Progress in the measurement and reduction of thermal noise in optical coatings for gravitational-wave detectors

Coating thermal noise is a fundamental limit for experiments involving optical and quantum transducers. We report on the latest research activity on low-noise coatings at the Laboratoire des Materiaux Avances, from updated values for the current Ta2O5, Ta2O5-TiO2 and SiO2 coatings of the Advanced LIGO, Advanced Virgo and KAGRA detectors to recent results from sputtered Nb2O5, TiO2-Nb2O5, Ta2O5-ZrO2, MgF2, AlF3 and Si3N4 coatings. Some alternative options, like amorphous-silicon and AlGaAs crystalline coatings among others, are also briefly discussed.