of germania (GeO2) films giving rise to the lowest-loss amorphous oxide film other than silica. Going forward, this structural guide will serve as a paradigm informing the development of high-refractive index amorphous coatings with lower elastic loss. In addition, thermo-optically-optimized AlGaAs crystalline coatings have demonstrated a coating thermal noise well below the requirements for A+, and the CCR has generated a development schedule to scale up these coatings to LIGO mirror sizes and will continue investigations into these materials. Other research paths include: exploring deposition techniques to produce “ultrastable glasses” using amorphous metal-oxides; and stabilizing amorphous coatings against crystallization in order to allow elastic loss reduction via high temperature annealing either with nano-layering or with different doping materials.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.