  Polaritons are quasi-particles describing the coupling between a photon and a
material excitation, which can carry large momentum and confine electromagnetic
fields to small dimensions, enabling strong light-matter interactions. In the
visible (VIS) to near-infrared (NIR) spectral ranges, the intraband response of
metals gives rise to surface-plasmon-polaritons (SPPs), which have practically
governed polaritonic response and its utilization in nanophotonics. Recently,
the concept of interband-based VIS/NIR in-plane exciton polaritons has been
introduced in two-dimensional materials, such as
transition-metal-dichalcogenides (TMDs), thus providing an excitonic
alternative to plasmonic systems. Here, we compare the properties of such
in-plane exciton polaritons supported by monolayer TMDs to the equivalent
configuration of SPPs supported by thin metallic films, known as the
short-range-SPPs (SRSPPs). Taking into account both excitonic and plasmonic
nonlocal corrections, which play a major role in large momentum modes, we find
that in-plane exciton polaritons provide confinement factors that are an order
of magnitude larger than those of SRSPPs, and with six times lower propagation
losses. In addition, we show that unlike SPPs, in-plane exciton polaritons are
coupled to the TMD's valley degree of freedom, leading to directional
propagation that depends on the exciton's valley. These properties make
in-plane exciton polaritons promising candidates for VIS/NIR nanophotonics and
strong light-matter interaction.
