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Communication Dans Un Congrès Année : 2020

Coherent dynamics of resonantly excited excitons in monolayers of transition metal dichalcogenides

Résumé

We investigate dynamics of resonantly excited excitons in single-layers of MoSe_2 and WS_2 down to 4.5 K. To this end, we measure the delay dependence of the heterodyne four-wave mixing (FWM) amplitude induced by three, short laser pulses. This signal depends not only on the population of optically active excitons, which affects the absorption of the probe, but also on the population of optically inactive states, by interaction-induced energy shift, influencing the refractive index experienced by the probe. As such, it offers insight into density dynamics of excitons which do not directly couple to photons. Reproducing the coherent signal detected in amplitude and phase, the FWM delay dependence is modeled by a coherent superposition of several exponential decay components , with characteristic time constants from 0.1 picosecond up to 1 nanosecond. With increasing excitation intensity and/or temperature, we observe strong interference effects in the FWM field amplitude, resulting in progressively more complex and nonintuitive signal dynamics. We attribute this behaviour to increasingly populated exciton dark states, which change the FWM field phase by the relative effect on absorption and refractive index. We observe that exciton recombination occurs on a significantly longer timescale in WS_2 with respect to MoSe_2 , which is attributed to the dark character of exciton ground state in the former and the bright in the latter.
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Dates et versions

hal-03012880 , version 1 (18-11-2020)

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Tomasz Jakubczyk, Miroslav Bartos, Lorenzo Scarpelli, Karol Nogajewski, Wolfgang Langbein, et al.. Coherent dynamics of resonantly excited excitons in monolayers of transition metal dichalcogenides. Ultrafast Phenomena and Nanophotonics XXIV, Feb 2020, San Francisco, United States. pp.47, ⟨10.1117/12.2549229⟩. ⟨hal-03012880⟩
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