Mechanical Characterisation of Silicon for the ETpathfinder Test Masses

TL;DR

This paper characterizes the mechanical loss of silicon at cryogenic temperatures for use in gravitational-wave detector test masses, addressing anisotropy effects and predicting noise contributions for the ETpathfinder project.

Contribution

It provides the first detailed measurement of silicon's mechanical loss from room temperature down to 53K, considering anisotropic effects for gravitational-wave detector applications.

Findings

Mechanical loss decreases with temperature, reaching low levels at cryogenic temperatures.

Anisotropic properties of silicon significantly influence loss measurements.

Predicted substrate noise contribution informs ETpathfinder's sensitivity estimates.

Abstract

The next generation of gravitational-wave detectors, such as the Einstein Telescope, is designed to reduce noise in a wide band of frequencies compared to the current generation, through the use of new technologies. ETpathfinder, designed as an R&D facility for these technologies, is a prototype for which the mirrors were chosen to be made of crystalline silicon, produced by the Leibniz-Institut f\"ur Kristallz\"uchtung. This material choice was made to pave the way for a low thermal noise level at cryogenic temperatures in the Einstein Telescope. This paper shows the mechanical loss of silicon designated to become the test masses for ETpathfinder in the range between room temperature and 53K. In addition, the effect of the anisotropic nature of silicon on the measurement procedure is addressed. Predictions are made of the contribution of the mirror substrate material to the overall ETpathfinder noise budget.