Stimulated Emission of Surface Plasmons at the Interface between a Silver Film and an Optically Pumped Dye Solution

Surface plasmons (SPs) are surface-bound electromagnetic waves supported by metals, offering the possibility of strong spatial confinement of electromagnetic fields on the micro- and nanoscales. They suffer, however, from strong damping caused by internal absorption and radiation losses. Here we demonstrate amplification of SPs by stimulated emission, which marks a possible solution to this problem. We 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 SPs can be considered analogous to photon amplification in a laser, thereby suggesting novel approaches in the field of nano-optics.

Figure 1

Twin ATR method for SP 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 (see text). 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.

Figure 2

Differential reflectance curves proving stimulated emission of surface plasmons for different metal film thicknesses and dyes; experiment (a), (b) and theory (c), (d). (a) and (c) refer to cresyl violet, whereas (b) and (d) depict results for rhodamine 101. The respective film thickness (measured by atomic force microscopy) is indicated for each curve. The modulation amplitude of the pump power was 10 mW root mean square (rms) for cresyl violet and 9 mW rms for rhodamine 101. The reflectance change $\Delta R$ is also given as an rms quantity. The number density of dye molecules was $7\times {10}^{17}{\mathrm{cm}}^{-3}$ for both dye solutions.