  We present the first observation of optomechanical coupling in ultra-high Q
crystalline whispering-gallery-mode (WGM) resonators. The high purity of the
crystalline material enables optical quality factors in excess of 10^{10} and
finesse exceeding 10^{6}. Simultaneously, mechanical quality factors greater
than 10^{5} are obtained, still limited by clamping losses. Compared to
previously demonstrated cylindrical resonators, the effective mass of the
mechanical modes can be dramatically reduced by the fabrication of CaF2
microdisc resonators. Optical displacement monitoring at the 10^{-18}
m/sqrt{Hz}-level reveals mechanical radial modes at frequencies up to 20 MHz,
corresponding to unprecedented sideband factors (>100). Together with the weak
intrinsic mechanical damping in crystalline materials, such high sindeband
factors render crystalline WGM micro-resonators promising for backaction
evading measurements, resolved sideband cooling or optomechanical normal mode
splitting. Moreover, these resonators can operate in a regime where
optomechanical Brillouin lasing can become accessible.
