Volume 95 Issue 4 | pp. 26-27
Issue Date: January 23, 2017 | Web Date: January 18, 2017

Specialized coatings help detect gravitational waves

A 2-µm-thick film helped produce the ultrareflective mirrors needed to confirm Einstein’s century-old prediction
Department: Science & Technology
Keywords: coatings, materials, optics, LIGO, gravitational wave, Einstein
Enormous mirrors such as the one being examined here have enabled the LIGO detectors to spot gravitational waves.
Credit: LIGO
This photo gives an aerial view of the LIGO facility in Livingston, La.
Enormous mirrors such as the one being examined here have enabled the LIGO detectors to spot gravitational waves.
Credit: LIGO

It’s only a slight exaggeration to say that a few layers of molecules helped make possible one of the most important physics observations of the past several decades.

Those layers of metal oxides—which barely add up to 2 μm in thickness—coat the highly specialized mirrors that make up the Laser Interferometer Gravitational-Wave Observatory, or LIGO. The coatings make LIGO’s mirrors exceptionally reflective, enabling the enormous pair of optical instruments to detect gravitational waves.

Gravitational waves are curious entities described as “ripples in spacetime” that radiate for billions of light-years as they traverse the universe. Albert Einstein predicted their existence in 1916 in his general theory of relativity. But it took a century until scientists were able to confirm that prediction experimentally. Last year, researchers reported that the ultrasensitive LIGO detectors, one of which is located in Livingston, La., the other in Hanford, Wash., finally detected these elusive waves (Phys. Rev. Lett. 2016, DOI: 10.1103/PhysRevLett.116.061102).

“There was very little room for doubt we actually detected gravitational waves. I could not believe this was really happening.”

Gregory M. Harry, physicist, American University

In his theory, Einstein described gravity in terms of warped spacetime. Some physicists like to depict that warping like the way a trampoline sags under the weight of a heavy bowling ball. If someone rolled a second, lighter ball across the warped trampoline, that ball would orbit and spiral in toward the heavier one, in a manner similar to the effects of gravity on celestial bodies. The 100-year-old theory predicts that as gargantuan bodies such as supernovae and black holes accelerate because of gravity, they will emit vast amounts of energy and generate gravitational waves that propagate through the universe, sometimes passing through Earth.

Detecting those waves turned out to be an extraordinary experimental challenge. Scientists at LIGO started looking for gravitational waves in the early 2000s. For years, they monitored their instruments, searching for telltale but minuscule signs of the waves. But they saw none.

So over the course of several years starting in 2008, the team of more than 1,000 scientists from California Institute of Technology, Massachusetts Institute of Technology, and many other research centers overhauled LIGO’s equipment. These improvements included replacing the mirrors with new advanced ones coated in that thin layer of metal oxides.

To understand why that 2-µm-thick coating played such an important role in spotting gravitational waves, consider the measurement that LIGO’s instruments were tasked with. Like Fourier Transform infrared (FTIR) spectrometers, LIGO’s detectors send light through a beam splitter that directs the two beams along a pair of arms oriented 90° apart. But unlike benchtop FTIRs, which use centimeter-sized interferometers, the arms in LIGO’s instruments measure a whopping 4 km. And the LIGO detectors don’t probe molecules. They’re designed instead to measure with extreme accuracy momentary subatomic-scale changes in the length of one arm relative to the other.

The 4-km-long arms of the LIGO interferometer seen here extend into the green countryside of Livingston, La. Hanford, Wash., hosts a second, nearly identical instrument.
Credit: LIGO
A photo of an enormous mirror.
The 4-km-long arms of the LIGO interferometer seen here extend into the green countryside of Livingston, La. Hanford, Wash., hosts a second, nearly identical instrument.
Credit: LIGO


As laser light bounces back and forth along arms between its mirrors, LIGO senses gravitational waves as minuscule fluctuations in the lengths of the arms.
This schematic shows the design of the LIGO interferometers.


As laser light bounces back and forth along arms between its mirrors, LIGO senses gravitational waves as minuscule fluctuations in the lengths of the arms.

What causes such minuscule changes in the arm’s lengths? According to Einstein’s theory, as gravitational waves pass through Earth, the spacetime ripples will subtly change the length of objects such as LIGO’s arms.

Here’s how LIGO detects those events. A 120-mm-wide beam of laser light bounces back and forth between a pair of mirrors suspended at both ends of the 4-km arms. A portion of the bouncing beams combine and travel to a photodetector. In the absence of gravitational waves, the beams interfere destructively, yielding no signal. A tiny change in the relative lengths of the arms results in constructive interference, leading, in principle, to signals that can be detected.

But actually measuring those signals requires heroic effort in eliminating multiple tiny sources of noise, says LIGO chief detector scientist Peter K. Fritschel. For example, the bouncing laser beam can heat and deform the mirrors ever so slightly and can cause them to move a tiny distance. Heavier mirrors are less prone to generating those types of noise than lighter ones. So during the overhaul, the LIGO team replaced the original 11-kg mirrors with wider, thicker 40-kg fused silica ones. And they upgraded the equipment that prevented environmental vibrations and seismic activity from jostling the mirrors.

They also sought the best coatings possible to ensure the mirrors remain exceptionally reflective to keep the light trapped in the interferometer arms. Doing so maintains the beams’ intensity, thereby maximizing the chance of detecting tiny gravitational wave signals. “The coatings present their own set of challenges and have multiple constraints that need to be addressed,” Fritschel stresses. For example, the material must minimize losses resulting from absorption and the scattering caused by room temperature molecular vibrations in the coating—thermal noise. And the material must be compatible with technologies that apply thin coatings.

Working with LMA, a specialty coatings research center based at Claud Bernard University Lyon 1, in France, the team decided to apply a total of 30 alternating layers of SiO2 and TiO2-doped Ta2O5. Those two materials pair well for this application, Fritschel explains, because in addition to their low absorption, they exhibit a large difference in index of refraction, which enhances the coating’s reflectivity. Doping with TiO2 further boosts the difference in index of refraction. LMA deposited the films via ion-beam sputtering, which produced ultrasmooth coatings on the mirror surfaces. Various tests show that LIGO’s overhaul improved its sensitivity for detecting gravitational wave signals by roughly a factor of three, according to Gregory M. Harry, an American University physicist and former chair of the LIGO optics working group.

So, in September 2015, just days after the upgraded instruments—referred to as Advanced LIGO—came online, the team saw exactly what it had been waiting for. At first, no one believed it was real.

When Harry first saw the signal, he thought it was just another dry run—fake results injected in the data stream to test the equipment and the team’s ability to recognize such signals.

Caltech’s GariLynn Billingsley thought the same thing, “or maybe that it was a hoax,” she says. Billingsley is a senior optical engineer who managed the design, fabrication, and test of some of LIGO’s optical components.

The team quickly ruled out those possibilities, and as Harry says excitedly, “there was very little room for doubt we actually detected gravitational waves. I could not believe this was really happening.”

The researchers then scrutinized the signals, which were detected nearly simultaneously by the twin instruments on opposite sides of the U.S. They concluded that they had detected gravitational waves that were generated 1.3 billion years ago as two black holes—roughly 29- and 36-times the mass of the sun—merged, forming a single black hole.

That detection feat marked the first time gravitational waves were observed directly and the first observation of a pair of black holes merging.

LIGO statisticians cautioned the team that it might take months to detect another event. But the next one occurred just two weeks later. Those signals were less convincing, Harry says, so the team did not publish the results.

But near the end of December 2015, LIGO researchers conclusively detected a second black-hole merger. Dubbed the Boxing Day event, the energy burst, which the team reported last summer, was caused by the coalescence of black holes eight- and 14-times the mass of our sun 1.4 billion years ago (Phys. Rev. Lett. 2016, DOI: 10.1103/PhysRevLett.116.241103).

With those first big successes behind them, the LIGO team is eager to learn more about gravitational waves and black holes, as well as possibly detect neutron-star mergers and other astrophysical events that generate the spacetime ripples. And to do that, they want to further improve their equipment, including the mirror coatings. That may come from advances in molecular-level understanding of the coatings, which remains an area with several unanswered questions.

For example, doping with TiO2 reduced thermal noise in the coating, but the mechanism is unclear. Understanding that mechanism could guide further coating improvements. According to Harry, doping seems to make the average bond distances between tantalum, titanium, and oxygen somewhat more regular, even though the material remains noncrystalline.

“We have clear data showing the material becomes less amorphous upon doping,” he says, but how this structural change is related to reducing thermal noise remains an open research question. Also not well understood is the effect of annealing, which further improves the film’s properties.

LIGO scientists are ecstatic that after decades of searching for gravitational waves, they detected them conclusively with their refurbished detectors, thereby launching a new era of astronomy. But they are hoping to further improve their detection sensitivity—and ideal optical coatings seem to be the key.

As Billingsley puts it, “We are appealing to chemists, materials scientists, and others to help us make better coatings and figure out what’s going on in these films at the molecular level.”

CORRECTION: This story was updated on Jan. 23, 2017, to correct the description of LMA. It is a specialty coatings research center based at Claud Bernard University Lyon 1, in France, not a specialty coatings firm.

Chemical & Engineering News
ISSN 0009-2347
Copyright © American Chemical Society
Alexander Wei (Wed Jan 18 23:30:49 EST 2017)
I have an insider story that goes nicely with this very well-written article on the detection of gravitation waves by LIGO, made possible by the invention of a special coating for mirrors with ultrahigh reflectivity. It involves my father, David Wei, a physicist retired nearly 20 years ago.

In the 1970s, Dad worked for Litton Industries (now part of Northrop Grumman) on a project requiring high-reflectance mirrors for a device known as a ring laser gyro (nowadays an essential component in precision aircraft, like fighter jets). Dad invented the technique that made it possible to produce coatings capable of 99.999% reflectance, pushing the state of the art of laser mirrors to an entirely new level. Unfortunately, politics in the workplace undermined Dad’s contributions and he left the company under a cloud, with his name removed from the patent (his experience is the main reason why I work in academia, and never considered working in industry).

About 10 years later, Litton got involved in a patent fight with Honeywell Industries for rights over the ring laser gyro, forcing them to hire Dad as an expert witness to testify in their favor. In addition to this bittersweet irony, Dad’s name was restored as the primary inventor of the mirror coating method. The Litton-Honeywell lawsuit was a landmark case in patent law for the sheer size of the stakes involved (the first billion-plus dollar lawsuit in history). More importantly, Dad got back the recognition he richly deserved, and rekindled his interest in scientific research.

As a retiree, Dad became involved as a consultant (or maybe just a bystander, but I like to think otherwise) on the Laser Interferometer Gravitational-Wave Observatory, or LIGO, headed by physicists at Caltech. He was very excited to be part of a monumental effort to directly observe gravitational waves, or ripples in space-time, as predicted by Einstein 100 years ago. Gravitational waves were at last detected in September 2015, with new occurrences every few months or so. The key enabling technology, of course, were mirrors that could pick up infinitesimally weak signals across an extremely wide frequency range, by as much as 20 orders of magnitude according to some estimates.

I rarely get a chance to brag about my father’s accomplishments as a scientist; in fact, this might be the first time ever. I guess you might say he is one of the unsung heroes in proving Einstein’s last theory.

Good job, Dad.
Guido Bognolo (Thu Jan 19 02:37:35 EST 2017)
Excellent article and exciting subject. Two questions:
How was it possible to identify the original event that caused the waves (when,where in space and the size of the bodies involved)
The first event was detected at both LIGO labs. Was it also the case for the second event?
A (Fri Jan 20 17:09:21 EST 2017)
I wondered the same thing. I learned a long time ago that scientists just make stuff up, especially regarding origins of whatever they are observing or measuring. They're great storytellers.
J-F Gal (Thu Jan 19 02:48:16 EST 2017)
LMA, is in fact not "a French specialty coatings firm", but a public lab: "Laboratoire des Matériaux Avancés" (Advanced Materials Laboratory, a research lab and technical facility) linked to the University Claude Bernard in Lyon, France. Have a look at their English site: http://lma.in2p3.fr/Lmagb.htm, for their high-tech activities related to very low loss mirror coatings.
Ciosek Jerzy (Thu Jan 19 04:24:44 EST 2017)
Gravitional waves are in fact only a noise effect. Every one who says that gravitational waves propagate through the universe, doesn't understand the quantum physics.
D H (Fri Jan 20 02:40:08 EST 2017)
Adding to this: this article seems also to say that LIGO team decided the design of the coating, the materials to use, the dopping and so on ... That is not what happened: LIGO gave his specifications on the coating (transmission/reflection levels, absorption, etc..) and the LMA did the whole reasearch and developpement for years on these coatings vastly improving the IBS deposition technique on large substrates with very low absorption layers and reaching ultimate coating uniformity levels on the surface. They also worked on the thermal noise leading to the Ti02-dopped Ta2O5 (a PhD was done on this at the LMA), thus lowering the effect of the thermal noise in the coatings and increasing the detector sensitivity. LMA also did all this reasearch work for VIRGO.

The LIGO team (in collaboration with the VIRGO team) has done an excellent work building and finely tuning their interferometers. This work was a colossal one and no one doubt about their scientific and technical capabilities when one sees what they have done. The 2 or 3 micrometers of coating such as said in this article, may be seen as a very little thing comparing to the the whole gigantic interferometer, but it enabled the optical part of the detector.
Correctly crediting other laboratories for their also excellent work and their capital contribution to the project is normal in every scientific community. But acting as if the LMA was only pushing a button to make the coating on LIGO direction is falsifying the truth.
Oh, and yeah, LMA is not a "firm", it is one of the public nationnal laboratories of France (CNRS) as the above comment has said so it is a reasearch laboratory and not just a vendor.

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