Thermal Noise Reduction and Absorption Optimisation via Multi-Material Coatings

TL;DR

This paper proposes a multi-material coating design for cryogenic gravitational wave detectors that reduces thermal noise by 25% and achieves low optical absorption, improving mirror performance at 1550 nm.

Contribution

It introduces a novel three-material coating stack combining Ta2O5 and amorphous silicon to optimize thermal noise reduction and optical absorption in GWD mirrors.

Findings

Achieved an optical absorption of 5.3 ppm at 1550 nm.

Predicted a 25% reduction in Brownian thermal noise.

Demonstrated feasibility of multi-material coatings for cryogenic GWDs.

Abstract

Future gravitational wave detectors (GWDs) such as Advanced LIGO upgrades and the Einstein Telescope are planned to operate at cryogenic temperatures using crystalline silicon (cSi) test-mass mirrors at an operation wavelength of 1550 nm. The reduction in temperature in principle provides a direct reduction in coating thermal noise, but the presently used coating stacks which are composed of silica (SiO2) and tantala (Ta2O5) show cryogenic loss peaks which results in less thermal noise improvement than might be expected. Due to low mechanical loss at low temperature amorphous silicon (aSi) is a very promising candidate material for dielectric mirror coatings and could replace Ta2O5. Unfortunately, such a aSi/SiO2 coating is not suitable for use in GWDs due to high optical absorption in aSi coatings. We explore the use of a three material based coating stack. In this multi-material design the low absorbing Ta2O5 in the outermost coating layers significantly reduces the incident light power, while aSi is used only in the lower bilayers to maintain low optical absorption. Such a coating design would enable a reduction of Brownian thermal noise by 25%. We show experimentally that an optical absorption of only (5.3 +/- 0.4)ppm at 1550 nm should be achievable.