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Thermal Noise and Material issues for ET

Explore challenges, material requirements, and mechanical properties of silicon for Enhanced Technology (ET) research, focusing on thermal noise limitations and mirror coatings. Delve into silicon research, surface loss, and suspension elements.

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Thermal Noise and Material issues for ET

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  1. Thermal Noise and Material issues for ET R. Nawrodt, N. Beveridge, A. Cumming, W. Cunningham, J. Hough, I. Martin, C. Schwarz†, D. Heinert†, S. Reid, S. Rowan Institute for Gravitational Research, Glasgow University †Institute of Solid State Physics, Jena University 2nd ET Annual Workshop, Erice/Italy 14th-16th October 2009 Nawrodt, Erice 10/2009

  2. Outline • challenges for ET • material requirements • mechanical properties of silicon • thermal properties of silicon • suspension issues (surface loss, TED, canceling) • cryogenic breaking strength (Nicola at the end) Nawrodt, Erice 10/2009

  3. Motivation • sensitivity enhancement by at least a factor of 10 between generations • thermal load within the optics causes unwanted effects (e.g. thermal lensing) • general change of infrastructure, topology and technology needs to be considered Nawrodt, Erice 10/2009

  4. Thermal noise contribution • thermal noise will limit between ~ 10 Hz … 400 Hz • main contributions to the overall thermal noise are: • Brownian noise (directly related to the mechanical loss) • thermoelastic noise (related to the thermal properties – mainly the linear coefficient of thermal expansion) • Silicon combines a low mechanical loss and a zero linear coefficient of thermal expansion (@ 18 K and ~ 125 K) Nawrodt, Erice 10/2009

  5. Mirror thermal noise • review of mirror thermal noise revealed a coating Brownian noise limited regime (see talk J. Franc or ET-021-09) Si(111) standard dielectric coating (Ta2O5 / SiO2) 20 K Nawrodt, Erice 10/2009

  6. Coating Thermal Noise • amorphous coating materials show a mechanical loss peak around 18 K which can be changed by special treatments (29  2) meV [Martin et al., 2009] (40  3) meV doping changes the microscopic structure  change in activation energy Nawrodt, Erice 10/2009

  7. Bulk Material Requirements • thermal noise issues • Brownian thermal noise (low mechanical loss) • thermoelastic noise (proper thermal parameters) • thermo-optical noise + effects (e.g. thermal lensing) (proper comb. of optical and thermal properties) • availability • large samples are needed • polishing, coating • engineering aspects • fabrication of suspension elements (fibres, ribbons) • available jointing techniques (@ low temperatures with high thermal conductivity) Nawrodt, Erice 10/2009

  8. Size estimate • further reduction by means of larger beam radius + improved coatings 4×10-4 for Ta2O5 2×10-4 for SiO2 18 K • max. beam radius determined by availability of silicon (dia. 18-20 inch) • keeping 1 ppm clipping loss results in a diameter of ~ 45 – 50 cm Nawrodt, Erice 10/2009

  9. Silicon Research Research on mechanical loss at low temperatures of Silicon and other bulk materials is carried out within the ET science team (collab. with Jena) (1) min = 5×10-10 (@ 3.5K) (2) (2) loss peaks around 18 and 120 K (1) (3) lower losses observed in current experiments (3) [McGuigan 1978] Nawrodt, Erice 10/2009

  10. Silicon Research • Temperature range limited to T > 5 K due to setup (collaboration with Legnaro for T<5 K) • Sample was only polished at the front surfaces • Influence of the surface treatment (oxidation and etching) and full polishing upcoming aim: understanding loss processes in silicon in order to give a suggestion for test mass and suspension elements Nawrodt, Erice 10/2009

  11. Choice of Orientation Selection of the crystal orientation for low noise performance: e.g. bulk Brownian noise: [e.g. Liu, Thorne 2000] 2 extreme values for the Young’s moduli of Si: Ymin = 130 GPa for Si(100) Ymax = 188 GPa for Si(111) [Wortman, Evans, J. Appl. Phys. 36 (1965)] Nawrodt, Erice 10/2009

  12. Thermal conductivity of Silicon • heat load needs to be removed through the suspension elements  thermal conductivity plays beside the thermal noise performance a central role • 4 identical fibres (L = 1 m) can extract: • fibre dia. driven by heat extraction T0 = 4 K Pmax T = 20 K Nawrodt, Erice 10/2009

  13. Thermal conductivity of Silicon • experimental results (double-log scale!): increasing impurity concentration (scattering of phonons on impurities) “recommended curve” (< 1014 cm-3 boron, approx. 1 mg B in 1 t Si) smaller structures (~ 1/L term) see Callaway 1958 or Casimir 1938 [Touloukian] Nawrodt, Erice 10/2009

  14. Mechanical Loss of Small Structures • total mechanical loss of a sample is modelled as consisting of 2 main contributions: surface loss bulk loss  … displacement field S … surface area V … volume Nawrodt, Erice 10/2009

  15. Mechanical Loss of Small Structures • mechanical loss obtained from different published papers for silicon oscillators with small dimensions (T<18 K) in pure bending  = sub× ds = 0.5 pm identical analysis compared to fused silica ( = 6 pm) Nawrodt, Erice 10/2009

  16. 100 K 300 K 20 K thermoelastic surface loss intrinsic loss Mechanical Loss of Small Structures Fused Silica Silicon Si(111) 300 K example: suspension fibre (diameter 1 mm) Nawrodt, Erice 10/2009

  17. Suspension thermal noise • simple TN estimate for 1 stage monolithic suspension • full treatment (with correlations) leads to slightly higher thermal noise in the pendulum noise (see e.g. Piergiovanni et al. VIR-0015C-09) 18 K circular Si(100) fibre ( 680 µm, 4 fibres for each optical element, L = 1 m) mirror (180 kg,  500 mm, Si(111)) beam radius 90 mm Nawrodt, Erice 10/2009

  18. Si 500 nm Options for ET • all-reflective topologies - coupling of lateral displacement into phase shift • monolithic waveguides – currently only small and thin samples (needs bonding!), surface loss dominates, but TN estimates show a very low contribution • combined materials (e.g. Si substrate, sapphire fibres?) – characterization of the bonds (mechanically, thermally) needed • Xylophone – concept (see talk S. Hild) • ………… Nawrodt, Erice 10/2009

  19. Summary – Material Selection • silicon is the promising material for 3rd generation detectors • material choice: • bulk: Si(111), maximum doping: ??? (R&D) dia. 45-50 cm, thickness: 30 cm • suspension: Si(100), maximum doping: < 1014 cm-3 4x dia. 680 µm (due to breaking) 4x dia. 4.5 mm (due to heat extraction) • BUT (!) Silicon needs – if transmissive optics are planned to be used - the 1550 nm technology at our current technology level. Nawrodt, Erice 10/2009

  20. Cryogenic bond strength tests • silicon pieces need to be joint • possible technique: hydroxide-catalysis bonding of oxidised silicon samples • important question: Is there any evidence of weakening the bonds at cryogenic temperatures? Nawrodt, Erice 10/2009

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