Exciton polaritons (EPs) are partial-light partial-matter quasiparticles in semiconductors demonstrating striking quantum phenomena such as Bose–Einstein condensation and single-photon nonlinearity. In these phenomena, the governing process is the EP relaxation into the ground states upon excitation, where various mechanisms are extensively investigated with thermodynamic limits. However, the relaxation process becomes drastically different and could significantly advance the understanding of EP dynamics for these quantum phenomena, when excited states of EPs are involved. Here, for the first time, we observe nonlinear optical responses at the EP excited states in a monolayer tungsten disulfide (WS2) microcavity, including dark excited states and dynamically metastable upper polariton bands. The nonlinear optics leads to unique emissions of ground states with prominent valley degree of freedom (DOF) via an anomalous relaxation process, which is applicable to a wide range of semiconductors from monolayer transition metal dichalcogenides (TMDs) to emerging halide perovskites. This work promises possible approaches to challenging experiments such as valley polariton condensation. Moreover, it also constructs a valley-dependent solid-state three-level system for terahertz photonics and stimulated Raman adiabatic passage.
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledged KAUST grant number(s): OSR-2016-CRG5-2996
Acknowledgements: The authors acknowledge the support from the National Science Foundation (NSF) under Grant 1753380 for sample measurements, the support from the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division under Contract No. DEAC02-05-CH11231 within the van der Waals Heterostructures program (KCWF16) for sample preparation, and the support
from King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research award OSR-2016-CRG5-2996 for theory modeling. X.L. also acknowledges the support from Wuhan University Faculty Startup Fund.
This publication acknowledges KAUST support, but has no KAUST affiliated authors.