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Resonant internal quantum transitions and femtosecond radiative decay of excitons in monolayer WSe2


Atomically thin two-dimensional crystals have revolutionized materials science1,2,3. In particular, monolayer transition metal dichalcogenides promise novel optoelectronic applications, owing to their direct energy gaps in the optical range4,5,6,7,8,9. Their electronic and optical properties are dominated by Coulomb-bound electron–hole pairs called excitons10,11,12,13,14,15,16,17,18, whose unusual internal structure13, symmetry15,16,17, many-body effects18 and dynamics have been vividly discussed. Here we report the first direct experimental access to all 1s A excitons, regardless of momentum—inside and outside the radiative cone—in single-layer WSe2. Phase-locked mid-infrared pulses reveal the internal orbital 1s–2p resonance, which is highly sensitive to the shape of the excitonic envelope functions and provides accurate transition energies, oscillator strengths, densities and linewidths. Remarkably, the observed decay dynamics indicates an ultrafast radiative annihilation of small-momentum excitons within 150 fs, whereas Auger recombination prevails for optically dark states. The results provide a comprehensive view of excitons and introduce a new degree of freedom for quantum control, optoelectronics and valleytronics of dichalcogenide monolayers19,20,21,22,23,24.

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Figure 1: Intra- and interband spectroscopy of single-layer WSe2.
Figure 2: Time-resolved response of the intra-excitonic 1s–2p transition.
Figure 3: Density-dependent renormalization of the internal excitonic response.
Figure 4: Ultrafast dynamics of exciton density in single-layer WSe2.

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We thank M. Eisele, T. Cocker, M. Huber, J. Fabian, A. Chernikov, S. Michaelis de Vasconcellos, R. Schmidt and C. Frankerl for helpful discussions and M. Furthmeier for technical assistance. Support by the European Research Council through ERC grant 305003 (QUANTUMsubCYCLE) and by Deutsche Forschungsgemeinschaft (DFG) through Research Training Group GK1570 and KO3612/1-1 is acknowledged.

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C.P., R.B., C.S., T.K. and R.H. planned the project; P.N., G.P., R.B., C.S. and T.K. provided, processed and characterized the samples; C.P., P.S., U.L. and M.P. performed the femtosecond measurements; C.P., P.S., U.L., M.P. and R.H. analysed the data; C.P., P.S. and R.H. elaborated the theoretical model; C.P., P.S. and R.H. wrote the paper with contributions from all authors.

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Correspondence to R. Huber.

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The authors declare no competing financial interests.

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Poellmann, C., Steinleitner, P., Leierseder, U. et al. Resonant internal quantum transitions and femtosecond radiative decay of excitons in monolayer WSe2. Nature Mater 14, 889–893 (2015).

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