Controllable Excitation of Surface Plasmons in End-to-Trunk Coupled Silver Nanowire Structures

In branched nanowire structures, the controllable excitation of surface plasmons is investigated by both experiments and simulations. By focusing the excitation light at the junction between the main wire and the branch wire, surface plasmons can be selectively launched to propagate to different output terminals, depending on the polarization of the excitation light. The parameters influencing the plasmon excitation and thus emission behavior are investigated, including the branch angle, the position of the branch and the nanowire radius. The different polarization dependence of the output light is determined by the surface plasmon modes selectively excited in the junction through end-excitation or/and gap-excitation manners. For the branch wire, when the branch angle is small, the end-excitation is dominant, which makes the branched wire behave like an individual nanowire. With the increase of the branch angle, the coupling between the branch wire end and the primary wire trunk is increased, which influences the plasmon excitation in the branch wire as evidenced by the rotation of the polarization angle for maximum output. For the primary wire, the SP excitation is dependent on the branch angle, position of the junction along the primary wire, and the radii of the nanowires. The results may be important for the design of a controllable surface plasmon launcher, one of the functional components in surface-plasmon-based nanophotonic circuits.