Nobuhiro Fukumoto, Yukinori Ono a, Masahiro Hori a, Ryo Chikaoka a, Yosuke Hayakawa a, Shigenori Moriwaki and Norikatsu Mio Photon Science Center, Univ. of Tokyo a Graduate School of Science and Engineering Univ. of Toyama, Toyama Electron spin resonance measurement of sapphire for KAGRA mirrors

Japanese gravitational wave detection project KAGRA 2 Gravitational wave Propagating as space strain Strain is very small No evidence of direct observation Gravitational wave interferometer Huge Michelson interferometer Efforts for decreasing noises ・ mono-crystal sapphire mirror space strain(plus mode) Sapphire mirror KAGRA interferometer

Mono-crystal sapphire mirror 3 We need information about impurities in Mono-crystal sapphire Merit ・ low mechanical loss （ lower at cryogenic measurement ） ・ high thermal conductivity Demerit ・ IR (infrared) absorption By impurities （ Fe 3+, Cr 3+,Ti 3+ … ） Sample IR absorption →30~680ppm /cm KAGRA requirement →50ppm /cm

Electron Spin Resonance （ ESR ） 4 Energy level transition (Free Electron)

Electron Spin Resonance （ ESR ） 5 Setup ・ Cavity Excite standing microwave ・ Electromagnet Sweep external field Inside of cavity Standing microwave fields High sensitive measurement observing each spin

Instruments Setup 6 ESR measurement instruments （ Graduate School of Science and Engineering Univ. of Toyama,Ono.lab ） microwave frequency ： 9.38 GHz （ X-band ） Sweeping magnetic Field ： 0 ～ 10 kG Electromagnet and cavity ESR instrument

Sample preparation 7 Crystals for IR absorption measurement High IR absorption crystal(680 ppm/cm): AC150 Low IR absorption crystal (30 ppm/cm): P401 Cut samples for ESR measurement Cut into 27 blocks(from A1 to C9) C-axis Sample cutting IR absorption crystal diameter: 10mm length: 40mm

Measurement Summary 8 ESR peak summary ESR peaks at 3 regions around 2 kG around 3 kG around 5 kG Comparison of peaks between samples among blocks Measurement condition Temperature: room temperature Microwave power: 1mW Microwave frequency: 9.38GHz We have measured 27 x 2=54 samples.

Around 2kG 9 Intensity(a.u.) Field(G) Each peak position is different→ different origin P401_A8 AC150_A3 Peak 1930G Peak 1705G

Around 3kG 10 Intensity(a.u.) P401_A8 AC150_A3 Peak: 3345G Peak at the same position Field(G)

Around 3kG, another block 11 Intensity(a.u.) No peak→ depend on position P401_C8 AC150_C3 Field(G)

Around 5kG 12 Intensity(a.u.) Field(G) Peak: 5233G Only high absorption sample has peak →possibility of contribution to IR absorption P401_A1 AC150_A3

Peak intensity distribution around 3 kG Ａ Ｃ Ｂ AC150 Ａ Ｃ Ｂ P401 Intensity is largest at red part and decreases with distance→possibility of local impurity/affix in cutting 13 Peak intensity distribution diagram Red: High Yellow: Middle Blue: Low

Identification of impurity 14 Around C1 and C2, high IR absorption sample has peak, but around C2 it has no peak. Around F3, both samples has peak, but around F1 and F2, they has no peak. (b):RT Around Ti 3+ peak, samples have peak, but, these peaks are same as Fe 3+ case. Cr 3+ Fe 3+ R. S. de BIAS1 and D. C. S. RODRIGUES J. Am. Cerum. Soc., 68 [7] 409 (1985) High Low Both 9.50GHz RT Conversion peak position (calculated from g-value) Ti 3+ Radiation Measurements 43 (2008) 295 – 299

Summary 15 We observed ESR signals of the samples fabricated from the mono-crystals of different IR absorptions. We observed peaks with intensity dependence on the block position around 3kG. It may come from local impurity or affix in cutting. We observed peaks from only high IR absorption crystal sample around 5kG. It can be related to IR absorption.

Future work 16 We have not identified the impurity yet. In order to do so, we will prepare mono-crystal samples in which impurities (Fe 3+, Ti 3+ Cr 3+ …) are intentionally doped and measure them for impurity identification.

Thank you for your attention 17

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Identification of impurity 19 At area1 and area3 AC150’s peak are close to Cr 3+, but C2 is not found. At area2, both samples’ peaks are close to F3, but at F1 and F2, neither has peak. (b):RT At area2, Ti 3+ and samples’ peaks are close. But we have not identified impurity. Cr 3+ Fe 3+ R. S. de BIAS1 and D. C. S. RODRIGUES J. Am. Cerum. Soc., 68 [7] 409 (1985) AC150 P401 Both 9.50GHz RT Conversion peak position (calculated from g-value) Ti 3+ Radiation Measurements 43 (2008) 295 – 299

Summary 20 ESR peak of samples with different IR absorption ・ difference between samples area1(~2000G) area3(~5000G) ・ difference between blocks area2(~3000G) Identification of impurity ・ failure to identify impurity ・ measurement of intentionally doped sample (Fe 3+, Cr 3+, Ti 3+ …)

Conclusion 21 We observed ESR peak of samples with different IR absorption ・ Peaks appearing at around 5200G might originate from impurities affecting the LASER absorption ・ Peak intensity at 3345G depends on the sample position; this fact indicates that the distribution of the origin is inhomogeneous We have not identified impurity yet ・不純物を意図的に入れたサンプル

Mono-crystal sapphire mirror 22 Merit ・ low mechanical loss （ lower at cryogenic measurement ） ・ high thermal conductivity Demerit ・ uniaxial anisotropy ・ LASER light absorption From impurities （ Fe 3+, Cr 3+,Ti 3+ … ） KAGRA requirement is 50ppm /cm We need information of impurities of in Mono-crystal sapphire sample absorption coefficient (Nd-YAG LASER : 1064 nm) Structure of sapphire crystal In-plane direction: a-axis Inter-plane direction: c-axis Mono-crystal sapphire mirror

Gravitational wave Predicted by general theory of relativity Distortion of space propagates at light speed relative displacement ～ ～ 100Hz) astronomical phenomenon （ high speed, high mass ） No evidence of direct observation What is gravitational wave ？ Extremely high sensitive measurement is Indispensable ＋ mode and × mode 23 Supernova explosion Pulsar

Sapphire ESR spectrum 24 Field(G) Intensity(a.u.) Magnetic field sweep with modulation Derivative of microwaves absorption RT Power: 1mW X-band(9.38GHz) AC150 A3 AC150 A3 No sapphire(background)

Quantum noise Shot noise Radiation pressure noise Thermal noise Mirror Suspension External disturbance Seismic noise Noise budget Frequency dependence of each noise on KAGRA Total noise minimization is important 25

26 Cr 3+

単結晶サファイアミラーの製法 27 サファイアミラーの製法 ・ HEM 法 He ガスで炉内の温度を制御 高純度・大型の結晶 炉内の結晶成長単結晶サファイア サファイアミラー実用化には不純物の知見が必要

Zero-field splitting ・ Effect of surrounding ions Depend on crystal structure orthorhombic field(in lattice) trigonal field （ [1 1 1]direction ） ・ Split depend on impurities Anisotropy of energy level ・ Depend on incident angle of magnetic field ESR in crystals 28 orthorhombic fieldtrigonal field In sapphire Angle dependence of Fe 3+ ESR peak ESR signals become complicated in crystals J. Phys.: Condens. Matter 14 (2002) 10331–10348

Comparison of samples AC150Peak position(G)g-value ① ② ③ P401Peak position(G)g-value ① ② peak position and g-value ①: both have peak but positions are separated →maybe different origin ②: both have peak at same position →both have same impurity ③: Only high absorption sample has peak →possibility of contribution to LASER light absorption