  A graphene sheet gated with a ridged ground plane, creating a soft-boundary
(SB) graphene nanoribbon, is considered. By adjusting the ridge parameters and
bias voltage a channel can be created on the graphene which can guide TM
surface plasmon polaritons (SPP). Two types of modes are found; fundemental and
higher-order modes with no apparent cutoff frequency and with energy
distributed over the created channel, and edge modes with energy concen-trated
at the soft-boundary edge. Dispersion curves, electric near-field patterns, and
current distributions of these modes are determined. Since the location where
energy is concentrated in the edge modes can be easily controlled
electronically by the bias voltage and frequency, the edge-mode phenomena is
used to propose a novel voltage controlled plasmonic switch and a plasmonic
frequency demultiplexer.
