Direct measurement of coating thermal noise in optical resonators

The best measurements of space and time currently possible (e.g., gravitational-wave detectors and optical reference cavities) rely on optical resonators, and are ultimately limited by thermally induced fluctuations in the reflective coatings which form the resonator. We present measurements of coating thermal noise in the audio band and show that for a standard ion-beam sputtered coating, the power spectrum of the noise does not have the expected power-law behavior.