New Mirror Coatings

New Mirror Coatings Will Increase the Volume of Space LIGO Can Observe in Its Next Run

Since LIGO's groundbreaking detection in 2015 of gravitational waves produced by a pair of colliding black holes, the observatory, together with its European partner Virgo facility, has identified dozens of similar cosmic rumbles sending waves through space and time. [caption id="attachment_134290" align="aligncenter"] Researchers test coatings for LIGO mirrors by placing them on glass disks that are smaller than actual mirrors and therefore easier to use. One of these test disks is shown being removed from its storage container here. Image source: Caltech[/caption] In the future, as increasingly more upgrades are made at the National Science Foundation-funded LIGO observatories, one at Hanford, Washington and the other at Livingston, Louisiana, these facilities are expected to detect an ever-growing number of extreme cosmic events. These observations will help solve fundamental mysteries about our universe, such as how black holes form and how the components of our universe are produced. An important factor in increasing the sensitivity of the observatories is the coatings on the glass mirrors at the heart of the instruments. [caption id="attachment_134291" align="aligncenter"] A view of the measurement system through one of the vacuum chamber windows. Red dots are produced by the probe laser beam. Image source: Caltech[/caption] Each 40-kilogram mirror (there are four in each detector at the two LIGO observatories) is coated with reflective materials that essentially transform the glass into a mirror. The mirrors reflect laser beams that are sensitive to passing gravitational waves. Generally, the more reflective the mirrors, the more sensitive the instrument, but there is an important consideration here: The coatings that make the mirrors reflective can also introduce noise in the device that masks the gravitational wave signals from the background. [caption id="attachment_134292" align="aligncenter"] The vacuum chamber is shown in close-up just before air is pumped out. The chamber must reach a pressure lower than one billionth of Earth's atmosphere before observations of the disk's vibrations begin and measurements of energy dissipation in the coating material are taken. Image source: Caltech[/caption] Now, a new study by the LIGO team describes a new type of mirror coating made of titanium oxide and germanium oxide and outlines how it can reduce the background noise in LIGO's mirrors by a factor of two, thus increasing the volume of space LIGO can observe by a total of eight times. Gabriele Vajente, senior research scientist at Caltech and lead author of the paper in Physical Review Letters, said, "We wanted to find a material at the boundary of what is possible today," and added "Our ability to study the astronomically large scale of the universe is limited by what happens in this very tiny microscopic space." David Reitze, director of the LIGO Laboratory at Caltech, said, "With these new coatings, we hope to increase the detection rate of gravitational waves from about once a week to once a day or more." The research, which may have future applications in telecommunications and semiconductors, was a collaboration between Caltech, Colorado State University, University of Montreal, and Stanford University, which used the synchrotron at SLAC National Accelerator Laboratory for characterization of the coatings. [caption id="attachment_134293" align="aligncenter"] A view of the inside of the measurement vacuum chamber: four samples with different materials can be measured simultaneously. Image source: Caltech[/caption] LIGO detects fluctuations in space-time using detectors called interferometers. In this setup, a powerful laser beam is split in two: each beam travels 4 kilometers down one arm of an L-shaped large vacuum enclosure toward mirrors. The mirrors reflect the laser beams back to their source. Gravitational waves, as they pass through, stretch and compress the void in nearly imperceptible but still detectable amounts (far less than the width of a proton). Perturbations (small deviations in orbit or path) change the timing of the two laser beams' return to the source. Any wobble in the mirrors themselves—even microscopic thermal vibrations of atoms in the mirrors' coatings—can affect the arrival timing of the laser beams and make it difficult to isolate gravitational wave signals. Vajente says, "Every time light passes between two different materials, some of that light is reflected. This is the same thing that happens in your windows; you can see the faint reflection of yourself in glass. By adding multiple layers of different materials, we can strengthen each reflection and make our mirrors reflective up to 99.999 percent." "What is important about this work is that we developed a new way to better test materials," he said. "While we could previously test the properties of a new material in about a week, we can now run the same test completely automatically in roughly eight hours." "This allowed us to discover the periodic table by experimenting with many different materials and many combinations. Some of the materials we tried did not work, but it gave us ideas about which properties might be important." Finally, the scientists discovered that a coating material made from a combination of titanium oxide and germanium oxide dissipated the least energy (equivalent to reducing thermal vibrations). Carmen Menoni, professor at Colorado State University and member of the LIGO Scientific Collaboration, said, "We adapted the production process to meet the stringent requirements for optical quality and the reduced thermal noise of mirror coatings." Menoni and her colleagues at Colorado State used a method called ion beam sputtering to coat the mirrors. In this process, titanium and germanium atoms are ejected from a source, combined with oxygen, and then deposited onto glass to form thin atomic layers. The new coating will be available for the fifth observing run, which will begin in the middle of the decade as part of LIGO's Advanced LIGO Plus program. Meanwhile, the fourth observing run, the last in LIGO's Advanced LIGO campaign, is expected to begin in the summer of 2022. Reitze noted, "This is a great example of how LIGO relies heavily on state-of-the-art research and development in optics and materials science." The study, titled "Low Mechanical Loss TiO2:GeO2 Coatings for Reduced Thermal Noise in Gravitational Wave Interferometers," was funded by the NSF and the Gordon and Betty Moore Foundation.   Source