Optical coatings for gravitational-wave detection

Gravitational waves are a prediction of Einstein's General Theory of Relativity. Astrophysical events like supernova and binary neutron star inspirais are predicted to create potentially detectable waves. The Laser Interferometer Gravitational-wave Observatory (LIGO) is an experiment to detect these waves using Michelson interferometers with 4 km long arms. The effect of gravitational waves, even on an interferometer with such a long baseline, is extremely small, with mirror displacements around 10 -18 m. Reducing noise is thus a primary design criterion. For the next generation interferometers now being designed, thermal noise from the optical coatings of the interferometer mirrors could prove a problematic limiting noise source. Reducing the mechanical loss of these coatings to improve thermal noise, while preserving the sub-ppm optical absorption, low scatter, and high reflectivity needed in the interferometer is an important area of research.