A very smooth glass sphere with a diameter of a few tens to hundreds of microns is an excellent optical cavity. Light can be trapped in the "whispering gallery modes", i.e. modes that span the "equator" of such a sphere. The spectrum of such a cavity tends to be very sparse, since an integer number of wavelengths has to fit around the sphere for resonance to occur. On the other hand, the finesse of the microresonator cavity is excellent with Q-factors that frequently exceed 10,000,000 and are typically 100,000,000.
We have shown that measurements of the ring-down time using the phase shift cavity ring down technique are possible on these tiny cavities. We also demonstrated that one can measure the absolute absorption cross section of a vibrational overtone is possible on these cavities even if the molecules that are adsorbed from less than one monolayer.
With Gianluca Gagliardi (INO, Naples, IT) we published on the detection of analytes in liquid droplet resonators using similar detection principles.