Silver Nanowires Terminated by Metallic Nanoparticles as Effective Plasmonic Antennas

Metallic nanowires constitute a distinctive class of nanostructures that are able to guide surface plasmons in subwavelength dimensions. The effective use of light in-and out-coupling in low dimensional systems, such as excitation of surface plasmon polaritons along metallic nanowires, has been proposed to reduce physical dimensions of opto-electronic and nano-optical components and for high-resolution microscopy applications. Our investigation of light in-and out-coupling on silver nanowire systems by scanning optical coupling microscopy (SOCM) performed in combination with atomic force microscopy (AFM) revealed that the maximum coupling was obtained when the exciting laser light is projected at the end of the nanowire with a positioning accuracy of approximately 100 nm. Furthermore, it was found that a nanoparticle positioned at the end of a nanowire imparts an enhanced (by almost the factor of 4) plasmon in-and out-coupling light efficiency as compared to a free nanowire under the same excitation conditions. These findings are supported by theoretical simulations, which in addition provide a correlation between the nanoparticle size and the out-coupling light efficiency. Our investigations demonstrate that a combination of SOCM and AFM methods provide reliable qualitative and quantitative evaluation of plasmon in-and out-coupling characteristics on metallic nanowire systems.