1. Synopsis by Maria Ruiz-Gonzalez 12/8/16
A study of coating mechanical and optical losses in view of reducing mirror thermal noise in gravitational waves
Synopsis by Maria Ruiz-Gonzalez
12/8/16

2. Gravitational waves What are GW? Fluctuations in space-time.
What causes GW? Massive objects moving with violent accelerations, colliding black holes, supernovae, birth of the Universe…
What can we do with them? Observe a different and complementary view of the Universe.

3. Gravitational-wave observatories
LIGO Scientific Collaboration (USA)
Virgo Collaboration (Italy)

4. Gravitational-wave detectors
The detector is a Michelson interferometer with 4-km Fabry-Perot cavity arms. The Fabry-Perot cavities increase distance traveled by each laser beam and sensitivity.

5. Noise in gravitational wave detectors
The interferometer have to measure changes of length on the order of m. Since the signals are read from noisy background, all sources of noise must be reduced as much as possible.
Mechanical and optical losses caused by mirror coatings produce thermal noise, which reduces sensitivity and optical interference.
1000 times less than the diameter of the nucleus of the hydrogen atom.

6. Objective
To test different high-index coating materials in order to reduce optical absorption and mechanical losses.
Advanced LIGO mirrors:
Diameter: 34 cm
Thickness: 20 cm
Mass: 40 kg
Material: fused silica.

7. Methods
Optical loss measurements: A 30 W Nd:YAG laser is sent to the coating material to be tested, which produces a thermal gradient in the silica substrate. A second laser (He-Ne, probe beam) is used to measure deflection.
Mechanical loss measurements: Thin silica cantilevers are coated with the material to test. The mechanical quality factor of the cantilever resonant modes in the coating are measured before and after the cantilever is coated.

8. High-index coating materials
Initially, the coating was alternate λ/4 layers of Ta2O5 and SiO2. It was previously found that noise caused by Ta2O5 was a limiting factor for the detector performance.
Material
Refraction index
Absorption (ppm)
Mechanical losses (x10-4)
Ta2O5
2.035
1.22 3
Ta2O5:Co
2.11
5000 11
Ta2O5:W
2.07
2.45
7.5
Ta2O5:W+Ti
2.06
1.65
3.3
Ta2O5:Ti
0.5
2.4
ZrO2
2.10
2.3
ZrO2:Ti
2.15 37
6.8
ZrO2:W
2.12 10
2.8
Nb2O5
2.21
2.2
4.6
Improve material by doping it with other oxide, which are materials kwon to have good transparency to NIR light.
Increase index of refraction with increases index contrast and reflectivity.
TaO: Tantalum pentoxide
ZrO: Zirconium dioxide
NbO: Niobium pentoxide
Co: Cobalt
Ti: Titanium
W: Tungsten

9. Thermal noise
Thermal noise for four different types of high-index materials combined with silica and alumina was simulated. The simulation includes other sources of thermal noise.

10. Discussion
Experiments and simulations indicate that the high-index coating material that gave the best result was Ta2O5:Ti.
This synopsis is from an article published in 2010.
In 2015, a gravitational wave was detected for the first time by both LIGO detectors, produced by the collision of two black holes.
The mirror coating was SiO2 /Ta2O5:Ti(25%) (thinner than λ/4).
A second gravitational-wave observation was announced earlier this year.

11. References
[1] R. Flaminio, J. Franc, C. Michel, N. Morgado, L. Pinard and B. Sassolas; "A study of coating mechanical and optical losses in view of reducing mirror thermal noise in gravitational wave detectors," Class. Quantum Grav, 27, (2010).
[2] Gregory M. Harry; "Advanced LIGO: the next generation of gravitational wave detectors," Class. Quantum Grav, 27, (2010).
[3] Gregory M. Harry et al.; ”Titania-doped tantala/silica coatings for gravitational-wave detection”, Class. Quantum Grav, 24, (2007).
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