  Plasmon coupling and hybridization in complex nanostructures constitutes a
fertile playground for controlling light at the nanoscale. Here, we present a
semi-analytical model to describe the emergence of hybrid plasmon modes guided
along 2D nano-slits. In particular, we find two new coupled plasmonic
resonances arising from symmetric and antisymmetric hybridizations of the edge
plasmons of the constituent half-sheets. These give rise to an antibonding and
a bonding mode, lying above and below the energy of the bare edge plasmon. Our
treatment is notably generic, being able to account for slits of arbitrary
width, and remains valid irrespective of the 2D conductive material (e.g.,
doped graphene, 2D transition metal dichalcogenides, or phosphorene). We derive
the dispersion relation of the hybrid modes of a 2D nano-slit along with the
corresponding induced potential and electric field distributions. We also
discuss the plasmonic spectrum of a 2D slit together with the one from its
complementarity structure, that is, a ribbon. Finally, the case of a nano-slit
made from an anisotropic 2D material is considered. Focusing on black
phosphorus (which is highly anisotropic), we investigate the features of its
plasmonic spectrum along the two main crystal axes. Our results offer insights
into the interaction of plasmons in complex 2D nanostructures, thereby
expanding the current toolkit of plasmonic resonances in 2D materials, and
paving the way for the emergence of future compact devices based on atomically
thin plasmonics.
