Characterization of Ion-Beam-Sputtered AlF3 Thin Films for Gravitational-Wave Interferometers

Thermal noise in amorphous coatings is a limitation for a wide range of precision experiments such as gravitational-wave detectors (GWDs). Mirrors for GWDs are composed of multiple thin layers of dielec-tric materials deposited on a substrate: the stack is made of layers with a high-refractive index interleaved with layers of a low-refractive index. The goal is to obtain high reflectivity and low thermal noise. In this paper we report on the optical and mechanical properties of ion-beam-sputtered aluminium fluoride (AlF3) thin films that have one of the lowest refractive indexes among the known coating materials and we discuss their application in current and future GWDs.