Direct observation of amplified spontaneous emission of surface plasmon polaritons at metal/dielectric interfaces

We report on direct observation of amplified spontaneous emission (ASE) of surface plasmon polaritons (SPPs) at the interface of a silver film and a gain medium. Based on a typical Kretschmann configuration incorporated with Rhodamine 6G molecules, the growing ASE spectra of SPPs have been clearly identified by carefully conducting a pump-dependent angle-resolved spectral measurement. Spectral narrowing effects induced by the SPP amplification are also demonstrated. The observed phenomena are helpful in understanding the fundamental interactions between SPPs and gain medium, which could enable wide applications on plasmonic sources and sensors.