  Broadband and tunable control of surface plasmon polaritons in the
near-infrared and visible spectrum is demonstrated theoretically and
numerically with a pair of phased nanoslits. We establish with simulations
supported by a coupled wave model that by dividing the incident power equally
between two input channels, the maximum plasmon intensity deliverable to either
side of the nanoslit pair is twice that for an isolated slit. For a broadband
source, a compact device with nanoslit separation of the order of a tenth of
the wavelength is shown to steer nearly all the generated plasmons to one side
for the same phase delay, thereby achieving a broadband unidirectional plasmon
launcher. The reported effect can be applied to the design of ultra-broadband
and efficient tunable plasmonic devices.
