  We study a plasmonic metasurface that supports pseudospin dependent edge
states confined at a subwavelength scale, considering full electrodynamic
interactions including retardation and radiative effects. The spatial symmetry
of the lattice of plasmonic nanoparticles gives rise to edge states with
properties reminiscent of the quantum spin Hall effect in topological
insulators. However, unlike the spin-momentum locking characteristic of
topological insulators, these modes are not purely unidirectional and their
propagation properties can be understood by analysing the spin angular momentum
of the electromagnetic field, which is inhomogenous in the plane of the
lattice. The local sign of the spin angular momentum determines the propagation
direction of the mode under a near-field excitation source. We also study the
optical response under far-field excitation and discuss in detail the effects
of radiation and retardation.
