  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.
