Measurement of the mechanical loss of a cooled reflective coating for gravitational wave detection

TL;DR

This study measures the mechanical loss of cooled dielectric reflective coatings used in gravitational wave detectors, finding it nearly temperature-independent and suggesting thermal noise can be reduced by cooling, which benefits future detector designs.

Contribution

It provides the first detailed measurement of mechanical loss in dielectric coatings at cryogenic temperatures, revealing temperature independence and implications for gravitational wave detector noise reduction.

Findings

Mechanical loss is nearly independent of temperature from 4 K to 300 K.

Loss angle is approximately (4 to 6) x 10^{-4}.

Cooling to 20 K significantly reduces thermal noise in mirrors.

Abstract

We have measured the mechanical loss of a dielectric multilayer reflective coating (ion-beam sputtered SiO$_2$ and Ta$_2$O$_5$) in cooled mirrors. The loss was nearly independent of the temperature (4 K $\sim$ 300 K), frequency, optical loss, and stress caused by the coating, and the details of the manufacturing processes. The loss angle was $(4 \sim 6) \times 10^{-4}$. The temperature independence of this loss implies that the amplitude of the coating thermal noise, which is a severe limit in any precise measurement, is proportional to the square root of the temperature. Sapphire mirrors at 20 K satisfy the requirement concerning the thermal noise of even future interferometric gravitational wave detector projects on the ground, for example, LCGT.