Gap plasmon mode of eccentric coaxial metal waveguide.

The gap plasmon mode of an eccentric coaxial waveguide is analyzed by the effective index method. The results agree-well with fully-vectorial numerical calculations. In the eccentric structure, there is extreme subwavelength field localization around the narrowest gap due to the gap plasmon. Furthermore, the effective index of the lowest-order waveguide mode increases considerably, for example, to 3.7 in the structure considered with a 2 nm minimum gap. The nanostructure waveguide geometry and wavelength (4 microm) are comparable with recent experiments on coaxial structures, except that that position of the center island is shifted for the eccentric coaxial structure; therefore, the proposed structure is a good candidate for future fabrication and experiments. In the visible regime, the effective index increases to over 10 for the same structure. The influence of symmetry-breaking in the eccentric coaxial structure is discussed as a way to enhance the local field and improve optical coupling.