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 (${\mathrm{SiO}}_2$) and tantala (${\mathrm{Ta}}_2{\mathrm{O}}_5$) 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 ${\mathrm{Ta}}_2{\mathrm{O}}_5$. Unfortunately, such an $\mathrm{aSi}/{\mathrm{SiO}}_2$ 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 multimaterial design the low absorbing ${\mathrm{Ta}}_2{\mathrm{O}}_5$ 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 $(5.3\pm 0.4)\mathrm{ppm}$ at 1550 nm should be achievable.

• Received 13 November 2014
• Corrected 4 March 2015

DOI:https://doi.org/10.1103/PhysRevD.91.042001