Comprehensive study of amorphous metal oxide and Ta$_2$O$_5$-based mixed oxide coatings for gravitational-wave detectors

TL;DR

This study systematically investigates amorphous metal oxide and Ta₂O₅-based mixed oxide coatings, revealing their potential for reducing thermal noise in gravitational-wave detectors by analyzing their mechanical and optical properties.

Contribution

It provides a comprehensive analysis of the relationship between amorphous structure and mechanical loss, and evaluates various mixed oxide coatings for low thermal noise applications.

Findings

Metal oxide loss correlates with amorphous morphology.

SiO₂ and GeO₂ have the lowest loss angles.

TiO₂-doped Ta₂O₅ is optimal for low thermal noise.

Abstract

High finesse optical cavities of current interferometric gravitational-wave detectors are significantly limited in sensitivity by laser quantum noise and coating thermal noise. The thermal noise is associated with internal energy dissipation in the materials that compose the test masses of the interferometer. Our understanding of how the internal friction is linked to the amorphous material structure is limited due to the complexity of the problem and the lack of studies that span over a large range of materials. We present a systematic investigation of amorphous metal oxide and Ta$_2$O$_5$-based mixed oxide coatings to evaluate their suitability for low Brownian noise experiments. It is shown that the mechanical loss of metal oxides is correlated to their amorphous morphology, with continuous random network materials such as SiO$_2$ and GeO$_2$ featuring the lowest loss angles. We evaluated different Ta$_2$O$_5$-based mixed oxide thin films and studied the influence of the dopant in the optical and elastic properties of the coating. We estimated the thermal noise associated with high-reflectance multilayer stacks that employ each of the mixed oxides as the high index material. We concluded that the current high index material of TiO$_2$-doped Ta$_2$O$_5$ is the optimal choice for reduced thermal noise among Ta$_2$O$_5$-based mixed oxide coatings with low dopant concentrations.