1. Measurement of coating mechanical loss Junko Katayama, K.Craig, K.Yamamoto, M.Ohashi ICRR 0

2. Outline Introduction Previous measurements Setup for our measurement Status report, next stage 1

3. Introduction Coating thermal noise is an important noise source for gravitational wave detectors. In particular, coating Brownian noise is expected to limit room temperature detectors at their most sensitive frequencies. The power spectral density of coating Brownian noise can be approximated as: The mirrors may be cooled, but the low temperature mechanical loss must be measured. 2 G. M. Harry, Class. Quantum Grav. 19 (2002) 897–917

4. Introduction Even after cooling the mirrors to 20K, mirror thermal noise is expected to be an important noise source in KAGRA. K. Somiya, Class. Quantum Grav. 29. 124007(2012) 3

5. Previous measurements Glasgow and LMA (2013) Samples Silicon cantilevers 35mm × 5mm, 70μm thickness coated by LMA, Lyon d =5.9μm (2.1 μm of titania doped tantala plus 3.8 μm of fused silica) Cantilever measurement schematic diagram Kieran Craig, PhD thesis The mechanical loss of the Advanced LIGO ETM coatings were measured at Glasgow and LMA, Lyon. Drive plate 4 Optical lever

6. Previous measurements - results Glasgow and LMA (2013) Loss peak observed at temperatures between 24-28K (frequency dependent) M. Granata et al., Opt. Lett. 38, 5268 (2013) 5

7. Previous measurements ICRR (2006, 2014) Samples Sapphire disks 100mm diameter, 0.5mm and 1.0mm thickness coated by NAOJ, JAE 30 layer silica/pure tantala coating Each layer was λ/4 thickness (λ=1064nm) Nodal support system E. Hirose et al., Phys. Rev. D 90, 102004 (2014) The mechanical loss of an NAOJ and JAE coating was measured by K. Yamamoto in 2006. The same sample was measured again before and after heat-treatment in 2014. 6

8. Previous report ICRR (2006, 2014) Both sets of measurements found the coating loss to be independent of temperature. E. Hirose et al., Phys. Rev. D 90, 102004 (2014) 7

9. 8 Glasgow/LMA silicon cantilevers coated by LMA titania doped ICRR sapphire disks coated by NAOJ,JAE titania undoped sapphire disks same coating Our measuremet sapphire disks coated by LMA titania doped/undoped

10. Measurement comparison The mechanical loss of an Advanced LIGO ETM coating will be measured using sapphire disks. The coating is identical to the one measured by Glasgow LMA using silicon cantilevers. This should provide insight cantilever vs disks for cryogenic mechanical loss measurements If a peak in coating loss is observed, agreement with the Glasgow/LMA results will be made. If no peak, it is likely that either the sample geometry, or differences in substrate material are responsible (sapphire vs. silicon) 9

11. Measurement comparison The Advanced LIGO ETM coating contains titania-doped tantala layers. Titania-doping of tantala has been shown to reduce mechanical loss However, the aLIGO ETM coating had higher mechanical loss than expected from measurements of silica and titania-doped tantala A coating with the same structure as the ETM coating, but with pure tantala was also produced to examine the effect of titania within this coating 10

12. New setup- ICRR Samples Sappire disks 100mm diameter 0.5mm thickness coated by LMA Will be measured using a Cryogen-free cryostat at ICRR laser PD heater thermometer 1 thermometer 2 thermometer 3 mirror disk electrostatic drive 11

13. Nodal support system 12 Cryostat copper stage

14. Status report Lock-in amplifier and signal generators have been tested. Wiring for heaters and temperature sensors is complete. Thermometers work, and a program for controlling temperature was made with LabView. A drive plate was made. 13

15. Next stage Vacuum gauge was broken, so we asked company to repair it and will get it back soon. Measurement software will be written using LabView. Excitation circuit needs to be made. After finishing setup, we will start measurement from the beginning of 2016 14

16. Summary We are preparing for measurement of coating loss at low temperature. It will clarify the cause of the differences between resuts of previous measurements. Measurement will be started from next month. 15