  Miniaturization of functional optical devices and circuits is a key
prerequisite for a myriad of applications ranging from biosensing to quantum
information processing. This development has considerably been spurred by rapid
developments within plasmonics exploiting its unprecedented ability to squeeze
light into subwavelength scale. In this study, we investigate on-chip plasmonic
systems allowing for synchronous excitation of multiple inputs and examine the
interference between two adjacent excited channels. We present a branchless
interferometer consisting of two parallel plasmonic waveguides that can be
either selectively or coherently excited via ultra-compact antenna couplers.
The total coupling efficiency is quantitatively characterized in a systematic
manner and shown to exceed 15% for small waveguide separations, with the power
distribution between the two waveguides being efficiently and dynamically
shaped by adjusting the incident beam position. The presented design principle
can readily be extended to other configurations, giving new perspectives for
highly dense integrated plasmonic circuitry, optoelectronic devices, and
sensing applications.
