Gravitational wave detection and reflective coating on mirrors

Gravitational wave detection and reflective coating on mirrors Kazuhiro Yamamoto Faculty of Science, Uniersity of Toyama Synchrotron Radiation Theory Workshop Gofuku campus, University of Toyama, Japan 15 Oct. 2019 1

Most important message Gravitational wave is one of branch of astrophysics. In order to detect gravitational wave, excellent detector is necessary. Excellent detector needs excellent material. Solid state physics can provide excellent material. 2

Contents 1. Gravitational wave 2. Gravitational wave detector 3. Thermal noise 4. Reflective coating 5. Our mission 6. Summary 3

1. Gravitational wave General Relativity 1915 A. Einstein “Gravity is curved space time. Matter makes space time curved. Trace of matter can not be straight 　　　　　　 in curved space time. ” A. Einstein (Wikipedis) 4

1. Gravitational wave　 1916 A. Einstein “When heavy matters move drastically, ripple of space time (gravitational wave) is generated. The speed is the same as light speed. ” Rotating heavy stars A. Einstein (Wikipedis) 5

1. Gravitational wave Since effect of gravitational wave is extremely tiny, it took one century to detect it directly. Finally, LIGO (Laser Interferometer Gravitationalwave Observatory) detected on September 2015. Measurement Simulation 6 6

1. Gravitational wave October 2017：It was announced that three scientists in LIGO won Nobel prize ! Rainer Weiss Barry C. Barish Kip S. Thorne 7

1. Gravitational wave Detection of gravitational wave revealed that there a lot of black hole binaries. Black hole binaries never emit anything. Gravitational wave is unique exception. So we were not sure that such binaries exist until first detection. Gravitational wave astronomy begins. 8

1. Gravitational wave Observation 3 : April 2019 – April 2020 LIGO and Virgo Gravitational wave signals were reported every week ! Almost all might come from black hole binaries … Gravitational wave astronomy would be more mature. 9

1. Gravitational wave After Galileo Galilei obseration, optical telescope were developed drastically to detect light which comes from farther parts of Universe. In order to detect gravitational wave which from farther parts of Universe, noise of gravitational wave detector would be reduced. Galileo Galilei (Wikipedia　Japanese) 10

2. Gravitational wave detector In short, gravitational wave detector is HUGE Michelson interferometer. 3 km-4 km 鏡 3 km-4 km 11

2. Gravitational wave detector In short, gravitational wave detector is HUGE Michelson interferometer. 3 km-4 km 鏡 3 km-4 km When gravitational wave comes, power at photo detector changes. 12

2. Gravitational wave detector Terrestrial gravitational wave detectors 4 km Arms a. LIGO Hanford (Observation) 600 m Arms GEO 600 -HF Advanced (Observation) VIRGO (Observation) a. LIGO Livingston 3 km Arms (Observation) 4 km Arms LIGO India (Constriction) KAGRA (Observation soon !) 3 km Arms 4 km Arms 13

2. Gravitational wave detector Timeline of gravitational wave detector 1 st generation：No detection 10 times smaller noise 2 nd generation : Current First detection 10 times smaller noise 3 rd generation：Future plans for mature astronomy 14

3. Thermal noise of mirror Mirror surface fluctuation caused by thermally excited elastic motion This is some kinds of Brownian motion. 15

3. Thermal noise Amplitude of thermal noise is proportional to 1/2 (T/Q) T : Temperature Q : Q-value (inverse number of amplitude of mechanical loss) 16

3. Thermal noise Q-value (Decay time of elastic resonance motion of mirror) Higher Q implies lower loss. 17

3. Thermal noise Amplitude of thermal noise is proportional to 1/2 (T/Q) High Q-value material for mirror is definitely necessary ! How about cooling ? 18

3. Thermal noise 2 nd generation detector LIGO and Virgo : Room temperature mirrors KAGRA : 20 K mirrors ! 3 rd generation detectors also will adopt cooled mirrors. KAGRA mirror 19

3. Thermal noise Amplitude of thermal noise is proportional to 1/2 (T/Q) High Q-value material for mirror is definitely necessary ! How about cooling ? It is effective. Note: In general, Q-value depends on T. We must investigate temperature dependence of Qvalues. 20

4. Reflective coating Thermal noise caused by coating mechanical loss Mirror consists of not only bulk ! Reflective coating 21

4. Reflective coating : Dielectric multilayer (amorphous) coating for high reflectivity Two kinds of layers (Ta 2 O 5/Si. O 2) Optical thickness is quarter of wavelength Reflected light : Constructive interference Transmitted light : Destructive interference http: //www. tokaioptical. com/product 01_en. php 22

4. Reflective coating : Dielectric multilayer coating for high reflectivity Advantages (1)Low optical loss (Absorption and scattering) (2)No break with high power http: //www. tokaioptical. com/product 01_en. php 23

4. Reflective coating Mirror consists of not only bulk ! Reflective coating Thickness of coating : ~ 5 mm Thickness of mirror : ~ 10 cm Why can coating contribute thermal noise ? Nobody cared until 1998. 24

4. Reflective coating Mirror consists of not only bulk ! Reflective coating Thickness of coating : ~ 5 mm Thickness of mirror : ~ 10 cm Loss angle f (1/Q) Bulk : 10 -8 Coating : 10 -4 Comparable ? This is not end of story … 25

4. Reflective coating Y. Levin, Physical Review D 57 (1998) 659. Laser beam This is qualitative discussion, but conclusion is amazing. Q-value of loss at A is the same as that at B. Loss at A can shake illuminated surface more largely owing to conservation of momentum. Contribution of loss near beam spot is quite larger ! 26

4. Reflective coating Quantitative evaluation Analytical formula about coating thermal noise G. Harry et al. , Classical and Quantum Gravity 19 (2002) 897. N. Nakagawa et al. , Physical Review D 65 (2002) 102001. Numerical evaluation (Finite Element Method) K. Yamamoto et al. , Physics Letters A 305 (2002) 18. Conclusion Coating thermal noise is larger than bulk thermal noise. 27

4. Reflective coating Old summary of coating mechanical loss Similar results (same order of magnitude) Structure damping Loss angle : 1/Q K. Yamamoto et al. , Physical Review D 74 (2006) 022002. 28

4. Reflective coating Current status 2 nd generation detector LIGO and Virgo : Detector noise will be dominated by coating thermal noise soon. Coating mechanical loss must be suppressed. KAGRA : Thanks for cryogenic technique, we need not to reduce coating mechanical loss. 3 rd generation detectors Coating mechanical loss reduction is necessary even if they adopt cooled mirrors. Solid state physics support is helpful. 29

4. Reflective coating How to reduce coating mechanical loss So far, no silver bullet … (We have to consider not only mechanical but also optical properties of coating) Two promising candidates with low mechanical loss (Somebody could give other candidates …) (a) Al. Ga. As/Al. Ga. P : Crystal coating Problem : Is it feasible for large mirror for long arm interferometer ? (b) Amorphous Silicon Problem : Large absorption 30

5. Our mission How to reduce coating mechanical loss So far, no silver bullet … Many groups are struggling to reduce mechanical loss. We intend to join. Our mission Measurement of mechanical loss at cryogenic temperature Place : Low Temperature Quantum Science Facility, University of Toyama Liquid nitrogen and helium is provided. We use dewar in this facility. We put vacuum chamber into dewar. This dewar is cooled down by liquid nitrogen and helium. 31

5. Our mission Measurement of mechanical loss at cryogenic temperature Vacuum chamber (supported by grants from Toyama daiichi Bank and president of our University) 16 cm in diameter, 34 cm in height 32

5. Our mission Measurement of mechanical loss at cryogenic temperature We have sapphire disks with and without coating. Measured Q-value difference implies coating mechanical loss. Our coating is exactly same as that on LIGO mirror. (Coating deposition at same time in LMA- Lyon, France) This sample is our first step. 33

5. Our mission Measurement of mechanical loss at cryogenic temperature Current status Three students Graduate student : Mori Undergraduate students : Hattori, Nakayama Vacuum chamber in liquid nitrogen has been checked. We are preparing electrical cables and so on. We will measure coating loss at room temperature soon. 34

6. Summary Gravitational wave was detected finally (100 years after Einstein’s prediction). This is beginning of gravitational wave astronomy. In order to mature gravitational wave astronomy, detector noise must be reduced, for example, thermal noise caused by mirror reflective coating mechanical loss. Many groups are struggling to reduce coating mechanical loss. Solid state physics support is necessary. We will measure coating loss at low temperature. As first step, we will measure coating loss at room temperature soon.

Thank you for your attention.