  Nonreciprocity and one-way propagation of optical signals is crucial for
modern nanophotonic technology, and is typically achieved using magneto-optical
effects requiring large magnetic biases. Here we suggest a fundamentally novel
approach to achieve unidirectional propagation of surface plasmon-polaritons
(SPPs) at metal-dielectric interfaces. We employ a direct electric current in
metals, which produces a Doppler frequency shift of SPPs due to the uniform
drift of electrons. This tilts the SPP dispersion, enabling one-way
propagation, as well as zero and negative group velocities. The results are
demonstrated for planar interfaces and cylindrical nanowire waveguides.

Optics