Thermal noise reduction and absorption optimization via multimaterial coatings

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 $(5.3\ifmmode\pm\else\textpm\fi{}0.4)\text{ }\text{ }\mathrm{ppm}$ at 1550 nm should be achievable.