  To enable multiple functions of plasmonic nanocircuits, it is of key
importance to control the propagation properties and the modal distribution of
the guided optical modes such that their impedance matches to that of nearby
quantum systems and desired light-matter interaction can be achieved. Here, we
present efficient mode converters for manipulating guided modes on a plasmonic
two-wire transmission line. The mode conversion is achieved through varying the
path length, wire cross section and the surrounding index of refraction.
Instead of pure optical interference, strong near-field coupling of surface
plasmons results in great momentum splitting and modal profile variation. We
theoretically demonstrate control over nanoantenna radiation and discuss the
possibility to enhance nanoscale light-matter interaction. The proposed
converter may find applications in surface plasmon amplification, index sensing
and enhanced nanoscale spectroscopy.
