Ultrasensitive surface spectroscopy with a miniature optical resonator

The number density and orientation of molecules at the surface of a total-internal-reflection-ring minicavity are probed with extremely high sensitivity in a novel realization of the cavity ring-down optical absorption experiment. The modes of the ultralow-loss cavity, which are excited by photon tunneling, have long lifetimes that are sensitive to the presence of absorbing species in the evanescent field near a cavity facet. The total-internal-reflection-ring cavity extends cavity ring-down spectroscopy to surfaces and condensed matter, permitting a wide range of novel fundamental studies and applications. Routine single molecule detection may ultimately be feasible.