The Stanford LIGO Group and collborators have published a new article in Physical Review Letters. Our team used X-ray scattering measurements at SSRL to probe and model the atomic structure of a promising set of LIGO coatings. Thermal noise from LIGO’s mirror coatings will limit the sensitivity of future detectors, and this work provides a pathway to reducing the thermal noise through a detailed understanding of the atomic structure.
LIGO, the Laser Interferometer Gravitational-wave Observatory, has launched the era of gravitational-wave astronomy. Colliding black-holes and neutron stars produce ripples in space-time called gravitational-waves that travel across the Universe at the speed of light. When they reach Earth, the signals are incredibly small, causing relative changes in distance of about 1000 times smaller than the size of an atom’s nucleus. To detect gravitational-waves, LIGO uses laser interferometers to form the world’s most sensitive measuring tool. With such small signals, atomic-scale movements in the highly reflective mirrors that the lasers bounce-off forms a noise source that limits the sensitivity of LIGO, known as Brownian thermal noise.
The next major upgrade to LIGO called Advanced LIGO Plus is due to start construction in 2022 and plans to double current sensitivity, which means we will see gravitational waves from eight-times more in volume of the Universe. Meeting this sensitivity improvement requires better mirrors with lower Brownian thermal noise. We use high energy X-rays and look at the scattering patterns they produce when we focus them on test coatings of zirconia-doped tantala (Zr:Ta2O5), a promising material. This type of X-ray scattering gives us information about the local atomic structure in the material, using a measurement known as pair-distribution functions (PDFs).
We find that when heating our coatings (annealing) up to 800°C, the mechanical loss, which is directly related to thermal noise, goes down. In addition, when our PDF measurements are combined with atomic modeling, we show that the atomic structure of the coating also changes with annealing, producing subtle rearrangements of polyhedral building-blocks within the material. This change gives rise to an increasing number of polyhedral building-blocks that are connected by sharing a corner (instead of an edge) as annealing temperature increases.
From these results, we suggest that to reduce mechanical loss, and therefore Brownian thermal noise, for room temperature detectors, materials with more corner-shared polyhedral building-blocks would be beneficial and will be the focus of future experiments.