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Résumé |
Because of weak dielectric screening, two-dimensional semiconductors exhibit a strong Coulomb interaction. As a consequence, they host robust excitons at room temperature leading to a strong light matter interaction. This makes them a promising platform for next generation optoelectronic devices and fundamental studies of many body phenomena. Understanding and manipulating energy flows in these materials is key to make full use of their promising properties. Recently, ultrafast transient absorption microscopy has emerged as a powerful tool to achieve this [1,2]. We recently demonstrated imaging spatiotemporal dynamics of excitons with ~15fs time resolution and few nanometres spatial precision [3]. In this talk, I will present our work on imaging spatiotemporal dynamics of excited states in 2d transition metal dichalcogenides, and I will present two examples where we have tuned the Coulomb interaction to control the transport of excitations. First, because they are very sensitive to their environment, it possible to engineer a potential landscape for excitons in the monolayer by introducing spatial variations in the surroundings. Here, through control of the dielectric environment we create a lateral junction and observe the excitons funnel through it. Photoexcitation can also be used to modify the energetics of the monolayer. Above a critical excitation threshold, we observe the complete ionization of the excitons into a dense electron-hole plasma which rapidly expands in the first 100 fs. We propose that this rapid expansion is driven by the Fermi pressure. These examples illustrate the opportunities offered by both transient absorption microscopy and 2d semiconductors.
[1] Delor, M et al Nat. Mater. 19, 56–62 (2020).
[2] Sung, J. et al. Nat. Phys. 16, 171–176 (2020).
[3] Ashoka, A .et al. Nat Commun 13, 5963 (2022). |
