A nano-scaled coupled-waveguide coupler based on the guidance of surface plasmon-polaritons (SPPs) is proposed,
designed and simulated at optical frequencies. The basic structure of the coupler comprises layered
dielectric materials and thin silver films, which serve as two stacked nano-transmission lines to achieve broadside
coupling. The key property of this design is that it operates based on the principle of contra-directional coupling
between a left-handed and a right-handed guided wave, giving rise to supermodes that are characterized by
complex-conjugate propagation constants (even in the absence of losses), where the attenuation constant signifies
the rate of coupling instead of the conventional power dissipation. The resulting exponential attenuation along
the coupler leads to dramatically reduced coupling lengths compared to previously reported co-directional SPP
couplers (e.g. from millimeters to sub-microns). Given its size, the device lends itself to form the building block
of a functional matrix such as a switching array in nanophotonics applications, for example. In order to verify the
contra-directional coupling theory and to characterize our design, we also propose and examine several possible
excitation schemes, such as using a plasmonic dipole antenna and a grating structure to excite the SPP mode.
Additionally, a measurement topology utilizing a curved plasmonic waveguide is also presented in this paper.