The LIGO coating materials are tested on thin glass disks that are much smaller than the LIGO Mirrors. The thin layer of metal oxide that has been deposited on the surface is responsible for the pinkish hue in the photo. Credit to Caltech
The Laser Interferometer Gravitational-wave Observatory's (LIGO), 2015 breakthrough detection of gravitational waves from a pair colliding black holes, has seen dozens of similar cosmic rumblings in space and time.
As more upgrades are made at the LIGO observatories funded by the National Science Foundation in Hanford, Washington and Livingston, Louisiana, the facilities will be able to detect more of these rare cosmic events. These observations will allow us to solve fundamental mysteries such as how black holes form or how our universe is made.
The coatings on the glass reflections at the heart of the observatories are an important factor in increasing their sensitivity. Each 40-kilogram (88-pound), mirror is coated with reflective materials. This transforms the glass into mirrors. Mirrors are sensitive to gravitational waves passing through them and reflect laser beams.
However, generally speaking, instruments that are more reflective will be more sensitive. But there's a catch. Background noise can also result from the coatings on the mirrors. This background noise masks the gravitational-wave signals that may be of interest.
A new study from the LIGO team has now described a type of mirror coating that is made of titanium oxide/germanium oxide. It can reduce background noise by two factors in LIGO's mirrors, increasing the space that LIGO can explore by eight.
Gabriele Vajente is a senior LIGO researcher scientist at Caltech. She was the lead author of the paper in Physical Review Letters. "What happens in the very small microscopic space is limiting our ability to study the vast, astronomically large universe."
"With these new coatings we expect to increase the detection rate gravitational waves from once per week to once per day or more," said David Reitze (executive director of LIGO Laboratory at Caltech).
This research may have future applications within the fields of telecommunications or semiconductors. It was a collaboration between Caltech, Colorado State University, the University of Montreal, and Stanford University. The coatings were characterized using the synchrotron at SLAC National Accelerator Laboratory.
LIGO uses interferometers to detect ripples in space and time. This setup allows a powerful laser beam to be split into two beams. Each beam travels down one end of a large L-shaped vacuum enclosed towards mirrors four kilometers away. Mirrors reflect laser beams back towards their source. Gravitational waves can pass through space and stretch it by almost imperceptible, but detectable amounts. This is much less than the width a proton. These perturbations can alter the timing of two laser beams returning to the source.
Any jiggling in mirrors, even the microscopic thermal vibrations caused by the atoms in mirror coatings, can affect the arrival time of laser beams and make it difficult to isolate gravitational-wave signals.
Vajente says that a fraction of the light that passes through two materials is reflected. This is exactly what happens in your windows. You can see the faint reflections in the glass. We can add multiple layers of different materials to strengthen each reflection, making mirrors 99.999 percent reflective.
Vajente says, "What's important in this work is that it developed a new method to better test materials." We can now test the properties a new material's properties in just eight hours. This is a significant improvement over what it used to take almost a week. We were able to explore the periodic tables by trying many different materials and combinations. Some materials didn't work out, but it gave us insight into which properties might be important.
The scientists eventually discovered that a coating material made of a mixture of titanium oxide, germanium oxide dissipated least energy (equivalent to reducing thermal vibrations).
Carmen Menoni, a professor at Colorado State University who is also a member of the LIGO Scientific Collaboration, says that she modified the fabrication process to meet stringent requirements in optical quality and lower thermal noise for the mirror coatings. Menoni and her Colorado State colleagues used an ion beam-sputtering method to coat mirrors. This involves atoms from titanium and germanium being removed from a source and combined with oxygen to form thin layers of atoms.
This new coating could be used by LIGO for its fifth observation run. It will take place as part of Advanced LIGO Plus and begin in the middle to the tenth decade. In the meantime, LIGO's fourth observation run, which is the last in Advanced LIGO campaign will begin in the summer 2022.
Reitze says, "This is a major breakthrough for Advanced LIGO Plus." "This is a wonderful example of how LIGO depends heavily on cutting-edge optical research and materials science research and design. This represents the greatest advance in precision optical coating technology for LIGO over the past 20-years.
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Gabriele Vajente and colleagues, Low Mechanical Loss TiO2 :GeO2 Covers for Reduced Thermal Noisy in Gravitational Wave Interferometers. Physical Review Letters (2021). Journal information: Physical Review Letters Gabriele Vajente et al, Low Mechanical Loss TiO2:GeO2 Coatings for Reduced Thermal Noise in Gravitational Wave Interferometers,(2021). DOI: 10.1103/PhysRevLett.127.071101
