Collaborative Research: Stanford-Florida Program in Support of LIGO on Coatings and Core Optics

In 2015, scientists detected ripples in spacetime called gravitational waves, created by two black holes merging, which launched the field of gravitational-wave astronomy. Improvements in the sensitivity in the Advanced LIGO detectors made this revolution possible. Planned future upgrades to improve detector sensitivity will require reduced-thermal-noise mirror coatings that are used in the detector optics. These improvements will continue to impact gravitational-wave astronomy for at least 20 years. A planned upgrade called A-sharp aims to reduce the thermal noise from the mirror coatings by at least half. The challenge of developing lower thermal noise coatings requires progress in the understanding of the physics of amorphous oxide materials, which is the core research focus of this collaborative project. The collaboration brings together experts in both experimental and theoretical aspects of this research at Stanford University and the University of Florida. The goal is to create mirror coatings that meet the necessary standards for use in future LIGO detectors. The team will continue to train the next generation of STEM researchers and professionals through their multidisciplinary activities and outreach activities.Future planned upgrades to LIGO will seek to install mirrors with further improved coatings, in particular with lower Brownian thermal noise (BTN), which is a key noise source limiting detector sensitivity. The baseline design for the potential A-sharp upgrade calls for a coating thermal noise reduced by a factor of at least two with respect to Advanced LIGO + (A+) levels. Synergies between the Stanford-Florida program and the Center for Coatings Research (CCR) have enabled considerable progress under previous support. Having identified, based on characterization via X-ray scattering and atomic structure modeling, the connection between room-temperature mechanical losses and edge- and face-shared polyhedral structural motifs, the team proposed Ti-doped GeO2 as a high refractive-index low-mechanical-loss coating. Subsequent experimental work in the CCR and LIGO Lab supported this identification, leading to the selection of this material for the A+ mirrors. The research conducted under this award builds on these results, with a goal of finding coating solutions with a further factor of two reduction in thermal noise for mirror upgrades and/or an A-sharp system. The group will work iteratively with deposition groups developing amorphous coatings, both refining the atomic structure models with data generated from the materials they deposit, and by providing those groups with guidance for next steps in their synthesis campaigns based on the results from the atomic structure and modeling efforts. While the major portion of our work will remain focused on amorphous coatings, the group will also continue to contribute to crystalline AlGaAs coatings by characterizing optical absorption and developing models for birefringence and observed excess noise above the expected Brownian contribution.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.