The atomic structure and properties of mirror coatings for use in gravitational wave detectors

Gravitational waves are a prediction of Einstein's General Theory of Relativity. They can be regarded as perturbations, or ripples, in the curvature of space-time that travel at the speed of light. Detectable gravitational waves are the result of the asymmetric acceleration of mass that occurs during massive astronomical events, such as coalescing compact binary systems and supernovae. The nature and detection of gravitational waves is the focus of Chapter 1. A direct detection of gravitational waves is still to be made, however, there is strong indirect evidence of their existence through the work Hulse and Taylor. They observed a binary pulsar system over a number of years and found it to have a decaying orbit that followed a decay rate consistent with a model in which energy is lost due to the production of gravitational waves. The most promising method for gravitational wave detection is through the use of long-baseline interferometric gravitational wave detectors, such as LIGO located in the US, GEO600 in Germany and Virgo in Italy. There are planned upgrades to current long-baseline interferometric gravitational wave detectors. These second generation of detectors will aim to improve sensitivity by a factor of around ten, allowing a much greater chance of detecting gravitational waves, particularly from sources such as coalescing compact binary systems. However, the sensitivity of these detectors will still be limited by noise sources, such as photon-shot, seismic and thermal noise, which could be further reduced by the development of new technologies. Chapter 2 discusses the current understanding of thermal noise arising from the mirror coatings in the detector test-masses. This will identify thermal noise as a particularly important noise source, limiting the sensitivity of detectors between the frequency range from a few tens Hertz to several hundred Hertz. There is an international network of scientists working on developing new technologies for future generations of interferometric gravitational wave detectors, which have the aim of increasing detector sensitivity and further reducing the effect of detector noise sources. The research presented in this thesis focuses on investigating the mechanical loss, which is directly related to the thermal noise, of the mirror coatings. In particular the first attempts at correlating changes in atomic structure of the coatings to the mechanical loss where various properties, such as heat-treatment and doping, of the coatings have been systematically changed will be presented. Chapter 3 will focus on the effect of heat-treatment of pure Ta2O5 coatings. The process of heat treating Ta2O5 coatings has observable effects on mechanical loss measured at low temperature, where loss peaks arise in the region of 10s of K and develop as the heat-treatment temperature rises. Heat-treatment also produces subtle changes to the averaged local atomic structure of the coatings where it can be seen that as the heat-treatment temperature is increased, the coatings became more ordered, moving towards crystallisation between heat-treatment at 300-600C coatings before fully crystallising at 800C. Atomic models show Ta2O2 ring fragments which are present in the crystalline phases of similar materials. In general it is observed that as heat-treatment temperature is increased there is an increase in the presence of the Ta2O2 ring fragments and a decrease in the presence of Ta-Ta bonds in the atomic structures. Changing the manufacturing deposition process for the Ta2O5 coatings also creates significant changes in the mechanical loss at low temperatures, where a `low water content' manufacturing processes gives rise to changes in the positions and shapes of the low temperature loss peaks. Preliminary investigations into the local atomic structure at different areas of a heat-treated coating shows that increasing heat-treatment temperature causes more ordered coating material nearer the substrate, compared with areas nearer the surface of the coating. Chapter 4 presents studies on the effect of doping Ta2O5 coatings with TiO2 with doping concentrations of 0, 8.3, 20.4, 25.7, 28.3, 53.8% (cation) TiO2. Mechanical loss measurements of multi-layer SiO2 and Ta2O5 doped with TiO2 coatings show that changing the TiO2 doping concentration reduces the mechanical loss of the coating by up to 40%. It is also shown that changing the TiO2 doping concentration can significantly change the local atomic structure of these coatings. Atomic models created for 20.4% and 53.8% Ti coatings indicate similar inter-atomic bond distances between the 20.4% and 53.8% Ti coatings. The models show that the distributions of Ta-Ti and Ti-O bonds in the atomic structure of the coatings as TiO2 doping is increased. There are also considerable contributions from Ta2O2 ring fragments that are seen in the pure Ta2O5 coatings, with the addition of TaTiO2 ring fragments. Further analysis of the atomic structures of these coatings revealed some preliminary correlations between the atomic structure and mechanical loss, were it is observed that 28.3% Ti coating is the most ordered atomic state out of all the Ti doped coatings and had the lowest measured mechanical loss. This suggests that there may be a link between slightly increased ordering in the atomic structures and a lower measurable mechanical loss. The amount of oxygen in a coating may play a key role important in the level of mechanical loss, as it is observed that the coating with the least oxygen deficiency coating is the coating with the lowest measured mechanical loss. Finally, Chapter 5 explores the material properties and atomic structures of HfO2 coatings, SiO2 coatings and substrates and HfO2 doped with SiO2 coatings. Pure HfO2 are studied as possible alternatives to Ta2O5 coatings. It appears that coatings subject to heat during the manufacturing process of just 100C or above appear part crystallised. Preliminary studies of a HfO2 coating doped with 30% (cation) SiO2 and heat-treated to 600C show that it is a promising coating for future study as it remains amorphous, with a room temperature mechanical loss value comparable to pure HfO2 coatings and therefore Ta2O5 coatings. SiO2 coatings deposited on SiO2 substrates are also studied and they show only subtle changes between them, which appear to lessen as the sample are heat-treated. Changes in the atomic structure of these coatings indicate an increase in order of the structure as heat-treatment temperature is increased, similar to the observed changes in the heat-treated Ta2O5 coatings

The atomic structure and properties of mirror coatings for use in gravitational wave detectors

Gravitational waves are a prediction of Einstein's General Theory of Relativity. They can be regarded as perturbations, or ripples, in the curvature of space-time that travel at the speed of light. Detectable gravitational waves are the result of the asymmetric acceleration of mass that occurs during massive astronomical events, such as coalescing compact binary systems and supernovae. The nature and detection of gravitational waves is the focus of Chapter 1. A direct detection of gravitational waves is still to be made, however, there is strong indirect evidence of their existence through the work Hulse and Taylor. They observed a binary pulsar system over a number of years and found it to have a decaying orbit that followed a decay rate consistent with a model in which energy is lost due to the production of gravitational waves. The most promising method for gravitational wave detection is through the use of long-baseline interferometric gravitational wave detectors, such as LIGO located in the US, GEO600 in Germany and Virgo in Italy. There are planned upgrades to current long-baseline interferometric gravitational wave detectors. These second generation of detectors will aim to improve sensitivity by a factor of around ten, allowing a much greater chance of detecting gravitational waves, particularly from sources such as coalescing compact binary systems. However, the sensitivity of these detectors will still be limited by noise sources, such as photon-shot, seismic and thermal noise, which could be further reduced by the development of new technologies. Chapter 2 discusses the current understanding of thermal noise arising from the mirror coatings in the detector test-masses. This will identify thermal noise as a particularly important noise source, limiting the sensitivity of detectors between the frequency range from a few tens Hertz to several hundred Hertz. There is an international network of scientists working on developing new technologies for future generations of interferometric gravitational wave detectors, which have the aim of increasing detector sensitivity and further reducing the effect of detector noise sources. The research presented in this thesis focuses on investigating the mechanical loss, which is directly related to the thermal noise, of the mirror coatings. In particular the first attempts at correlating changes in atomic structure of the coatings to the mechanical loss where various properties, such as heat-treatment and doping, of the coatings have been systematically changed will be presented. Chapter 3 will focus on the effect of heat-treatment of pure Ta2O5 coatings. The process of heat treating Ta2O5 coatings has observable effects on mechanical loss measured at low temperature, where loss peaks arise in the region of 10s of K and develop as the heat-treatment temperature rises. Heat-treatment also produces subtle changes to the averaged local atomic structure of the coatings where it can be seen that as the heat-treatment temperature is increased, the coatings became more ordered, moving towards crystallisation between heat-treatment at 300-600C coatings before fully crystallising at 800C. Atomic models show Ta2O2 ring fragments which are present in the crystalline phases of similar materials. In general it is observed that as heat-treatment temperature is increased there is an increase in the presence of the Ta2O2 ring fragments and a decrease in the presence of Ta-Ta bonds in the atomic structures. Changing the manufacturing deposition process for the Ta2O5 coatings also creates significant changes in the mechanical loss at low temperatures, where a `low water content' manufacturing processes gives rise to changes in the positions and shapes of the low temperature loss peaks. Preliminary investigations into the local atomic structure at different areas of a heat-treated coating shows that increasing heat-treatment temperature causes more ordered coating material nearer the substrate, compared with areas nearer the surface of the coating. Chapter 4 presents studies on the effect of doping Ta2O5 coatings with TiO2 with doping concentrations of 0, 8.3, 20.4, 25.7, 28.3, 53.8% (cation) TiO2. Mechanical loss measurements of multi-layer SiO2 and Ta2O5 doped with TiO2 coatings show that changing the TiO2 doping concentration reduces the mechanical loss of the coating by up to 40%. It is also shown that changing the TiO2 doping concentration can significantly change the local atomic structure of these coatings. Atomic models created for 20.4% and 53.8% Ti coatings indicate similar inter-atomic bond distances between the 20.4% and 53.8% Ti coatings. The models show that the distributions of Ta-Ti and Ti-O bonds in the atomic structure of the coatings as TiO2 doping is increased. There are also considerable contributions from Ta2O2 ring fragments that are seen in the pure Ta2O5 coatings, with the addition of TaTiO2 ring fragments. Further analysis of the atomic structures of these coatings revealed some preliminary correlations between the atomic structure and mechanical loss, were it is observed that 28.3% Ti coating is the most ordered atomic state out of all the Ti doped coatings and had the lowest measured mechanical loss. This suggests that there may be a link between slightly increased ordering in the atomic structures and a lower measurable mechanical loss. The amount of oxygen in a coating may play a key role important in the level of mechanical loss, as it is observed that the coating with the least oxygen deficiency coating is the coating with the lowest measured mechanical loss. Finally, Chapter 5 explores the material properties and atomic structures of HfO2 coatings, SiO2 coatings and substrates and HfO2 doped with SiO2 coatings. Pure HfO2 are studied as possible alternatives to Ta2O5 coatings. It appears that coatings subject to heat during the manufacturing process of just 100C or above appear part crystallised. Preliminary studies of a HfO2 coating doped with 30% (cation) SiO2 and heat-treated to 600C show that it is a promising coating for future study as it remains amorphous, with a room temperature mechanical loss value comparable to pure HfO2 coatings and therefore Ta2O5 coatings. SiO2 coatings deposited on SiO2 substrates are also studied and they show only subtle changes between them, which appear to lessen as the sample are heat-treated. Changes in the atomic structure of these coatings indicate an increase in order of the structure as heat-treatment temperature is increased, similar to the observed changes in the heat-treated Ta2O5 coatings.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Comparison of single-layer and double-layer anti-reflection coatings using laser-induced damage threshold and photothermal common-path interferometry

The dielectric thin-film coating on high-power optical components is often the weakest region and will fail at elevated optical fluences. A comparison of single-layer coatings of ZrO2, LiF, Ta2O5, SiN, and SiO2 along with anti-reflection (AR) coatings optimized at 1064 nm comprised of ZrO2 and Ta2O5 was made, and the results of photothermal common-path interferometry (PCI) and a laser-induced damage threshold (LIDT) are presented here. The coatings were grown by radio frequency (RF) sputtering, pulsed direct-current (DC) sputtering, ion-assisted electron beam evaporation (IAD), and thermal evaporation. Test regimes for LIDT used pulse durations of 9.6 ns at 100 Hz for 1000-on-1 and 1-on-1 regimes at 1064 nm for single-layer and AR coatings, and 20 ns at 20 Hz for a 200-on-1 regime to compare the //ZrO2/SiO2 AR coating

Optical absorption of ion-beam sputtered amorphous silicon coatings

Low mechanical loss at low temperatures and a high index of refraction should make silicon optimally suited for thermal noise reduction in highly reflective mirror coatings for gravitational wave detectors. However, due to high optical absorption, amorphous silicon (aSi) is unsuitable for being used as a direct high-index coating material to replace tantala. A possible solution is a multimaterial design, which enables exploitation of the excellent mechanical properties of aSi in the lower coating layers. The possible number of aSi layers increases with absorption reduction. In this work, the optimum heat treatment temperature of aSi deposited via ion-beam sputtering was investigated and found to be 450 °C. For this temperature, the absorption after deposition of a single layer of aSi at 1064 nm and 1550 nm was reduced by more than 80%

Medium range structural order in amorphous tantala spatially resolved with changes to atomic structure by thermal annealing

Amorphous tantala (a-Ta2O5) is an important technological material that has wide ranging applications in electronics, optics and the biomedical industry. It is used as the high refractive index layers in the multi-layer dielectric mirror coatings in the latest generation of gravitational wave interferometers, as well as other precision interferometers. One of the current limitations in sensitivity of gravitational wave detectors is Brownian thermal noise that arises from the tantala mirror coatings. Measurements have shown differences in mechanical loss of the mirror coatings, which is directly related to Brownian thermal noise, in response to thermal annealing. We utilise scanning electron diffraction to perform Fluctuation Electron Microscopy (FEM) on Ion Beam Sputtered (IBS) amorphous tantala coatings, definitively showing an increase in the medium range order (MRO), as determined from the variance between the diffraction patterns in the scan, due to thermal annealing at increasing temperatures. Moreover, we employ Virtual Dark-Field Imaging (VDFi) to spatially resolve the FEM signal, enabling investigation of the persistence of the fragments responsible for the medium range order, as well as the extent of the ordering over nm length scales, and show ordered patches larger than 5 nm in the highest temperature annealed sample. These structural changes directly correlate with the observed changes in mechanical loss.Comment: 22 pages, 5 figure

Investigating the medium range order in amorphous Ta₂O₅ coatings

Ion-beam sputtered amorphous heavy metal oxides, such as Ta2O5, are widely used as the high refractive index layer of highly reflective dielectric coatings. Such coatings are used in the ground based Laser Interferometer Gravitational-wave Observatory (LIGO), in which mechanical loss, directly related to Brownian thermal noise, from the coatings forms an important limit to the sensitivity of the LIGO detector. It has previously been shown that heat-treatment and TiO2 doping of amorphous Ta2O5 coatings causes significant changes to the levels of mechanical loss measured and is thought to result from changes in the atomic structure. This work aims to find ways to reduce the levels of mechanical loss in the coatings by understanding the atomic structure properties that are responsible for it, and thus helping to increase the LIGO detector sensitivity. Using a combination of Reduced Density Functions (RDFs) from electron diffraction and Fluctuation Electron Microscopy (FEM), we probe the medium range order (in the 2-3 nm range) of these amorphous coatings

Argon bubble formation in tantalum oxide-based films for gravitational wave interferometer mirrors

The argon content of titanium dioxide doped tantalum pentoxide thin films was quantified in a spatially resolved way using HAADF images and DualEELS. Films annealed at 300∘C, 400∘C and 600∘C were investigated to see if there was a relationship between annealing temperature and bubble formation. It was shown using HAADF imaging that argon is present in most of these films and that bubbles of argon start to form after annealing at 400∘C and coarsen after annealing at 600∘C. A semi-empirical standard was created for the quantification using argon data from the EELS atlas and experimental data scaled using a Hartree Slater cross section. The density and pressure of argon within the bubbles was calculated for 35 bubbles in the 600∘C sample. The bubbles had a mean diameter, density and pressure of 22Å, 870kg/m3 and 400MPa, respectively. The pressure was calculated using the Van der Waals equation. The bubbles may affect the properties of the films, which are used as optical coatings for mirrors in gravitational wave detectors. This spatially resolved quantification technique can be readily applied to other small noble gas bubbles in a range of materials

Structure and morphology of low mechanical loss TiO₂-doped Ta₂O₅

The exceptional stability required from high finesse optical cavities and high precision interferometers is fundamentally limited by Brownian motion noise in the interference coatings of the cavity mirrors. In amorphous oxide coatings these thermally driven fluctuations are dominant in the high index layer compared to those in the low index SiO₂ layer in the stack. We present a systematic study of the evolution of the structural and optical properties of ion beam sputtered TiO₂-doped Ta₂O₅ films with annealing temperature. We show that low mechanical loss in TiO₂-doped Ta₂O₅ with a Ti cation ratio = 0.27 is associated with a material that consists of a homogeneous titanium-tantalum-oxygen mixture containing a low density of nanometer sized Ar-filled voids. When the Ti cation ratio is 0.53, phase separation occurs leading to increased mechanical loss. These results suggest that amorphous mixed oxides with low mechanical loss could be identified by considering the thermodynamics of ternary phase formation