A group of the Plasmo-nano-devices Network of Excellence has demonstrated amplification of surface plasmons by stimulated emission, which marks a possible solution to the large propagation losses that they suffer. They use an attenuated-total-reflection setup to detect stimulated emission of SPs at the interface between a silver film and an optically pumped dye solution acting as the amplifying medium. Clear evidence of stimulated emission is provided by an excellent agreement of the experimental observations with a theoretical analysis. Amplification of surface plasmons can be considered analogous to photon amplification in a laser, thereby suggesting novel approaches in the field of nano-optics.

Twin atenuated total reflection method for surface plasmon amplification. Dye molecules, which are optically excited by the pump plasmon field, are made to coherently deliver their energy to the probe plasmon field by stimulated emission, thereby producing amplification. The signature of this process can be found in the reflected probe beam. The spontaneous decay channel causes directional plasmon-coupled light emission visible as a light cone containing all colors of the dye emission spectrum. The inset shows a simplified energy level diagram of the dye molecules.

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Images courtesy of L. Eng, Near-field Optics Group, Technische Universitaet Dresden, Germany.