Stimulated emission of surface plasmon polaritons on smooth and corrugated silver surfaces

We have observed two regimes of stimulated emission of surface plasmon polaritons (SPPs) propagating at interfaces between dye-doped polymeric films and silver substrates. (i) On top of smooth silver substrates, the lasing threshold is very soft, and the relatively broad stimulated emission peak gradually narrows with increase of the pumping. We infer that this stimulated emission regime is dominated by amplification of SPPs in open paths, with a possible contribution from a diffusion feedback provided by unintentional scattering. (ii) At the same time, the laser action on top of roughened silver substrates is characterized by a sharp threshold and multiple narrow spectral lines. This novel stimulated emission effect has strong coherent feedback supported by strong scattering of SPPs at a roughened silver surface. The observed phenomena provide an addition to the family of surface plasmon lasers, enabling active metamaterials and plasmonics.