An overview on ET-WP2 activities in Glasgow

1. An overview on ET-WP2 activities in Glasgow R. Nawrodt, A. Cumming, W. Cunningham, J. Hough, I. Martin, S. Reid, S. Rowan ET-WP2 Workshop, Genoa/Italy 15th September 2009 Nawrodt, Genoa 09/2009

2. Content • design study report • mirror thermal noise calculation • size estimate of the mirror • novel concept: nano-structured optics (surface loss of silicon) • suspension thermal noise • experimental work supporting the ET design • next steps Nawrodt, Genoa 09/2009

3. 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, Genoa 09/2009

4. Mirror thermal noise • reason for high coating Brownian noise is the increasing mechanical loss of the amorphous coating materials Nawrodt, Genoa 09/2009

5. Size estimate • further reduction by means of larger beam radius 18 K 18 K • unrealistic beam radius needed • combination of upscaling and better coatings needed • max. beam radius determined by availability of silicon (dia. 20 inch) Nawrodt, Genoa 09/2009

6. Mirror thermal noise • What coating loss is needed and how can it be achieved? • international coating research is ongoing, but so far we don’t have a full understanding of the origin of this loss example 4×10-4 for Ta2O5 2×10-4 for SiO2 Nawrodt, Genoa 09/2009

7. Si 500 nm Monolithic Waveguide Mirror • aim: no amorphous coating materials needed  low mechanical loss  low thermal noise • high reflectivity was shown (> 99.8%) • combined use at 1550 nm could reduce optical absorption and would thus reduce problems arising for the heat extraction surface area is roughly doubled by the structure  surface loss will contribute stronger Nawrodt, Genoa 09/2009

8. Surface Loss of Silicon • A thin lossy surface layer is assumed: • If the structure is thin only the top and bottom surface contribute: • Following Gretarsson and Harry (1999): with the dissipation depth ds which is obtained from measurements with oscillators and at temperatures where the surface loss will dominate (thin samples at low temperatures).  … displacement field Nawrodt, Genoa 09/2009

9. Surface Loss of Silicon • mechanical loss obtained from different published papers for silicon oscillators with small dimensions (T<18 K) in pure bending  = sub× ds = 0.5 pm loss can be influenced by different treatment techniques (e.g. hydrogen passivation)  currently under investigation Nawrodt, Genoa 09/2009

10. [Li et al., Appli. Phys. Lett. 83 (2003)] Monolithic Waveguide Mirror • Brownian thermal noise estimate  = 0.5 pm S/V = 4/t (t … thickness) µ ~ 3 • for very small structures the size dependence of the Young’s modulus needs to be considered • no additional losses are assumed (TE cancelled at 18 K) Nawrodt, Genoa 09/2009

11. Monolithic Waveguide Mirror • Brownian thermal noise estimate • monolithic waveguides have a smaller contribution than the bulk • thermal noise modelling still at an early stage 18 K total thermal noise, 18 K Nawrodt, Genoa 09/2009

12. Suspension thermal noise • The mechanical loss of silicon suspension elements arises from 3 main contributions – thermoelastic, surface and intrinsic bulk. • Thermoelastic peak shifts to higher frequencies while cooling. 300 K 50 K  680 µm  680 µm Nawrodt, Genoa 09/2009

13. Suspension thermal noise • simple TN estimate for 1 stage monolithic suspension • full treatment (with correlations) leads to higher thermal noise in the pendulum noise (see e.g. poster P. Puppo @ Amaldi8) 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, Genoa 09/2009

14. Experimental work • mechanical loss of coating materials (understanding and reduction of mechanical loss) • mechanical loss of silicon flexures (extraction of surface loss, influence of surface treatment) • silicon bonding, mechanical loss of bonds (bond loss values needed for realistic design) • strengths of bonds (300 and 80 K) (pieces need to be bonded, mechanical “stability”) Nawrodt, Genoa 09/2009

15. Bond experiments • bond technique needs to be characterised for the design: • mechanical properties (breaking strength, mechanical loss, Young’s modulus) • thermal properties (thermal conductivity, collaboration with the Florence group) • details at the ET annual meeting Nawrodt, Genoa 09/2009

16. Future work • refined suspension model based on realistic design information needed on exact design, temperature distribution, requirements due to cooling strategy • Xylophone “option” (optimisation, noise levels) • heat extraction concepts • combination of results  suggestions for design study Nawrodt, Genoa 09/2009

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