  We show that the surface plasmon polariton (SPP) radiation patterns of
point-dipole emitters in the vicinity of a metal-dielectric interface are
generally asymmetric with respect to the location of the emitter. In particular
rotating dipoles, which emit elliptically polarized light, produce highly
asymmetric SPP radiation fields that include unidirectional emission.
Asymmetric SPP radiation patterns also result when a dipole oscillates tilted
with respect to the plane of the interface and optical losses or gains are
present in the materials. These effects can be used to directionally control
SPP emission and absorption, as well as to study emission and scattering
processes close to metal-dielectric interfaces. Possible implementations of
asymmetrically emitting SPP sources are discussed.
