  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 dielectric 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 (AlF$_3$) thin
films which have one of the lowest refractive index among the known coating
materials and we discuss their application in current and future GWDs.
