Transition metal dichalcogenide (TMD) monolayers are direct bandgapsemiconductors that feature tightly bound excitons, strong spin-orbit coupling,and spin-valley degrees of freedom. Depending on the spin configuration of theelectron-hole pairs, intra-valley excitons of TMD monolayers can be eitheroptically bright or dark. Dark excitons involve nominally spin-forbiddenoptical transitions with zero in-plane transition dipole moment, making theirdetection with conventional far-field optical techniques challenging. Here, weintroduce a new method for probing the optical properties of two-dimensional(2D) materials via near-field coupling to surface plasmon polaritons (SPPs),which selectively enhances optical transitions with dipole moments normal tothe 2D plane. We utilize this method to directly detect dark excitons inmonolayer TMDs. When a WSe2 monolayer is placed on top of a single-crystalsilver film, its emission into near-field-coupled SPPs displays new spectralfeatures whose energies and dipole orientations are consistent with darkneutral and charged excitons. The SPP-based near-field spectroscopysignificantly enhances experimental capabilities for probing and manipulatingexciton dynamics of atomically thin materials.
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