Key points are not available for this paper at this time.
We experimentally demonstrate a crystalline CaF₂ optical whispering gallery mode resonator characterized by the quality factor Q>210^10 at =1. 3190. 3em{0ex}. We also report the demonstration of ultrahigh-Q resonators of lithium niobate and lithium tantalate (Q>210^8), and sapphire (Q>10^9). We discuss fundamental limits of the Q factor, and application of such resonators for measurement of ultrasmall optical losses in various materials. We further analyze the observation of thermo-optical instability in whispering gallery modes of crystalline resonators.
Savchenkov et al. (Tue,) studied this question.