Ultrafast dynamics of bright and dark excitons in monolayer WSe2 and heterobilayer WSe2/MoS2

Jan Philipp Bange; Paul Werner; David Schmitt; Wiebke Bennecke; Giuseppe Meneghini; AbdulAziz AlMutairi; Marco Merboldt; Kenji Watanabe; Takashi Taniguchi; Sabine Steil; Daniel Steil; R Thomas Weitz; Stephan Hofmann; G S Matthijs Jansen; Samuel Brem; Ermin Malic; Marcel Reutzel; Stefan Mathias

doi:10.1088/2053-1583/ace067

Journal article Ultrafast dynamics of bright and dark excitons in monolayer WSe₂ and heterobilayer WSe₂/MoS₂

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Jan Philipp Bange, Paul Werner, David Schmitt, Wiebke Bennecke, Giuseppe Meneghini, AbdulAziz AlMutairi, Marco Merboldt, Kenji Watanabe, Takashi Taniguchi, Sabine Steil, Daniel Steil, R Thomas Weitz, Stephan Hofmann, G S Matthijs Jansen, Samuel Brem, Ermin Malic, Marcel Reutzel, Stefan Mathias. Ultrafast dynamics of bright and dark excitons in monolayer WSe₂ and heterobilayer WSe₂/MoS₂. 2D Materials. 2023, 10 (3), 035039. https://doi.org/10.1088/2053-1583/ace067

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The energy landscape of optical excitations in mono- and few-layer transition metal dichalcogenides (TMDs) is dominated by optically bright and dark excitons. These excitons can be fully localized within a single TMD layer, or the electron- and the hole-component of the exciton can be charge-separated over multiple TMD layers. Such intra- or interlayer excitons have been characterized in detail using all-optical spectroscopies, and, more recently, photoemission spectroscopy. In addition, there are so-called hybrid excitons whose electron- and/or hole-component are delocalized over two or more TMD layers; so far, a detailed characterization of the energy landscape and dynamics of optically dark hybrid excitons is missing. In this work, using femtosecond momentum microscopy, we quantitatively compare the formation dynamics of momentum-forbidden intralayer excitons in monolayer WSe2 with the formation dynamics of momentum-forbidden hybrid excitons in heterobilayer WSe2/MoS2, and draw three key conclusions: First, we find that the energy of hybrid excitons is reduced when compared to excitons with pure intralayer character. Second, in direct comparison with microscopic modelling, we show that the momentum-forbidden intralayer and hybrid excitons are formed via exciton-phonon scattering from optically excited bright excitons. And third, we show that the efficiency for phonon absorption and emission processes in the mono- and the heterobilayer is strongly dependent on the energy alignment of the dark intralayer and hybrid excitons with respect to the optically excited bright exciton.

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