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 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 WSe 2, 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 A′ 1 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.
| Original language | English |
|---|---|
| Pages (from-to) | 323-327 |
| Number of pages | 5 |
| Journal | Nature Physics |
| Volume | 12 |
| Issue number | 4 |
| DOIs | |
| State | Published - Apr 1 2016 |
Funding
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. 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.