Coated with titanium and germanium oxides, the mirrors in the LIGO gravitational wave detector will halve the background noise level. This, in turn, will increase the amount of space that can be explored the next time the detector is launched. Since the first detection of gravitational waves in 2015, the LIGO laser-interferometric gravitational-wave observatory and the European Virgo collaboration have recorded dozens of such events. Albert Einstein predicted the existence of gravitational waves at the beginning of the last century. He suggested that the accelerated motion of objects with powerful gravity can cause waves that create curvatures in space-time. By studying gravitational waves, scientists hope to answer fundamental questions about our universe, such as how black holes form.
One of the limiting factors for detecting gravitational waves is the reflective coating properties of the 40 kg mirrors at each of LIGO’s two observatories (Livingston, Louisiana and Hanford, Washington). Huge mirrors are located perpendicular to each other at a distance of four kilometers from the laser source. In each of the arms of such an L-shaped system, light reaches the mirror, is reflected, returns and enters the detector. If on its way the laser beam encounters gravitational waves that stretch and compress space, this affects the time of its arrival at the detector. The better the mirrors reflect the laser light, the higher the sensitivity. The difficulty is that any movement of the mirrors, even the thermal vibrations of the atoms in their coating, increase the noise level, and it becomes more difficult to separate the signal from gravitational waves. This problem was partially solved by researchers from the California Institute of Technology, Colorado State University, Montreal University and Stanford University. They described a new type of titanium oxide and germanium oxide coating (44% TiO2 and 56% GeO2) that will halve the background noise in the LIGO mirrors, thereby eight times the amount of space that LIGO can explore on its next launch. The coating was applied by ion-beam sputtering, in which the titanium and germanium atoms are separated from the source, then combined with oxygen and then deposited on the glass to create thin layers of atoms. With the new coating, scientists will detect gravitational waves significantly more often: instead of one event per week, one or even several per day, according to David Reitze, executive director of the LIGO laboratory at California Institute of Technology. There is a chance that it will be possible to test this in practice at the fifth launch of LIGO, which is scheduled to start in 2024/2025.