Light emission from surface plasmon polaritons mediated by metallic fine particles

A system of Ag nanoparticles placed very close to an Al surface was prepared by depositing an Ag-${\mathrm{SiO}}_2$ composite film on an Al film. Surface plasmon polaritons (SPP's) on the Al surface were excited by an attenuated total reflection (ATR) method and light emission from the SPP's caused by the presence of the Ag nanoparticles was measured. The intensity of emitted light was found to rapidly decrease as the distance between the particles and the surface increases. From a good qualitative agreement between experimental results and those of electromagnetic calculations, the following processes of light emission are suggested: (i) The SPP's excited by the ATR method excite the electromagnetic normal modes localized in between the Ag particles and the Al surface (gap modes), (ii) the gap modes then induce localized surface current on the Al surface, and (iii) the induced surface current emits light.