We have developed a method to determine the internal friction over a wide range of frequencies in transparent materials such as fused silica. We measure the anelastic aftereffect, the relaxation of a sample after a steady stress is removed, by sensing the polarization of a beam of light that traverses the sample. The fractional relaxation of the sample on a given time scale is a direct measure of the anelastic loss angle of the material at the corresponding frequency. To date, we have achieved sensitivity to loss angles as small as 10-6. We discuss the relevance of these measurements for the design of interferometric detectors of gravitational waves.
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