![]() ![]() ![]() b Nanoparticle-on-mirror (NPoM) plasmonic cavity with WSe 2 defining a gap between a bottom facet of the nanoparticle and the Au mirror. The optical near-fields of plasmonic antennas enable direct coupling to the out-of-plane transition dipole of the dark exciton and enhance the normal Stokes PL emission, which was demonstrated for metallic tips as well as nanoparticle-on-mirror cavities 25, 27, 28.Ī Band-structure of spin-allowed bright ( X B) and spin-forbidden dark ( X D) A excitons in WSe 2. ![]() Promising routes to activate the dark exciton are through strong magnetic fields or through metallic nanostructures that sustain surface plasmons 24, 25, 26. Excitation with far-field radiation is however inefficient because the dark intravalley exciton has an out-of-plane transition dipole 22, 23. The energetic ordering makes dark excitons on the other hand a potential excitation channel for anti-Stokes PL. In WSe 2 and WS 2 they have lower energies than the bright excitons, which leads to quenching of the bright exciton emission at low temperature due to fast non-radiative relaxation to the dark exciton 20, 21. The dark excitons are either momentum-forbidden (intervalley) or spin-forbidden (intravalley) for excitation at normal incidence 15, 16. ![]() Another possible excitation channel arises from the spin-orbit splitting of the conduction band in TMDs, which leads to the formation of bright and dark excitons with an energy splitting of several tens of meV (Fig. Initial experiments on anti-Stokes PL in monolayer WSe 2 were explained by doubly resonant processes involving charged and neutral excitons 2, 18, as well as A- and B-excitons 5. Anti-Stokes PL in transition metal dichalcogenides (TMDs) is thus mostly explained by phonon-assisted processes and the interplay of different excitons 2, 18, 19.Įfficient anti-Stokes PL requires a condition in which both the excitation laser and the emission are resonant with a material excitation. The reduced dielectric screening and enhanced Coulomb attraction make optical transition dipoles and exciton-phonon coupling an order of magnitude larger than in conventional bulk semiconductors or quantum wells 15, 16, 17. Excitons in two-dimensional semiconductors are a promising platform for anti-Stokes PL 2, 5, 12, 13, 14. This leads to industrially-relevant applications in optical refrigeration 6, bioimaging 7, lasing 8, quantum information 9, and the detection of infrared light 10, 11. The upconversion occurs through a variety of mechanisms including the absorption of phonons 1, 2, Auger processes 3, or multi-photon absorption 4, 5. Our work introduces the dark exciton as an excitation channel for anti-Stokes PL in WSe 2 and paves the way for large-area substrates providing nanoscale optical cooling, anti-Stokes lasing, and radiative engineering of excitons.Īnti-Stokes photoluminescence (PL) is a process in which light is emitted at a higher energy than the excitation laser by extracting energy from the material. Finally, we demonstrate a selective and reversible switching of the upconverted PL via electrochemical gating. We show that a precise nanoparticle geometry is key for a consistent enhancement, with decahedral nanoparticle shapes providing an efficient PL upconversion. This is further corroborated by experiments in which the laser excitation wavelength is tuned across the dark exciton. Through statistical measurements on hundreds of plasmonic cavities, we show that coupling to the dark exciton leads to a near hundred-fold enhancement of the upconverted PL intensity. The optical near-fields of the plasmonic cavities excite the out-of-plane transition dipole of the dark exciton, leading to light emission from the bright exciton at higher energy. Here, we show how plasmonic nano-cavities activate anti-Stokes PL in WSe 2 monolayers through resonant excitation of a dark exciton at room temperature. Anti-Stokes photoluminescence (PL) is light emission at a higher photon energy than the excitation, with applications in optical cooling, bioimaging, lasing, and quantum optics. ![]()
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