A Systematic Error in the Internal Friction Measurement of Coatings for Gravitational Waves Detectors

TL;DR

This paper identifies a systematic error in measuring the coating loss angle for gravitational wave detectors caused by sample inhomogeneity, and proposes a model to improve measurement accuracy.

Contribution

It introduces a new model for disk-shaped resonators that accounts for inhomogeneity, revealing a systematic error in previous coating loss angle measurements.

Findings

Systematic error due to edge inhomogeneity affects loss angle measurements.

The proposed model improves the accuracy of coating loss angle estimation.

Inhomogeneity impacts the design of coatings for gravitational wave detectors.

Abstract

Low internal friction coatings are key components of advanced technologies such as optical atomic clocks and high-finesse optical cavity and often lie at the forefront of the most advanced experiments in Physics. Notably, increasing the sensitivity of gravitational-wave detectors depends in a very large part on developing new coatings, which entails developing more suitable methods and models to investigate their loss angle. In fact, the most sensitive region of the detection band in such detectors is limited by the coating thermal noise, which is related to the loss angle of the coating. Until now, models which describe only ideal physical properties have been adopted, wondering about the use of one or more loss angles to describe the mechanical properties of coatings. Here we show the presence of a systematic error ascribed to inhomogeneity of the sample at its edges in measuring the coating loss angle. We present a model for disk-shaped resonators, largely used in loss angle measurements, and we compare the theory with measurements showing how this systematic error impacts on the accuracy with which the loss model parameters are known.