  Strong optical nonreciprocity at the nanoscale, relying on extreme one-way
modes and backscattering suppression, can enable fundamentally new approaches
in optoelectronics and plasmonics. Of special interest is achieving
nonreciprocity in systems devoid of magnetic couplings. We describe a new
approach based on the plasmonic Doppler effect which takes place for plasmons
propagating in the presence of an electrical DC current. Large carrier drift
velocities reachable in high-mobility electron systems, such as graphene, can
enable strongly nonreciprocal or even fully one-way modes. Striking effects
such as mode isolation and one-way transmission in DC-current-controlled
Mach-Zehnder interferometers provide clear manifestations of plasmonic
nonreciprocity. Phenomena such as plasmon resonance splitting into a doublet,
induced by a DC current, afford new ways to generate and exploit
unidirectionally propagating plasmon modes.
