Photoluminescence upconversion in monolayer WSe2 activated by plasmonic cavities through resonant excitation of dark excitons.
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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. Here, we show how plasmonic nano-cavities activate anti-Stokes PL in WSe2 monolayers through resonant excitation of a dark exciton at room temperature. 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. 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. This is further corroborated by experiments in which the laser excitation wavelength is tuned across the dark exciton. We show that a precise nanoparticle geometry is key for a consistent enhancement, with decahedral nanoparticle shapes providing an efficient PL upconversion. Finally, we demonstrate a selective and reversible switching of the upconverted PL via electrochemical gating. Our work introduces the dark exciton as an excitation channel for anti-Stokes PL in WSe2 and paves the way for large-area substrates providing nanoscale optical cooling, anti-Stokes lasing, and radiative engineering of excitons.
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Acknowledgements: The authors acknowledge funding from the EPSRC (EP/L027151/1 and EP/R013012/1), and the EU (883703 PICOFORCE, 861950 POSEIDON). B.d.N. acknowledges support from the Winton Program for the Physics of Sustainability and from Royal Society University Research Fellowship URF∖R1∖211162. L.M.L.-M. acknowledges funding from the Spanish Ministerio de Ciencia e Innovacion, MCIN/AEI/10.13039/501100011033 (Grant PID2020-117779RB-100). N.S.M. acknowledges support from the German National Academy of Sciences Leopoldina. R.A. acknowledges support from the Rutherford Foundation of the Royal Society Te Apārangi of New Zealand, the Winton Program for the Physics of Sustainability, and Trinity College Cambridge. L.A.J. acknowledges support from the Cambridge Commonwealth, European & International Trust, and EPSRC award 2275079. J.B.D. acknowledges support from the Blavatnik fellowship. F.L. acknowledges support from the Terman Fellowship and startup funds from the Department of Chemistry at Stanford University. We thank Angela Demetriadou and Demelza Wright for the helpful discussions.
Funder: University of Cambridge | Trinity College, University of Cambridge; doi: https://doi.org/10.13039/501100000727
Funder: Rutherford Foundation of the Royal Society Te Apārangi of New Zealand; Winton Programme for the Physics of Sustainability
Funder: Blavatnik fellowship
Funder: Terman Fellowship; Department of Chemistry at Stanford University;
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2041-1723
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European Commission Horizon 2020 (H2020) Research Infrastructures (RI) (861950)
European Commission Horizon 2020 (H2020) Future and Emerging Technologies (FET) (829067)
European Commission Horizon 2020 (H2020) ERC (883703)
Engineering and Physical Sciences Research Council (2275079)