Crystalline Whispering Gallery Mode Resonators: In Search of The Optimal   Material
  Different applications of crystalline whispering gallery mode resonators call for different properties of the resonator host material. We report on our recent study of resonators made out of sapphire, diamond, and quartz crystals and discuss possible applications of these resonators. In particular, we demonstrate Kerr frequency comb generation in sapphire microresonators. 
V. S. Ilchenko, A. A. Savchenkov, A. B. Matsko, and L. Maleki
physics.optics