Extending LIGO's reach into the cosmos

Date:
September 29, 2021
Source:
California Institute of Technology
Summary:
New mirror coatings will increase the volume of space LIGO can
probe in its next run.



FULL STORY ========================================================================== 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 facility Virgo, has detected dozens of similar
cosmic rumblings that send ripples through space and time.


==========================================================================
In the future, as more and more upgrades are made to the National Science Foundation-funded LIGO observatories -- one in Hanford, Washington,
and the other in Livingston, Louisiana -- the facilities are expected to
detect increasingly large numbers of these extreme cosmic events. These observations will help solve fundamental mysteries about our universe,
such as how black holes form and how the ingredients of our universe
are manufactured.

One important factor in increasing the sensitivity of the observatories involves the coatings on the glass mirrors that lie at the heart of
the instruments. Each 40-kilogram (88-pound) mirror (there are four in
each detector at the two LIGO observatories) is coated with reflective materials that essentially turn the glass into mirrors. 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 a catch: The coatings that make the mirrors
reflective also can lead to background noise in the instrument -- noise
that masks gravitational- wave signals of interest.

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 background noise in LIGO's mirrors by a factor of two, thereby increasing
the volume of space that LIGO can probe by a factor of eight.

"We wanted to find a material at the edge of what is possible today,"
says Gabriele Vajente, a LIGO senior research scientist at Caltech
and lead author of a paper about the work that appears in the journal
Physical Review Letters.

"Our ability to study the astronomically large scale of the universe
is limited by what happens in this very tiny microscopic space."
"With these new coatings, we expect to be able to increase the detection
rate of gravitational waves from once a week to once a day or more,"
says David Reitze, executive director of LIGO Laboratory at Caltech.



==========================================================================
The research, which may have future applications in the fields of telecommunications and semiconductors, was a collaboration between
Caltech; Colorado State University; the University of Montreal; and
Stanford University, whose synchrotron at the SLAC National Accelerator Laboratory was used in the characterization of the coatings.

LIGO detects ripples in space-time using detectors called
interferometers. In this setup, a powerful laser beam is split into two:
each beam travels down one arm of a large L-shaped vacuum enclosure
toward mirrors 4 kilometers away. The mirrors reflect the laser beams
back to the source from which they originated.

When gravitational waves pass by, they will stretch and squeezes space
by nearly imperceptible and yet detectable amounts (much less than the
width of a proton). The perturbations change the timing of the arrival
of the two laser beams back at the source.

Any jiggling in the mirrors themselves -- even the microscopic thermal vibrations of the atoms in the mirrors' coatings -- can affect the
timing of the laser beams' arrival and make it hard to isolate the gravitational-wave signals.

"Every time light passes between two different materials, a fraction
of that light is reflected," says Vajente. "This is the same thing
that happens in your windows: you can see your faint reflection in the
glass. By adding multiple layers of different materials, we can reinforce
each reflection and make our mirrors up to 99.999 percent reflective."
"What's important about this work is that we developed a new way to better
test the materials," says Vajente. "We can now test the properties of
a new material in about eight hours, completely automated, when before
it took almost a week.

This allowed us to explore the periodic table by trying a lot of different materials and a lot of combinations. Some of the materials we tried didn't work, but this gave us insights into what properties might be important."
In the end, the scientists discovered that a coating material made from
a combination of titanium oxide and germanium oxide dissipated the least
energy (the equivalent of reducing thermal vibrations).



==========================================================================
"We tailored the fabrication process to meet the stringent demands in
optical quality and reduced thermal noise of the mirror coatings," says
Carmen Menoni, professor at Colorado State University and member of the
LIGO Scientific Collaboration. Menoni and her colleagues at Colorado State
used a method called ion beam sputtering to coat the mirrors. In this
process, atoms of titanium and germanium are peeled away from a source, combined with oxygen, and then deposited onto the glass to create thin
layers of atoms.

The new coating may be used for LIGO's fifth observing run, which
will begin in the middle of the decade as part of the Advanced LIGO
Plus program. Meanwhile, LIGO's fourth observing run, the last in the
Advanced LIGO campaign, is expected to commence in the summer of 2022.

"This is a game changer for Advanced LIGO Plus," says Reitze. "And this
is a great example of how LIGO relies heavily on cutting-edge optics and materials science research and development. This is the biggest advance
in precision optical coating development for LIGO in the past 20 years." ========================================================================== Story Source: Materials provided by
California_Institute_of_Technology. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Gabriele Vajente, Le Yang, Aaron Davenport, Mariana Fazio, Alena
Ananyeva, Liyuan Zhang, Garilynn Billingsley, Kiran Prasai, Ashot
Markosyan, Riccardo Bassiri, Martin M. Fejer, Martin Chicoine,
Franc,ois Schiettekatte, Carmen S. Menoni. Low Mechanical Loss
TiO2:GeO2 Coatings for Reduced Thermal Noise in Gravitational
Wave Interferometers. Physical Review Letters, 2021; 127 (7) DOI:
10.1103/PhysRevLett.127.071101 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210929155704.htm

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