Abstract
Photon upconversion is an elementary light–matter interaction process in which an absorbed photon is re-emitted at higher frequency after extracting energy from the medium. This phenomenon lies at the heart of optical refrigeration in solids1, where upconversion relies on anti-Stokes processes enabled either by rare-earth impurities2 or exciton–phonon coupling3. Here, we demonstrate a luminescence upconversion process from a negatively charged exciton to a neutral exciton resonance in monolayer WSe2, producing spontaneous anti-Stokes emission with an energy gain of 30 meV. Polarization-resolved measurements find this process to be valley selective, unique to monolayer semiconductors4. Since the charged exciton binding energy5 closely matches the 31 meV A1′ optical phonon6,7,8,9, we ascribe the spontaneous excitonic anti-Stokes to doubly resonant Raman scattering, where the incident and outgoing photons are in resonance with the charged and neutral excitons, respectively. In addition, we resolve a charged exciton doublet with a 7 meV splitting, probably induced by exchange interactions, and show that anti-Stokes scattering is efficient only when exciting the doublet peak resonant with the phonon, further confirming the excitonic doubly resonant picture.
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Acknowledgements
We thank R. Merlin and D. Cobden for helpful discussions. This work is mainly supported by the Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division (DE-SC0008145 and SC0012509). H.Y. and W.Y. are supported by the Croucher Foundation (Croucher Innovation Award), and the RGC and UGC of Hong Kong (HKU17305914P, HKU9/CRF/13G, AoE/P-04/08). J.Y. and D.G.M. are supported by US DoE, BES, Materials Sciences and Engineering Division. H.D. is supported by Department of Energy under Contract No. DE-SC0014349 and National Science Foundation under Contract No. DMR-1503601. X.X. acknowledges a Cottrell Scholar Award, support from the State of Washington-funded Clean Energy Institute, and support from the Boeing Distinguished Professorship in Physics. Device fabrication was performed at the University of Washington Microfabrication Facility and NSF-funded Nanotech User Facility.
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X.X. and W.Y. conceived and supervised the experiments; A.M.J. fabricated the devices and performed measurements, assisted by J.R.S.; A.M.J., H.Y., X.X., W.Y. analysed the data with discussion from H.D.; J.Y. and D.G.M. provided and characterized the bulk WSe2; T.T. and K.W. provided BN crystals; A.M.J., X.X., W.Y., H.Y. wrote the paper. All authors discussed the results.
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Jones, A., Yu, H., Schaibley, J. et al. Excitonic luminescence upconversion in a two-dimensional semiconductor. Nature Phys 12, 323–327 (2016). https://doi.org/10.1038/nphys3604
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DOI: https://doi.org/10.1038/nphys3604