Controlling the radiation direction of propagating surface plasmons on silver nanowires

Metal nanowires supporting propagating surface plasmons (SPs) can be used as nanowaveguides and nanoantennas for light manipulation beyond the diffraction limit. Here the control of the propagation and radiation of SPs on silver nanowires is investigated. By covering an Al₂O₃ layer onto a silver nanowire to change the local dielectric environment, the wave vector of the propagating SPs is increased. Thus, the radiation direction of SPs into the substrate is changed according to the phase matching condition, which is experimentally shown by Fourier imaging method. The radiation angle is sensitively dependent on the Al₂O₃ thickness. By depositing 1 nm Al₂O₃, the increase of the radiation angle can be close to 1 degree. These results show that dielectric-layer-coating provides a simple and effective method to control the propagation and radiation of SPs, which will be of great importance for designing plasmonic circuits, antennas and sensors based on silver nanowires.