Light emission from tunnel junctions: The role of the fast surface polariton

We have carried out calculations of light emission from tunnel junctions biased by a dc voltage to explore the role of "fast surface polaritons" (i.e., modes with dispersion curve close to the light line) in the emission process. This may be done within the framework of the formalism described in our earlier papers. For roughness with transverse scale from ten to a few hundred angstroms, we find the emission mediated by this mode to be orders of magnitude weaker than that mediated by the "slow mode" explored in our previous work. However, if the sinusoidal grating with a period of several thousand angstroms is impressed on the junction, then, in agreement with recent experiments by Kirtley, Theis, and Tsang, we find intense, $p$-polarized emission lines with frequency controlled by the dispersion relation of the fast mode. We present selected examples of calculated emission spectra.