Measurement of the Mechanical Loss of Test Mass Materials for Advanced Gravitational Wave Detectors Peter Murray

Q

03 April 2014IGR Lunchtime Talk3 Summary Thermal Noise Measurement of Quality Factor Calculation of Residual Coating Loss Experimental Results Silica Silicon Electron Energy Loss Spectroscopy Cryogenics Future Work Conclusions

03 April 2014IGR Lunchtime Talk4 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

03 April 2014IGR Lunchtime Talk5 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

03 April 2014IGR Lunchtime Talk6 Quality Factor Q factor or Q, in resonant systems, is measurement of the effect of resistance to oscillation Fusion energy gain factor, Q value or Q, the ratio of fusion power produced in a reactor to the power required to maintain the plasma in steady state Q factor, in recumbent bicycle mechanics, refers to the width between the cranks and should be small when building a streamliner Q factor, in marketing and pop culture commentary, sometimes used as a casual synonym for Q Score The higher the Q Score, the more well-known and well thought of the item or person being scored is

03 April 2014IGR Lunchtime Talk7 Measurement of Quality Factor Larger Qs result in less off resonance loss by equipartition theorem Levin approach proportional to ( ) at frequencies well below resonance Apparatus being used to measure the mechanical dissipation of bulk samples at room temperature Samples are suspended in loops of silk thread Resonant modes exited using electrostatic actuator

03 April 2014IGR Lunchtime Talk8 Measurement of Quality Factor

03 April 2014IGR Lunchtime Talk9 Evaluating Quality Factor Resonant mode modelled as freely decaying harmonic oscillator Rate of decay of amplitude is measured Q and mechanical dissipation can be determined Many different suspensions are measured to obtain as high a Q as possible

03 April 2014IGR Lunchtime Talk10 Dielectric coatings formed from layers of Ta 2 O 5 and SiO 2 Introduction of these coatings introduces another source of mechanical dissipation Assuming all other losses have been reduced to a negligible level, total measured loss of a coated test mass can be expressed as is the fraction of energy stored within the coating Calculation of Residual Coating Loss

03 April 2014IGR Lunchtime Talk11 The magnitude of the loss introduced by the dielectric coating can be calculated Compare the losses of test mass before and after coating is applied Subtracting the loss attributable to thermoelastic damping The residual loss can be expressed as having a small frequency dependence Calculation of Residual Coating Loss

03 April 2014IGR Lunchtime Talk12 Coating Split Analysis undertaken to calculate individual losses Masses produced coated with single layer of Silica or Tantala Reasonably consistent with above Suggests doping tantala reduces residual coating loss Experimental Results for Silica Samples Un-doped tantala had small cracks which formed during annealing Could have introduced excess losses Control sample used in analysis had not gone through the same annealing process as the silica coated mass May attribute to difference between the two loss values for silica Single thick layer of silica may not have same structure as thinner silica in a multi-layer coating

03 April 2014IGR Lunchtime Talk13 Experimental Results for Silica Samples LMA produced series of multi-layer tantala-silica coatings with increasing percentages of TiO 2 30 alternating /4 layers of Ta 2 O 5 and SiO 2 Adding any TiO 2 to the Ta 2 O 5 reduces the mechanical loss by ~30 to 50% Formula 4 has a residual loss higher than the first three formulae Originally unclear whether difference was due to the different coating chamber Formula 5 coating produced using this second chamber has a residual loss comparable to that of the first three Formula masses Suggests use of different chamber has no significant effect on coating losses

03 April 2014IGR Lunchtime Talk14 Electron Energy Loss Spectroscopy Material is exposed to beam of electrons with a known, narrow range of kinetic energies Some electrons will lose energy by inelastic scattering Interaction of beam electron with an electron in sample Results in both a loss of energy and a change in momentum Interactions may be Phonon excitations Inter and intra band transitions Inner shell ionisations Latter are particularly useful for detecting elemental components of a material Energy transferred is related to the ionization potential of atom Therefore the spectrum can be compared to that of known samples

03 April 2014IGR Lunchtime Talk15 Electron Energy Loss Spectroscopy LMA originally able to provide only approximate value of TiO 2 in Ta 2 O 5 layers EELS being used to produce a definitive composition of all these coatings Formula 1 silica-tantala coating is uniformly doped with 8.5±1.2% titania F3 doped with 22.5±2.9% TiO 2 F4 doped with 54±5% TiO 2 EELS analysis in agreement with LMA values Light layers are doped Ta 2 O 5 layers Brighter the pixel, the more TiO 2 is present

03 April 2014IGR Lunchtime Talk16 Empirical Model of Mechanical Losses in Silica Previous models for substrate loss assumed a frequency independent loss. Empirical model developed, by S. Penn et al, to model the mechanical loss in differing types of fused silica: (V/S) 1 is the surface to volume ratio of a sample (in mm) th is thermoelastic loss f is frequency C 1, C 2, C 3 and C 4 constants related to specific type of fused silica Resonant frequencies measured well away from frequency at which thermoelastic loss is maximum Therefore th can be assumed to be negligible

03 April 2014IGR Lunchtime Talk17 Empirical Model of Mechanical Losses in Silica Qs for a 65 mm diameter and 70 mm long Suprasil 311 silica test mass compared to the empirical model Several Qs lie close to the empirical model Some Q values however lie below the empirical model Suggests some other factor limiting the quality factor most likely frictional losses associated with the suspension

03 April 2014IGR Lunchtime Talk18 Nodal support being developed to improve Q values for suspension limited modes Qs of 6.17x10 7 already achieved on Sapphire using Super Noodle Ellies Talk (15 th March) will discuss this in more detail Nodal Support

03 April 2014IGR Lunchtime Talk19 Finite Element Analysis of Mode Shapes

03 April 2014IGR Lunchtime Talk20 Future Work Residual Coating Loss Measurements Continue investigations into losses of different coatings applied to silica and sapphire masses Introduction of Nodal support should improve Q values for suspension limited modes EELS analysis will produce a definitive composition of coatings Diffraction gratings etched onto test masses may be used as non-transmissive beam splitters in future detectors

03 April 2014IGR Lunchtime Talk21 Future Work Silicon at room temperature Continue to investigate: Losses of different aspect ratios of silicon Losses of different orientations of silicon The effects of doping silicon on losses Begin to investigate losses introduced by applying coatings to silicon Testing bulk samples at cryogenic temperatures

03 April 2014IGR Lunchtime Talk22 Conclusions Study of coating loses helping inform decisions towards upgrades for advanced gravitational wave detectors Needs to also be investigated at cryogenic temperature Intrinsic coating loss dominated by loss associated with tantala Doping the tantala reduces this loss EELS has helped to find the definitive composition of some of these coatings

03 April 2014IGR Lunchtime Talk23 Conclusions Current apparatus is capable of obtaining high quality factors However, need to ensure not limited by suspension losses Nodal Support may improve Qs of some resonance modes Should allow better analysis of losses introduced by coating samples Results suggest [111] silicon has lower loss than [100] silicon and work will continue to investigate this

Thank You For Your Attention Any Qs?

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