Theory for light emission from a scanning tunneling microscope

We have calculated the rate of light emission from a scanning tunneling microscope with an Ir tip probing a silver film. We find a considerable enhancement of the rate of spontaneous light emission compared with, for example, inverse-photoemission experiments. This enhancement is the result of an amplification of the electromagnetic field in the area below the microscope tip due to a localized interface plasmon. One can estimate that one out of ${10}^4$ tunneling electrons will emit a photon. We also find that the experimentally observed maximum in the light emission as a function of bias voltage is directly related to the detailed behavior of tip-sample separation versus bias voltage.