Surface plasmons at nanoscale relief gratings between a metal and a dielectric medium with optical gain

Surface plasmons at the interface between metal and a dielectric with strong optical amplification are analyzed theoretically. It is shown that proper choice of optical indices of the dielectric medium results in an infinitely large effective refractive index of surface waves. Such resonant plasmons have extremely low group velocity and are localized within a vanishingly small distance near the interface. The plasmon-related anomalies in the UV reflection spectra are predicted for nanoscale gratings on a surface of a silver film covered by a concentrated dye solution with high optical amplification.