Optical and mechanical properties of ion-beam-sputtered Nb$_2$O$_5$ and TiO$_2$-Nb$_2$O$_5$ thin films for gravitational-wave interferometers

Brownian thermal noise associated with highly-reflective mirror coatings is a fundamental limit for several precision experiments, including gravitational-wave detectors. Recently, there has been a worldwide effort to find mirror coatings with improved thermal noise properties that also fulfill strict optical requirements such as low absorption and scatter. We report on the optical and mechanical properties of ion-beam-sputtered niobia and titania-niobia thin films, and we discuss application of such coatings in current and future gravitational-wave detectors. We also report an updated direct coating thermal noise measurement of the HR coatings used in Advanced LIGO and Advanced Virgo.