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In 2D semiconducting quantum materials, organic semiconductors and their heterostructures, the energy of absorbed light is stored in Coulomb-bound electron-hole pairs, which are called excitons. For future technological applications of these classes of materials, for instance in optoelectronics and for energy harvesting, it is crucial to study the initial exciton formation and also the subsequent relaxation and dissipation processes at the fundamental level and on the relevant length and time scales. In our research, we have built a new photoemission-based experiment [1] that is capable of studying excitons at the space-time limit corresponding to nanometers and femtoseconds. In a series of experiments, we identified characteristic signatures in the exciton formation process and the pathways of subsequent energy conversion and thermalization. In addition, the new experiment gives us access to the so-called “dark exciton energy landscape”. In the first part of my talk, I will present the ultrafast formation dynamics of dark interlayer excitons in twisted WSe2/ MoS2 heterostructures [2-6]. In particular, I will report on the identification of a key signature of the moiré superlattice that is imprinted on the momentum-resolved interlayer exciton photoemission signal [2] and how such photoemission data can be used to reconstruct the real- space wavefunction of the non-equilibrium excited state [2,7]. In the second part, I will discuss our recent efforts to monitor the interlayer exciton formation dynamics with spatiotemporal resolution using femtosecond photoelectron dark-field microscopy [6]. By probing the WSe2/ MoS2 heterostructure with 50 fs time- and 500 nm spatial resolution, we identify an astonishing inhomogeneity in the interlayer exciton formation dynamics, a quantity that so far has not been addressed by any other experiment. These first proof-of-principle experiments of femtosecond photoelectron dark-field microscopy can be considered as a door opener for future research addressing non-equilibrium many-body interactions on the ultrashort time- and length-scales. References [1] Keunecke et al., Rev. Sci. Ins. 91, 063905 (2020), Time-resolved momentum microscopy with a 1 MHz high-harmonic extreme ultraviolet beamline. [2] Schmitt et al., Nature 608, 499 (2022), Formation of moiré interlayer excitons in space and time. [3] Bange et al., Science Advances 10, eadi1323 (2024), Probing correlations in the exciton landscape of a moiré heterostructure. [4] Bange et al., 2D Materials 10, 035039 (2023), Ultrafast dynamics of bright and dark excitons in monolayer WSe2 and heterobilayer WSe2/MoS2. [5] Düvel et al., Nano Letters 22, 4897-4904 (2022), Far-from-equilibrium electron-phonon interactions in optically-excited graphene. [6] Schmitt et al., arXiv:2305.18908 (2023), Ultrafast nano-imaging of dark excitons. [7] Bennecke et al., Nat. Commun. 15, 1804 (2024), Multiorbital exciton formation in an organic semiconductor.

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Marcel Reutzel, Georg-August-Universität Göttingen
Seminar Room of the Institute of Physics II
Contact: Erwann/Matteo