Light-matter interactions in conventional nanophotonic structures typically
lack directionality. Furthermore, surface waves supported by conventional
material substrates do not usually have a preferential direction of
propagation, and their wavefront tends to spread as it propagates along the
surface, unless the surface or the excitation are properly engineered and
structured. In this article, we theoretically demonstrate the possibility of
realizing \emph{unidirectional and diffractionless surface-plasmon-polariton
modes} on a nonreciprocal platform, namely, a gyrotropic magnetized plasma.
Based on a rigorous Green function approach, we provide a comprehensive and
systematic analysis of all the available physical mechanisms that may bestow
the system with directionality, both in the sense of one-way excitation of
surface waves, and in the sense of directive diffractionless propagation along
the surface. The considered mechanisms include (i) the effect of strong and
weak forms of nonreciprocity, (ii) the elliptic-like or hyperbolic-like
topology of the modal dispersion surfaces, and (iii) the source polarization
state, with the associated possibility of chiral surface-wave excitation
governed by angular-momentum matching. We find that three-dimensional
gyrotropic plasmonic platforms support a previously-unnoticed wave-propagation
regime that exhibit several of these physical mechanisms simultaneously,
allowing us to theoretically demonstrate, for the first time, unidirectional
surface-plasmon-polariton modes that propagate as a single ultra-narrow
diffractionless beam. We also assess the impact of dissipation and nonlocal
effects. Our theoretical findings may enable a new generation of plasmonic
structures and devices with highly directional response
