Near-field enhancement of multipole plasmon resonances in Ag and Au nanowires.

In this paper, we investigate theoretically the electromagnetic field enhancement arising from excitation of silver and gold nanowires (NWs) of finite length, capable of sustaining surface plasmon resonances of different multipole order, using the Discrete Dipole Approximation (DDA). The influence of NW length on the degree of enhancement and confinement of the electromagnetic field for each surface plasmon mode is analyzed by a 3D mapping of the near field for different planes around the NW as well by calculating its variation with distance along two different directions, one parallel to and the other perpendicular to the NW axis, outside of the NW. It was found that the enhancement is still significant at relative large distances from the NW end, its decay being of much longer range than that predicted by a simple dipole approximation, especially at near-infrared wavelengths.