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

Coating thermal noise is a fundamental limit for precision experiments based on optical and quantum transducers. In this review, after a brief overview of the techniques for coating thermal noise measurements, we present the latest worldwide research activity on low-noise coatings, with a focus on the results obtained at the Laboratoire des Matériaux Avancés. We report new updated values for the T a 2 O 5 , T a 2 O 5 − T i O 2 , and S i O 2 coatings of the Advanced LIGO, Advanced Virgo, and KAGRA detectors, and new results from sputtered N b 2 O 5 , T i O 2 − N b 2 O 5 , T a 2 O 5 − Z r O 2 , M g F 2 , A l F 3 , and silicon nitride coatings. Amorphous silicon, crystalline coatings, high-temperature deposition, multi-material coatings, and composite layers are also briefly discussed, together with the latest developments in structural analyses and models.