Microwave characterisation of CaF2 at cryogenic temperatures using a   dielectric resonator technique
  Properties of calcium fluoride (CaF2) have been well researched at UV, visible and IR range of frequencies, but not at ultra high frequencies. In this work we report the loss tangent and the real part of relative permittivity of CaF2, measured in the temperatures range of 15 - 81 K and the frequency of 29.25 GHz. The loss tangent and the real part of relative permittivity were determined by measurements of the resonant frequency and the Qo - factor of a TE011 mode cylindrical copper cavity with superconducting plates containing the sample under test. The measured real part of relative permittivity of CaF2 was found to change from 6.484 to 6.505, and the loss tangent from 3.1x10^(-6) to 22.7x10^(-6), when the temperature was varied from 15 to 81 K. Due to the low losses CaF2 can be useful in construction of high Q-factor microwave circuits and devices, operating at the cryogenic temperatures. 
Mohan V. Jacob, Janina E. Mazierska, Dimitri O. Ledenyov, Jerzy Krupka
cond-mat.mtrl-sci physics.optics