Investigating the medium range order in amorphous Ta2O5 coatings

Riccardo Bassiri,Ian Maclaren,Sheila Rowan,Roger K Route,Keith Evans,Martin M Fejer,Iain W Martin,Martin Hart,Angie C Lin,Robert L Byer,Ashot S Markosyan,Konstantin B Borisenko

doi:10.1088/1742-6596/522/1/012043

Abstract

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.

Highlights

Amorphous heavy metal oxides, such as Ta2O5, are widely used as the high refractive index layer of highly reflective dielectric coatings
This is thought to be the result of changes in the atomic structure, and recent observations have shown a direct correlation between the mechanical loss and local atomic structure [4]
We aim to find ways to reduce the levels of mechanical loss in the coatings, and help to increase the Laser Interferometer Gravitational-wave Observatory (LIGO) detector sensitivity

Summary

Introduction

Amorphous heavy metal oxides, such as Ta2O5, are widely used as the high refractive index layer of highly reflective dielectric coatings. It has previously been shown that heat-treatment and TiO2 doping of the Ta2O5 coatings can cause noticeable changes to the levels of mechanical loss measured [2, 3]. This is thought to be the result of changes in the atomic structure, and recent observations have shown a direct correlation between the mechanical loss and local atomic structure [4]. The atomic structure of heat-treated Ta2O5 coatings is studied and compared to previously measured values of mechanical loss, and the first results of the medium range order from a heat-treated Ta2O5 coating is presented. Each diffraction pattern was radially averaged before the variance was computed through the stack

Linking atomic structure and mechanical loss

Conclusions and future work