Unidirectional plasmon propagation in metallic nanowires

We report the observation of unidirectional plasmon propagation in metallic nanowires over distances > 10 mu m. Through control of the incident excita tion wavelength and rod composition, we demonstrate the selective coupling of photons into the plasmon mode of a 20 nm diameter nanowire. This mode th en propagates in a nonemissive fashion down the wire length before being em itted as an elastically scattered photon at the distal end. As expected fro m previous studies of plasmon excitation in nanoparticles and thin films, w e observe a strong wavelength and material dependence of this phenomenon. T his metal-dependent plasmon propagation is exploited to produce a wire thro ugh which plasmons propagate unidirectionally. A bimetallic wire with a sha rp Au/Ag heterojunction is shown to display both wavelength dependence and unidirectionality with respect to plasmon propagation across the heterojunc tion. It is expected that these results will contribute to the growing inte rest in optical energy transport in molecular-level and nanoscale devices.