Low temperature dissipation in coating materials
S. Reid1, I. Martin1, E. Chalkley1, H. Armandula3, R. Bassiri1, C. Comtet4, M.M.Fejer5, A. Gretarsson6, G. Harry7, J. Hough1, P. Lu5, I. McLaren1, J-M.M.Mackowski4, N. Morgado4, R. Nawrodt2, S. Penn8, A. Remillieux4, S. Rowan1, R.Route5, A. Schroeter2, C. Schwarz2, P. Seidel2, K. Vijayraghavan5, W. Vodel2,A. Woodcraft1
1SUPA, University of Glasgow, Scotland. 2Friedrich-Schiller University, Jena, Germany. 3LIGOLaboratory, California Institute of Technology, USA. 4LMA, Lyon, France. 5Stanford University, USA.6Embry-Riddle Aeronautical University, USA. 7LIGO Laboratory, Massachusetts Institute ofTechnology, USA. 8Hobart and William Smith Colleges, USA.
Introduction
Mechanical dissipation from dielectric mirrorcoatings is predicted to be a significant source ofthermal noise for advanced detectors.
Experiments suggest
Ta2O5 is the dominant source of dissipation incurrent SiO2/Ta2O5 coatings
Doping the Ta2O5 with TiO2 can reduce themechanical dissipation
Mechanism responsible for the observed mechanicalloss in Ta2O5 as yet not clearly identified
GEO600 mirrorsuspension, with HRcoating on front face.
Studying dissipation as a function of temperature of interest to:
Determine dissipation mechanisms in the coatings, possibly allowingdissipation to be reduced
Evaluate coating for possible use in proposed cryogenic gravitationalwave detectors
Single layer coating samples for lowtemperature studies
Thin silicon substrates used for coating
Loss of silicon decreases at low temperature
Coating will dominate the loss
34mm
500m
48m
Coating appliedhere
Titania doped tantala coatedsilicon cantilever in clamp
uncoated silicon cantilever in clamp
ratio of energy stored incantilever to energy storedin coating
difference in loss betweencoated and un-coatedcantilevers
Typical amplitude ringdown
Mechanical loss measurements
Comparison of the mechanical loss of the third bending mode (~1000 Hz)for a cantilever coated with Ta2O5 with 14.5 % TiO2, and an identical un-coated cantilever (in collaboration with Jena University)
Low temperature coating loss peak
A dissipation peak at ~18-20 K observed in both TiO2-doped Ta2O5 andpure Ta2O5
Most internal friction mechanisms may be thought of as relaxation processesassociated with transitions between equilibrium states, and typically:
where is the relaxation time
is a constant related to height of peak
Thermally activated processes follow Arrhenius equation:
where 0-1 is the rate factor and Ea is the activation energy for the process
At the dissipation peak, =1 and
…hence:
Interpretation and analysis
Fitting to Arrhenius equation
This gives an activation energy associated with the dissipation peakin doped tantala of (42 ± 2) meV, and a rate factor of 3.3×1014 Hz.
Glasgow (I. Martin et al)low T dissipation in single-layers Ta2O5 and SiO2
Dissipation peak observed at ~ 20 K in both pure and TiO2 doped (14.5%) Ta2O5
Activation energy of 42 meV for dissipation in Ta2O5 is similar to that in bulk fusedsilica i.e. dissipation possibly arises from double well potential corresponding tostable Ta-O bond angles
Evidence that TiO2 doping reduces height of peak in addition to reducing loss at roomtemperature
Loss of Ta2O5 is higher than loss of SiO2 between 10 and 290 K
Scatter close to room temperature due to clamping loss, measurements to berepeated
Glasgow (I. Martin et al)Heat treatment of un-doped tantala films
(c)
(a)
(b)
Temperature (K)
Loss of coated cantilever
0
50
100
300
150
200
250
1E-5
1E-4
Large loss peak at ~ 80 to 90 K in un-doped Ta2O5 coating heat treated at 800 °C,perhaps due to (expected) onset of polycrystalline structure
Smaller, broader peak at with activation energy of 138 ± 4 meV at ~35 K in un-dopedTa2O5 heat treated at 300°C
Above: Electron diffraction measurement ofTa2O5 heat treated at (i) 600 °C, showingamorphous structure and (ii) 800 °C, showingcrystalline structure.
Left: Loss at 1.9 kHz of 0.5 mm Ta2O5 coatingsheat treated at (a) 600, (b) 300 and (c) 800 °C
Glasgow (E. Chalkley et al)Alternative high index coating materials
Investigation of possible alternativehigh index coating materials
Initial studies of 0.5 mm thick HfO2coating, heat treated at 300°Cshow:
a broad dissipation peak at ~ 50 K
a plateau, or possibly a secondpeak, at ~200 K.
Study of the effect of heattreatment on loss of hafniaplanned.
500nm HfO2coating on asilicon cantilever
uncoated siliconcantilever
Initial HfO2 compared to undoped Ta2O5
Below ~50 K, dissipation in HfO2 and Ta2O5 broadly equivalent (both heat treated 300°C)
Note: the higher Young’s modulus of Hafnia should lead to lower thermal noise for thesame .d (loss-thickness product) for silicon optics (not necessarily true for othermaterials e.g. fused silica) (using typical HfO2 material property data).
Initial room temperature studies on a multi-layer silica-hafnia coating on a fused silicasubstrate were found to be hafnia=(5.7±0.3)×10-4.
Conclusions - 1
Dissipation peak observed at ~ 20 K in both pure Ta2O5 and in Ta2O5 doped with 14.5 %TiO2
Activation energy of dissipation process calculated to be:
42 ± 2 meV for doped coating (LMA).
24 ± 2 meV for undoped coating
Possible dissipation mechanism is thermally activated transitions of the oxygen atoms,similar to that in fused silica
Some evidence that TiO2 doping reduces the height of the dissipation peak in Ta2O5,inaddition to reducing the loss at room temperature.
A full understanding of the dissipation mechanism may allow
Mechanical loss at room temperature to be further reduced
Reduction of loss at particular temperatures of interest for future cryogenic detectors
Ta2O5 has consistently higher loss than SiO2 between 10 and 300 K
Collaboration ongoing with INFN lab at Legnaro to study the loss of coating materials atultra-cryogenic temperatures – may yield more information on loss mechanisms.
Conclusions - 2
Material properties of coatings being investigated by ellipsometry, AFM andelectron microscopy techniques.
ADX Reduced Density Function analysis of TEM images to help quantify structurein amorphous thin-films.
Preliminary results of the mechanical loss of Hafnia show similar levels ofmechanical loss to the equivalent Tantala coating below ~50 K.
However material properties for thin-film Hafnia are not well studied and anychanges over bulk properties will change the results presented here.
The optical properties also require further investigation, where initial recentabsorption studies of a multilayer silica-hafnia coating by Markosyan et al.(Stanford University) lie in the range 60-80ppm, which is considerably higher thanrequired.