Direction-resolved radiation from polarization-controlled surface plasmon modes on silver nanowire antennas.

Metallic nanowires (NWs) support multiple surface plasmon (SP) modes, which lead to extraordinary SP propagation behaviors. The leaky SP modes in metallic NWs connect the guiding and radiation of light at the nanometer scale. Understanding and controlling these modes are of vital importance for various nanophotonic applications. Here, we investigate the radiation from two polarization-controlled SP modes on supported silver NWs by using leakage radiation imaging and Fourier imaging techniques. The radiation directions from these modes can be clearly resolved from the Fourier images. The radiation polarization of the SP modes is related to the polarization of the excitation light. By depositing thin Al2O3 films onto silver NWs or decreasing the excitation wavelength, the radiation angles and wave vectors of the two modes are increased, and the longitudinal mode is more sensitive to Al2O3 thickness. Moreover, the propagation length of the longitudinal mode is obtained by analyzing the leakage radiation images, which is decreased with the decrease of the excitation wavelength and the increase of the Al2O3 layer thickness. These results show that leakage radiation from different SP modes on silver NWs can be resolved directly and controlled effectively. The supported silver NWs can thus be applied to designing plasmonic circuits, nanoantennas and nanosensors.