  2D materials support unique excitations of quasi-particles that consist of a
material excitation and photons called polaritons. Especially interesting are
in-plane propagating polaritons which can be confined to a single monolayer and
carry large momentum. In this work, we report the existence of a new type of
in-plane propagating polariton, supported on monolayer
transition-metal-dicalcogonide (TMD) in the visible spectrum, which has not yet
been observed. This 2D in-plane exciton-polariton (2DEP) is described by the
coupling of an electromagnetic light field with the collective oscillations of
the excitons supported by monolayer TMDs. We expose the specific experimental
conditions required for the excitation of the 2DEP and show that these can be
created if the TMD is encapsulated with hexagonal-boron-nitride (hBN) and
cooled to cryogenic temperatures. In addition, we compare the properties of the
2DEPs with those of surface-plasmons-polaritons (SPPs) at the same spectral
range, and find that the 2DEP exhibit over two orders-of-magnitude larger
wavelength confinement. Finally, we propose two configurations for the possible
experimental observation of 2DEPs.
