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In recent years an increasing number of devices and experiments are shown tobe limited by mechanical thermal noise. In particular sub-Hertz laser frequencystabilization and gravitational wave detectors, that are able to measurefluctuations of 1E-18 m/rtHz or less, are being limited by thermal noise in thedielectric coatings deposited on mirrors. In this paper we present a newmeasurement of thermal noise in low absorption dielectric coatings deposited onmicro-cantilevers and we compare it with the results obtained from themechanical loss measurements. The coating thermal noise is measured on thewidest range of frequencies with the highest signal to noise ratio everachieved. In addition we present a novel technique to deduce the coatingmechanical losses from the measurement of the mechanical quality factor whichdoes not rely on the knowledge of the coating and substrate Young moduli. Thedielectric coatings are deposited by ion beam sputtering. The results presentedhere give a frequency independent loss angle of (4.70 $\pm$ 0.2)x1E-4 with aYoung's modulus of 118 GPa for annealed tantala from 10 Hz to 20 kHz. Foras-deposited silica, a weak frequency dependence (~ f^{-0.025}) is observed inthis frequency range, with a Young's modulus of 70 GPa and an internal dampingof (6.0 $\pm$ 0.3)x1E-4 at 16 kHz, but this value decreases by one order ofmagnitude after annealing and the frequency dependence disappears.

Measurements of mechanical thermal noise and energy dissipation in optical dielectric coatings