Surface plasmon-mediated energy transfer in heterogap Au-Ag nanowires.

We report the observation of energy transfer from a gold (Au) nanodisk pair to a silver (Ag) nanowire across a 120 nm gap via surface plasmon resonance (SPR) excitation. The enhanced electromagnetic (EM) fields generated by Au SPR excitation induce oscillation of the conduction electrons in the Ag segment, transferring energy to it even though the Ag segment has only weak resonant interactions with the incident EM radiation. The induced Ag SPR produces strong EM fields at the position of the Ag segment, leading to a Raman signal approximately 15 times greater than when the Ag segment is alone (not adjacent to the Au nanodisk pair). The Raman intensity is found to depend nonlinearly on the incident laser intensity for laser power densities of 10 kW/cm(2), which is consistent with the results of EM theory calculations which are not able to account for the factor of 15 enhancement based on a linear mechanism. This suggests that energy transfer from the Au disk pair to the Ag segment involves an enhanced nonlinear polarization mechanism such as can be produced by the electronic Kerr effect or stimulated Raman scattering.