Publication Date:
2016-04-02
Description:
Nature Physics 12, 323 (2016). doi:10.1038/nphys3604 Authors: Aaron M. Jones, Hongyi Yu, John R. Schaibley, Jiaqiang Yan, David G. Mandrus, Takashi Taniguchi, Kenji Watanabe, Hanan Dery, Wang Yao & Xiaodong Xu 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 solids, where upconversion relies on anti-Stokes processes enabled either by rare-earth impurities or exciton–phonon coupling. 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 semiconductors. Since the charged exciton binding energy closely matches the 31 meV A1′ optical phonon, 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.
Print ISSN:
1745-2473
Electronic ISSN:
1745-2481
Topics:
Physics
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