  In recent years, enhanced light-matter interactions through a plethora of
dipole-type polaritonic excitations have been observed in two-dimensional (2D)
layered materials. In graphene, electrically tunable and highly confined
plasmon-polaritons were predicted and observed, opening up opportunities for
optoelectronics, bio-sensing and other mid-infrared applications. In hexagonal
boron nitride (hBN), low-loss infrared-active phonon-polaritons exhibit
hyperbolic behavior for some frequencies, allowing for ray-like propagation
exhibiting high quality factors and hyperlensing effects. In transition metal
dichalcogenides (TMDs), reduced screening in the 2D limit leads to optically
prominent excitons with large binding energy, with these polaritonic modes
having been recently observed with scanning near field optical microscopy
(SNOM). Here, we review recent progress in state-of-the-art experiments, survey
the vast library of polaritonic modes in 2D materials, their optical spectral
properties, figures-of-merit and application space. Taken together, the
emerging field of 2D material polaritonics and their hybrids provide enticing
avenues for manipulating light-matter interactions across the visible, infrared
to terahertz spectral ranges, with new optical control beyond what can be
achieved using traditional bulk materials.
