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

The detections of gravitational waves from coalescing black holes by the Advanced LIGO detectors has launched the field of gravitational wave astronomy. Increasing the sensitivity of the LIGO detector several times would increase the number of gravitational waves and the types of events observed. Future detectors, such as the A+ LIGO detector, planned for 2021, will be limited by thermal noise associated with the mirror coatings used in the detector optics. The proposed work is a collaborative effort between Martin Fejer's group at Stanford University and Hai-Ping Cheng's group at the University of Florida to develop mirror coatings with lower thermal noise to address this problem for A+ LIGO and beyond. The Stanford gravitational wave research program has been involved for more than two decades in research to enable gravitational wave detectors by working closely with the LIGO Science Collaboration (LSC) to do critical research and mitigate difficult challenges. In the past, Stanford has contributed broadly to the development of novel interferometer components and design, detailed studies of the optics and mitigating optical and thermal noise, and the advanced seismic isolation systems used in Advanced LIGO (aLIGO). Hai-Ping Cheng's group at Florida is involved in computational materials simulations. In support of LIGO, she has modeled atomic structure of amorphous films and evaluated mechanical losses associated with those structures as part of a broader LSC effort to develop low-thermal-noise mirror coatings.

While Advanced LIGO has now operated with adequate sensitivity to detect black hole coalescences, its mid-band sensitivity will be limited by thermal noise resulting from mechanical dissipation in the mirror coatings. Stanford has had a leading role within the LSC in developing experimental methods to characterize the optical, elastic, and structural properties of the amorphous materials composing multilayer dielectric mirrors. Florida carries out the current computational materials modeling effort within LSC. The proposed program is a synergistic teaming to combine these skill sets to address a critical issue to meet the design goals of A+ LIGO, developing mirrors with 2-4 times less mechanical loss than the best currently available. The mechanical losses in amorphous materials depend on subtle, preparation-dependent features in their atomic structure. Data on these structural features obtained via the electron diffraction and X-ray scattering methods proposed here is challenging to interpret, as are molecular dynamics predictions of the structure. Methods exist to use the modeling to help interpret the data and the data to help constrain the modeling, which led to the teaming arrangement proposed here. The structural data and predictions for dependence of elastic losses on material composition and process conditions, will become a major contributor to the broader LSC program to develop mirrors for A+ LIGO, guiding the others working on this problem through the thicket of possible synthesis and characterization experiments. Another long-standing effort at Stanford has been in the optical characterization of low-optical loss materials at the sub-ppm/cm level, dating back to the selection between silica and sapphire for initial LIGO test masses. The group has recently begun using the interferometric tool developed for those studies to characterize cryogenic losses in single-crystal silicon samples to evaluate their suitability as test masses in the planned cryogenic LIGO Voyager.