Mechanical Losses of Coatings for use in Advanced Gravitational Wave Detectors

1. Mechanical Losses of Coatings for use in Advanced Gravitational Wave Detectors Peter Murray

2. Collaborators • P. Murray, S. Reid, E. Chalkley, D. Crooks, J. Hough, S. Rowan • University of Glasgow • M.M. Fejer, R. Route • Stanford University • A. Remillieux, JM Mackowski, L Pinard, Ch Michel, N Morgado • LMA Lyon • G.Harry • LIGO MIT • H. Armandula • LIGO Caltech • S. Penn • Hobart and William Smith 3rd Year PhD Talk

3. Summary • Gravitational Waves • Thermal Noise • Measurement of Quality Factor • Calculation of Coating Loss • Contributors to the Coating Mechanical Loss • Titania-doped Tantala/Silica Coatings • Conclusions 3rd Year PhD Talk

4. Gravitational Waves • Einstein’s General Theory of Relativity predicted the existence of gravitational waves • Rapid motion of mass will generate ripples in space-time • Radiate outward as gravitational waves • For the effect of a gravitational wave to be significant need to examine astronomical events • Collapse of star to black hole or neutron star • Coalescing binaries • Interaction from the early universe • Highly sensitive detectors required to observe gravitational waves 3rd Year PhD Talk

5. Detection of Gravitational Waves • Two main types of gravitational wave detector • Resonant Bar Detector • Long Baseline Interferometers • Based on Michelson interferometer • Offers possibility of high sensitivityover a much wider frequency rangeof a few tens of Hz to kHz • Several long baseline interferometric detectors have been, or are being, commissioned • LIGO, GEO600, VIRGO, TAMA 3rd Year PhD Talk

6. Noise Sources • There are many sources of noise that can limit the sensitivity of interferometric detectors • Shot Noise • Radiation Pressure • Seismic Noise • Thermal Noise • Ground-based detectors high frequency limit of a few kHz, set by photon shot noise 3rd Year PhD Talk

7. Thermal Noise • Random Brownian motion of atoms in test mass mirrors appears as thermally driven motion of the mechanical system • Thermal noise significant noise source at the lower operating frequency range • Low loss suspension materials ensure that thermal noise level over operating bandwidth of detectors is kept to a minimum 3rd Year PhD Talk

8. Measurement of Quality Factor • Apparatus being used to measure the mechanical dissipation of bulk samples at room temperature • Resonant mode modelled as freely decaying harmonic oscillator 3rd Year PhD Talk

9. The resonant frequencies for the modes of each sample are found using FEA modelling in ANSYS FEA also provides useful information about the shape of each resonant mode Finite Element Analysis of Mode Shapes 3rd Year PhD Talk

10. Calculation of Coating Loss • Reducing thermal noise from optical coatings is crucial to reaching the required sensitivity in next generation interferometric GW detectors • Dielectric coatings formed from layers of Ta2O5 and SiO2 • Internal friction in coating materials introduce another source of mechanical dissipation • The loss of a coated test mass can be expressed as Multi-layer coating on face of mass 3rd Year PhD Talk

11. Coating Split Analysis undertaken to calculate individual losses Substrates coated with single layer of Silica or Tantala Preliminary Results suggest trend is consistent with above Suggests doping tantala reduces residual coating loss Contributors to the Coating Mechanical Loss 3rd Year PhD Talk

12. Coating Research • Study of multilayer dielectric optical coatings is an on-going research project • The next stage of coating research has been to improve the mechanical loss without significantly degrading the optical absorption • Adding TiO2 as a dopant to the Ta2O5 was tried • High Young's modulus • Atomic size allows for dense packing in the Ta and O matrix • Melting point of TiO2/Ta2O5 alloy is relatively high, indicating a stable amorphous structure 3rd Year PhD Talk

13. LMA produced series of multi-layer tantala-silica coatings with increasing percentages of TiO2 Adding any TiO2 to the Ta2O5 reduces the mechanical loss by ~30 to 50% Titania-doped Tantala/Silica Coatings 3rd Year PhD Talk

14. Conclusions • Intrinsic coating loss dominated by loss associated with tantala • Mechanical loss results in show adding TiO2 to the Ta2O5 reduces the mechanical loss • Differing concentrations of TiO2 do not effect the loss nearly as much as simply the presence or absence of TiO2 • This reduction of nearly half in the loss angle of the coating corresponds to a significant improvement in thermal noise • Translates into greater astronomical reach for advanced interferometers 3rd Year PhD Talk

15. Thank You For Your Attention Any Qs?