Report on WP1/M4: Investigation of losses on substrates
S. Rowan 1, P. Murray 1, S. Reid 1, A. Heptonstall 1, I. Martin 1, A. Cumming 1,
J. Hough 1,F. Travasso 2, M. Punturo 2,
A. Remillieux3, J-M Mackowski3
1Institute for Gravitational Research, University of Glasgow, Glasgow (UK)
2 INFN Sezione di Perugia and Universita’ di Perugia, Perugia (IT)
3Laboratoire des matériaux avancés, Université Claude Bernard, Lyon (FR)
Coordinators: S. Rowan (Glasgow), M. Punturo (INFN)
Abstract
This task has as its objective the measurement of the mechanical loss, optical loss and index of refraction of dielectric coatings of the type necessary for use in the mirrors for advanced gravitational wave detectors. Here we report on the results of recent work carried to study the mechanical and optical properties of low loss dielectric mirror coatings.
Introduction
All current interferometric gravitational wave detectors use suspended test masses to which ion-beam-sputtered dielectric multi-layer coatings are applied to form ultra-high quality mirrors. The mechanical losses of coatings currently available would result in a level of thermal displacement noise expected to be a significant limit to the performance of future ‘advanced’ gravitational wave detectors. The optical power incident on the coatings may be of the order of 100’s of kW, thus the optical absorption of these coatings must be of a low enough level to avoid significant thermally induced distortions of the interferometer mirrors. To ensure that future detectors reach desired levels of sensitivity and operate robustly it is essential that both the optical and mechanical loss of dielectric mirror coatings be quantified and, where possible, minimised.
The work over the second year of this project has been targeted at several tasks.
(1)Production of substrates and coatings
(2)The study of the mechanical loss of coatings formed from alternating multi-layers of SiO2/Ta2O5 doped with TiO2
(3)Studies of different types of coatings applied to fused silica at room and low temperatures
(4)Preparation of substrates necessary to study the properties of coatings applied to silicon at room and low temperature across a wide range of frequencies.
(5)Investigations of the losses of substrates necessary to support diffractive coatings
This work is described in more detail in the report that follows.
The study of the mechanical loss of coatings formed from alternating multi-layers of SiO2/Ta2O5 doped with TiO2
A large amount of R&D work has been carried out towards improving the mechanical quality factor (Q) of the coatings, in order to reduce the coating thermal noise.
Previous work has shown that in SiO2/Ta2O5 coatings the Ta2O5 is the dominant sources of mechanical loss in the coatings and suggested that doping the Ta2O5 with TiO2 reduced the loss.
Under a separate collaboration of the University of Glasgow with the LIGO Scientific Collaboration, a considerable number of experiments have been carried out on coatings applied to samples of 3” diameter by 1” thick, where the Ta2O5 component of the coating was doped with TiO2 of varying percentage, with the exact value for the percentage initially unknown.
The mechanical loss results showed that adding TiO2to the Ta2O5reduces themechanical loss, by approximately a factor of two.
In Glasgow initial studies to quantify the exact percentage of dopant were been carried out using Electron Energy Loss Spectroscopy (EELS) (19). Small coated ‘witness’ samples were thinned and examined using the electron microscopy facilities in Glasgow.
Figure 1 Image obtained using EELS of a section through a sample of silica/doped tantala coating (I. MacLaren, University of Glasgow).
From these measurements amount of TiO2 present in the Ta2O5 layers was found.
Using a separate technique an independent measurement of the amount of TiO2 present was carried out by LMA Lyon. The results are shown in table 1.
Coating numberpercentage of TiO2 present in Ta2O5
From index of refractionFrom EELS
0 0-
1 60.6 8.51.2
2 131-
3 24222.52.9
454.55 545
Table 1: Concentration of TiO2 in Ta2O5 as measured by change in index of refraction and by electron energy loss spectroscopy (EELS).
Initial loss measurements suggest there is no strong correlation between the amount of dopant used and the measured loss. The EELS technique was also used to map the physical location of the TiO2 present, and indicated the dopant was spread uniformly throughout the doped regions, see figure 2.
This work has shown adding TiO2to the Ta2O5reduces the mechanical loss, by approximately a factor of two. This reduction of nearly half in the loss angle of the coating corresponds to a significant improvement in thermal noise, which translates into greater astronomical reach for advanced interferometers, and a publication describing this work is in preparation. [1]
Studies of different types of coatings applied to fused silica at room and low temperatures
Work is being carried out in a number of institutions under the STREGA project to allow coating losses to be measured at low temperatures. Two systems have been developed at the University of Glasgow to measure the losses of coatings at low temperature. One cryostat is designed to measure the losses of thin coated membrane-like samples and the other to measure the losses of larger, cylindrical ‘test-mass’ like samples. (19).
Work has been carried out to characterise the loss factors of fused silica membranes, and in particular measurements have shown that careful design of the joint between the membrane a thicker silica block to which it is welded can significantly reduce excess mechanical losses in the system which can otherwise be problematic when trying to quantify losses associated with coatings added to the membranes for study. (19).
At Glasgow a number of samples have been fabricated using a laser welding technique for use in joint studies of coating loss with LMA Lyon.
Smaller coated samples have also been studied at Perugia in a setup where a fused silica slab is held using a copper clamp. Preliminary measurements have been made on three separate samples. The first one with a standard SiO2/Ta2O5 multilayer coating; the second with a Ta2O5 mono-layer coating doped with TiO2 and the last one with a Ta2O5 mono-layer coating doped with Cobalt. An FE model has been developed to aid analysis of the results.
Preparation of substrates necessary to study the properties of coatings applied to silicon at room and low temperature across a wide range of frequencies.
Work has been carried out to characterise the mechanical loss factor of thin silicon membranes both at room and low temperature – see [2].
These have been transferred to LMA where tests have been carried out to check the quality of a doped Ta2O5 layer applied to the silicon membranes. The test indicated that annealing of the membranes was necessary to achieve good surface roughness and help the adhesion of the doped Ta2O5. Coatings are now being added to the membranes for studies of mechanical loss to be carried out.
In addition the loss factors of larger silicon samples with resonant modes at higher frequencies have also been studied and the samples transferred to LMA for coating.
Investigations of the losses of substrates necessary to support diffractive coatings
Measurements have been made of the mechanical Q factors of (a) a blank fused silica
disk of Hereaus Suprasil fused silica and (b) a companion fused silica disk with anetched diffraction grating added by colleagues in Jena.
In each case the mechanical loss factors of a set of resonant modes of a fused silicadisk were measured by exciting the resonant modes of the sample electrostaticallyusing a high voltage plate held behind the sample whilst the sample was suspended,under vacuum, at the bottom of a fused silica suspension – see figure 3
The sample is suspended from double cantilever blades holding wire loops around anintermediate mass to provide isolation from seismic motion. The disk sample itself iswelded to a bi-filar fibre suspension beneath the intermediate mass.
A complete set of measurements have been carried out on the control sample disk formode frequencies between 1 and 30kHz. Measurements have also been carried out on a disk with an etched optical diffraction grating of 200nm in depth.
No systematic difference in the loss factors of the two samples has been observed.
A diffractive etched substrate has been delivered to LMA Lyon in order to check the adhesion of the coating on this special kind of substrate, and measurements of the loss factor will be carried out over the next period.
Figure 3. (a) disk with etched diffraction grating (b) Welding of disk to base of double pendulum suspension (picture shows A. Heptonstall) (c) silica suspension attached to cantilever blades
.
[1] G. Harry et al ‘Titania-doped tantala/silica coatings for gravitational-wave detection’, in press