Frequency tuning of the whispering gallery modes of silica microspheres   for CQED and spectroscopy
  We have tuned the whispering gallery modes of a fused silica microresonator over nearly 1 nm at 800 nm, i.e. over 0.5 FSR or 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 high-Q optical resonator when it is desirable to optimize its coupling to emitters with given transition frequencies. The tuning capability demonstrated here is compatible with both UHV and low temperature operation, which should be useful for future experiments with laser cooled atoms or single quantum dots. 
Wolf von Klitzing, Romain Long, Vladimir S. Ilchenko, Jean Hare,   Valerie Lefevre-Seguin