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Low-Noise Optical Coatings Could Swell LIGO’s Reach

photo of hand holding test sample

LIGO researchers have developed new coatings for its gigantic mirrors that could cut thermal noise in half—potentially vastly increasing the amount of space that the LIGO facilities can probe. [Image: Caltech]

Laser-interferometer gravitational-wave observatories—including the two LIGO facilities in the United States, the Virgo facility in Europe and a slowly increasing number of sites in other countries—are already the most sensitive devices on the planet. In their current configuration, they can pick up impossibly faint ripples in spacetime emanating from cosmic events billions of light years away from Earth.

But LIGO and Virgo scientists want to take things further. To do so, they’ve labored on a variety of improvements, ranging from the use of quantum squeezed vacuum states to new materials to cut back on stray light in the detectors. These upgrades have steadily ratcheted down detector noise and thereby expanded the volume of the cosmos that the facilities can sample. Many of these improvements have come under the informal project heading of “Advanced LIGO Plus” (LIGO A+), an undertaking that aims to push the sensitivity of the existing facilities as far as it can go.

Now, researchers have announced that a key final piece of the A+ puzzle may have fallen into place—the development of ultra-low-noise coatings for the 40-kg mirrors, or “test masses,” that form the endpoints of the interferometer arms (Phys. Rev. Lett., doi: 10.1103/PhysRevLett.127.071101). The mirror coatings could, the scientists report, reduce thermal noise in the interferometers by a factor of two. That improvement, if realized, would result in an eightfold boost in the volume of space the LIGO and Virgo facilities can probe by the middle of this decade.

Giant interferometers

Gravitational-wave observatories are essentially gigantic Fabry-Pérot interferometers, designed for a singular purpose. The interferometer arms are several kilometers long, with the big test masses—large silica disks coated with high-reflectivity stacks of dielectric thin films—delicately suspended on wires at the end. High-powered lasers within the interferometer arms bounce light off of the test masses. The light from the two arms is then routed to the facility’s “dark port,” where it is combined and the interference signal is read.

aerial photo of LIGO Livingston

The LIGO site near Livingston, LA, USA. [Image: Caltech/MIT/LIGO Lab]

As a gravitational wave from a distant cosmic event passes Earth, it nudges the test masses, infinitesimally shortening one interferometer arm and lengthening the other. The differing length changes show up in the interference signal at the dark port. This configuration enables an almost ridiculous level of sensitivity; in one commonly cited statistic, the LIGO and Virgo detectors can pick up strains a thousand times smaller than the width of a proton.

Improving on this mind-bending sensitivity ultimately boils down to methodically rooting out sources of noise in the detector. Much recent attention has gone to hammering down one noise source: quantum noise in the laser signal. In recent upgrade rounds, the LIGO team has addressed this noise source by injecting so-called squeezed light into the detector, to lower quantum shot noise—and, more recently, via “frequency-dependent squeezing”, to minimize quantum radiation-pressure noise at the mirrors.

Tackling mirror thermal noise

But there’s another, pesky source of background noise in the observatories: thermal noise in the dielectric mirror films that coat the heavy test masses. This noise is generated by Brownian motion—the random thermal vibration of the atoms within the coatings. The team behind the recently reported work, which includes scientists from the California Institute of Technology, Colorado State University, the University of Montreal and Stanford University, aimed to develop a next-gen coating that could slash that thermal noise.

The mirror coatings currently used on the LIGO facilities are built up from alternating layers of low-refractive-index silica (SiO2) thin films and high-refractive-index layers of a titanium–tantalum oxide mix (TiO2:Ta2O5). Previous studies had shown that the latter component was the dominant contributor to Brownian noise in the coating. So the team focused on finding a suitable replacement for TiO2:Ta2O5 in the thin-film stack.

One candidate was germanium oxide (GeO2), the detailed properties of which suggested that it would produce much lower Brownian noise. The problem, however, was that GeO2 has a much lower refractive index than TiO2:Ta2O5. Because of that, building up a sufficiently reflective coating using GeO2 instead of TiO2:Ta2O5 would require so many additional layers that it would effectively cancel out the GeO2’s thermal-noise advantage.

Exploring new materials

The researchers found a solution by co-depositing GeO2 with TiO2, and tweaking the proportions until they arrived at the optimum combination of high refractive index and low Brownian noise. Researchers at Colorado State, led by Optica Fellow Carmen Menoni, used carefully controlled ion-beam sputtering to coat glass disks with various proportions of the two materials for evaluation. The disks were then tested by the group of team leader Gabriele Vajente at Caltech.

photo of tester at console

A member of the testing team scans a display of the vibrational modes in one of the test samples. Lead author Gabriele Vajente of Caltech said that improved, automated testing processes slashed the amount of time necessary to converge on the best coating material. [Image: Caltech]

Vajente, in a press release accompanying the research, noted that improvements in testing were one key to finding the new material. “We can now test the properties of a new material in about eight hours, completely automated, when before it took almost a week,” he said. “This allowed us to explore the periodic table by trying a lot of different materials and a lot of combinations.”

A “game changer”

According to the LIGO collaborators, the test masses at the two LIGO facilities’ could be equipped with the improved coatings by the middle of the decade, in synch with the LIGO A+ program—and in time for the sites’ fifth observing run. Optica Fellow David Reitze, executive director of the LIGO Laboratory at Caltech, predicted in a press release that the reduction in thermal noise with the coatings could “increase the detection rate of gravitational waves from once a week to once a day or more.”

“This is a game changer for Advanced LIGO Plus,” said Reitze. “This is the biggest advance in precision optical coating development for LIGO in the past 20 years.”

Publish Date: 03 October 2021

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