PRISM-COUPLED LIGHT-EMISSION FROM A SCANNING TUNNELING MICROSCOPE

We have investigated the light emission mechanisms from a scanning tunneling microscope in which the sample surface is a thin film of Au (22.5 nm thick) evaporated on a microscope cover glass. This sample was attached to the flat surface of a hemicylindrical coupler prism. The tunneling electrons from a Pt-Ir tip were directed to the Au film from the air side, and the angle dependence of light emission intensity was measured through the coupler prism using a photon counting system. We found a sharp emission peak centered at 43-degrees from the film surface normal. This is the angle at which the fast mode of surface plasmon polaritons (SPP) mostly localized at the Au/air interface can radiate by conserving the wave vector parallel to the interface. Thus we conclude that most of the light on the prism side is emitted by the delocalized fast mode of SPP of the Au/air interface.