The emission of light from metal-oxide-metal tunnel junctions has been studied as a probe of the interaction of the tunneling current with the surface plasmon polaritons of the tunneling junction structure. The slow mode, with large fields in the oxide layer, is most directly coupled to the tunneling current. In this experiment, diffraction gratings with periods between 70 and 100 nm provide the momentum for the slow mode to radiate. We report the unambiguous measurement of the direct emission from the slow mode showing that it does contribute to the light emission.

• Received 4 February 1992

DOI:https://doi.org/10.1103/PhysRevLett.68.2668