Tunable whispering gallery modes for spectroscopy and CQED experiments
  We have tuned the whispering gallery modes of a fused silica micro- resonator over nearly 1 nm at 800 nm, i.e.over half of a free spectral range or the equivalent of 10^6 linewidths of the resonator.This has been achieved by a new method based on the stretching of a two-stem microsphere.The devices described below will permit new Cavity-QED experiments with this ultra high .nesse optical resonator when it is desirable to optimise its coupling to emitters with given transition frequencies. The tuning capability demonstrated is compatible with both UHV and low temperature operation which should be useful for future experiments with laser cooled atoms or single quantum dots.A general overview of the current state of the art in microspheres is given as well as a more general introduction. 
Wolf von Klitzing, Romain Long, Vladimir S. Ilchenko, Jean Hare,   Valerie Lefevre-Seguin