In-plane exciton polaritons versus plasmon polaritons: Nonlocal corrections, confinement, and loss

Polaritons are quasi-particles describing the coupling between a photon and a\nmaterial excitation, which can carry large momentum and confine electromagnetic\nfields to small dimensions, enabling strong light-matter interactions. In the\nvisible (VIS) to near-infrared (NIR) spectral ranges, the intraband response of\nmetals gives rise to surface-plasmon-polaritons (SPPs), which have practically\ngoverned polaritonic response and its utilization in nanophotonics. Recently,\nthe concept of interband-based VIS/NIR in-plane exciton polaritons has been\nintroduced in two-dimensional materials, such as\ntransition-metal-dichalcogenides (TMDs), thus providing an excitonic\nalternative to plasmonic systems. Here, we compare the properties of such\nin-plane exciton polaritons supported by monolayer TMDs to the equivalent\nconfiguration of SPPs supported by thin metallic films, known as the\nshort-range-SPPs (SRSPPs). Taking into account both excitonic and plasmonic\nnonlocal corrections, which play a major role in large momentum modes, we find\nthat in-plane exciton polaritons provide confinement factors that are an order\nof magnitude larger than those of SRSPPs, and with six times lower propagation\nlosses. In addition, we show that unlike SPPs, in-plane exciton polaritons are\ncoupled to the TMD's valley degree of freedom, leading to directional\npropagation that depends on the exciton's valley. These properties make\nin-plane exciton polaritons promising candidates for VIS/NIR nanophotonics and\nstrong light-matter interaction.\n