Measurements of low-loss crystalline materials for high-Q temperature stable resonator applications

Whispering gallery modes were used for very accurate permittivity and dielectric loss tangent measurements for low loss isotropic and uniaxially anisotropic materials. We present the measurements of several specimens including sapphire, YAG, quartz, rutile and SrLaAlO₄. The total absolute uncertainty in real part of the permittivity tensor was estimated to be less than 0.1% and was limited by the uncertainty in the dimensions of the samples. Imaginary parts of the permittivity tensor were measured to about 10% accuracy, limited by the accuracy of Q-factor measurements in whispering gallery modes. The anisotropy ratio of the measured materials varied from 1 (isotropic YAG) to 2.2 (rutile). All anisotropic materials exhibited anisotropy in the imaginary part of the permittivity tensor as well as the real part. For most crystals dielectric losses can be approximated by a power function of absolute temperature in only a limited temperature range. At very low temperatures (4-50 K) properties of both the real and imaginary permittivity tensor are often affected by impurities which are always present in real crystals