Theory of surface plasmon-polariton amplification in planar structures incorporating dipolar gain media

We investigate theoretically surface plasmon-polariton (SPP) amplification in planar metallic structures taking into account the nonuniformity of the gain medium close to the metal surface due to position-dependent dipole lifetime and pump irradiance. We propose a model that accounts for these nonuniformities and apply it to a physically realizable structure consisting of a thin silver film cladded on one side by a lossless dielectric and on the other by an optically pumped Rhodamine 6G dye solution. We study amplification of the supported long-range SPP mode and show that net amplification is possible at visible wavelengths using reasonable pump power and molecular concentration. It is shown that the gain nonuniformity close to the metal surface must be considered to describe adequately the SPP amplification phenomenon.