Coating thermal noise

1. Coating thermal noise S. Reid, I. Martin, J. Hough, S. Rowan Univ. of Glasgow M.M. Fejer, R. Route Stanford University A. Remillieux, J.M. Mackowski, C. Comtet, N. Morgado L. Pinard, Ch. Michel LMA, Lyon G. Harry LIGO MIT H. Armandula, LIGO Caltech S. Penn, Hobart and William Smith Colleges 3rd ILIAS-GW Annual General Meeting – Imperial College, 27th October 2006 • GEO

2. Temperature dependence of the mechanical dissipation in single layer tantalum pentoxide coatings on thin single-crystal silicon substrates • GEO • Experiments suggest • Silicon is of significant interest as a mirror substrate material for future detectors and its mechanical loss is predicted to decrease at low temperatures. • Ta2O5 is the dominant source of dissipation in current ion-beam-sputtered SiO2/Ta2O5 coatings • Doping the Ta2O5 with TiO2 can reduce the mechanical dissipation • Mechanism responsible for the observed mechanical loss in Ta2O5 as yet not clearly identified • Use of thin substrates means coating loss becomes dominant – interpretation of results is easier

3. Sample fabrication • The silicon samples have been fabricated by wet etching from silicon wafers at Stanford (Stefan Zappe) • The coatings were deposited by LMA. • A pair of silicon cantilevers, 52 mm thick were measured. • control sample (underwent same oxidation/annealing process as coated sample) coated cantilever, 52 mm thick, 57 mm long • Silicon substrate properties: • ~ 52 mm thick, P-type Boron doped, resistivity = 10-20 W-cm • Tantala coating properties: • 0.5 mm single layer tantala, doped with 14.5 ± 1 % titania (Formula 5)

4. Measured mechanical loss • Results – 4th bending mode at ~777 Hz (b) (a) (c) Temperature dependence of (a) measured uncoated loss, (b) measured coated loss and (c) calculated thermoelastic loss for the second bending mode at ~777Hz.

5. Measured mechanical loss ctd • Results – 5th bending mode at ~1280 Hz (b) (a) (c) Temperature dependence of (a) measured uncoated loss, (b) measured coated loss and (c) calculated thermoelastic loss for the third bending mode at ~1280Hz.

6. Coated silicon cantilever – energy distribution • Interpretation: calculating the energy stored in the coating layer • The the energy ratios were calculated analytically and evaluated by Finite Element analysis and found to agree within a few %. • The energy ratio used:Ecoating/Esubstrate= 0.0249 deflected beam with a lossy surface layer (Heptonstall et al PLA 2006) Young’s modulus of silicon substrate: 162.4 GPa Reid et al, PLA 2005 Young’s modulus of tantala coating: 140.0 GPa K. Srinivasan et al, LIGO-T970176-00-D 2001, “Coating Strain Induced Distortion in LIGO Optics” http://www.ligo.caltech.edu/docs/T/T970176-00.pdf

7. Second cantilever • Calculated coating loss Temperature dependence of the measured loss in a single layer doped tantala coating applied to a 52mm silicon substrate at ~777 Hz and ~1280 Hz.

8. Conclusions - coatings • Measurements of the loss of a single layer of tantala coating have been made on silicon cantilever substrates • Results at low temperature are consistent with: the mechanical loss of tantala increases as temperature decreases ~1.5x10-4 at 290K → ~4.0x10-4 at 80Kpossible dissipation peak at lower temperatures (?) - compare to Yamamoto et al (Phys. Rev. D 74, 022002, 2006) where a similar increase in mechanical loss for silica-tantala (undoped) multilayer coatings is observed at the measured points at 300K and 77K. • Further work: in process of adapting cryostats to cool to liquid helium temperature to study coating loss down to 4 K