Integrated optical microcavities for enhanced evanescent-wave spectroscopy

The use of integrated optical microcavities (MCs) for enhanced optical spectroscopy and sensing is investigated. The MC sustains high-$Q$ whispering-gallery modes, in which the energy of the optical field can be efficiently stored. The resulting enhanced field can be used to probe fluorescent molecules in the cladding of the MC. Enhanced fluorescence excitation with an integrated optical MC is demonstrated experimentally for what is believed to be the first time. A comparison between a MC and a straight waveguide shows that the MC may give an increase of 40 times in fluorescence excitation. Because of the ultrasmall size of the MC ($15 \mathit{\mu }\mathrm{m}$ in radius), the fluorescence signal may be observed from only 20 molecules in the cladding.