Optical properties of silicon-nitride atomic-force microscopy tips in scanning tunneling optical microscopy: experimental study

Atomic-force SiN probe tips (atomic-force microscopy) can be conveniently used for scanning-tunnelingoptical-microscopy experiments because of their transparency in the visible domain. They are known to provide a satisfying transmission yield and spatial resolution in spite of their complex structural shape. Nevertheless the photon collection mechanism is not so clearly understood. We give some experimental information on the conversion of the evanescent waves into a propagating mode; we show experimentally (1) that optical coupling satisfactorily obeys a classical global and macroscopic dielectric model, and (2) that the collected photons dominantly follow the photon momentum conservation rule that makes these devices directionally selective.