Main Content

Submitted – in-review process

  • Y. Yen, M. Reutzel, A. Li, Z. Wang, H. Petek, M. Schüler, arXiv:2502.18269 (2025).

    Electromagnetic fields not only induce electronic transitions but also fundamentally modify the quantum states of matter through strong light-matter interactions. As one established route, Floquet engineering provides a powerful framework to dress electronic states with time-periodic fields, giving rise to quasi-stationary Floquet states. With increasing field strength, non-perturbative responses of the dressed states emerge, yet their nonlinear dynamics remain challenging to interpret. In this work we explore the emergence of non-adiabatic Landau-Zener transitions among Floquet states in Cu(111) under intense optical fields. At increasing field strength, we observe a transition from perturbative dressing to a regime where Floquet states undergo non-adiabatic tunneling, revealing a breakdown of adiabatic Floquet evolution. These insights are obtained through interferometrically time-resolved multi-photon photoemission spectroscopy, which serves as a sensitive probe of transient Floquet state dynamics. Numerical simulations and the theory of instantaneous Floquet states allow us to directly examine real-time excitation pathways in this non-perturbative photoemission regime. Our results establish a direct connection the onset of light-dressing of matter, non-perturbative ultrafast lightwave electronics, and high-optical-harmonic generation in the solids.

    https://arxiv.org/abs/2502.18269, https://arxiv.org/abs/2503.04431

  • W. Bennecke, T. L. Dinh, J.P. Bange, D. Schmitt, M. Merboldt, L. Weinhagen, B. van W., F. Frassetto, L. Poletto, M. Reutzel, D. Steil, D.R. Luke, S. Mathias, GS Jansen, arXiv:2502.18269 (2025).

    Two-dimensional transition metal dichalcogenides (TMDs) and organic semiconductors (OSCs) have emerged as promising material platforms for next-generation optoelectronic devices. The combination of both is predicted to yield emergent properties while retaining the advantages of their individual components. In OSCs the optoelectronic response is typically dominated by localized Frenkel-type excitons, whereas TMDs host delocalized Wannier-type excitons. However, much less is known about the spatial and electronic characteristics of excitons at hybrid TMD/OSC interfaces, which ultimately determine the possible energy and charge transfer mechanisms across the 2D-organic interface. Here, we use ultrafast momentum microscopy and many-body perturbation theory to elucidate a hybrid exciton at an TMD/OSC interface that forms via the ultrafast resonant Förster energy transfer process. We show that this hybrid exciton has both Frenkel- and Wannier-type contributions: Concomitant intra- and interlayer electron-hole transitions within the OSC layer and across the TMD/OSC interface, respectively, give rise to an exciton wavefunction with mixed Frenkel-Wannier character. By combining theory and experiment, our work provides previously inaccessible insights into the nature of hybrid excitons at TMD/OSC interfaces. It thus paves the way to a fundamental understanding of charge and energy transfer processes across 2D-organic heterostructures.

    https://arxiv.org/abs/2502.18269, https://arxiv.org/abs/2411.14993

  • W. Bennecke, I. Oliva, J. P. Bange, P. Werner, D. Schmitt, M. Merboldt, A. Seiler, K. Watanabe, T. Taniguchi, D. Steil, R.T. Weitz, P. Puschnig, C. Draxl, GS Jansen, M. Reutzel, S. Mathias, Hybrid Frenkel-Wannier excitons facilitate ultrafast energy transfer at a 2D-organic interface, arXiv:2411.14993 (2024).

    Two-dimensional transition metal dichalcogenides (TMDs) and organic semiconductors (OSCs) have emerged as promising material platforms for next-generation optoelectronic devices. The combination of both is predicted to yield emergent properties while retaining the advantages of their individual components. In OSCs the optoelectronic response is typically dominated by localized Frenkel-type excitons, whereas TMDs host delocalized Wannier-type excitons. However, much less is known about the spatial and electronic characteristics of excitons at hybrid TMD/OSC interfaces, which ultimately determine the possible energy and charge transfer mechanisms across the 2D-organic interface. Here, we use ultrafast momentum microscopy and many-body perturbation theory to elucidate a hybrid exciton at an TMD/OSC interface that forms via the ultrafast resonant Förster energy transfer process. We show that this hybrid exciton has both Frenkel- and Wannier-type contributions: Concomitant intra- and interlayer electron-hole transitions within the OSC layer and across the TMD/OSC interface, respectively, give rise to an exciton wavefunction with mixed Frenkel-Wannier character. By combining theory and experiment, our work provides previously inaccessible insights into the nature of hybrid excitons at TMD/OSC interfaces. It thus paves the way to a fundamental understanding of charge and energy transfer processes across 2D-organic heterostructures.

    https://arxiv.org/abs/2411.14993

  • P. Werner, W. Bennecke, J. P. Bange, G. Meneghini, D. Schmitt, M. Merboldt, A. M. Seiler, A. A. AlMutairi, K. Watanabe, T. Taniguchi, G. S. M. Jansen, J. Liu, D. Steil, S. Hofmann, R. T. Weitz, E. Malic, S. Mathias, M. Reutzel, The role of non-equilibrium populations in dark exciton formation

    In two-dimensional transition metal dichalcogenide structures, the optical excitation of a bright exciton may be followed by the formation of a plethora of lower energy dark states. In these formation and relaxation processes between different exciton species, non-equilibrium exciton and phonon populations play a dominant role, but remain so far largely unexplored as most states are inaccessible by regular spectroscopies. Here, on the example of homobilayer 2H-MoS , we realize direct access to the full exciton relaxation cascade from experiment and theory. By measuring the energy- and in-plane momentum-resolved photoemission spectral function, we reveal a distinct fingerprint for dark excitons in a non-equilibrium excitonic occupation distribution. In excellent agreement with microscopic many-particle calculations, we quantify the timescales for the formation of a non-equilibrium dark excitonic occupation and its subsequent thermalization to 85~fs and 150~fs, respectively. Our results provide a previously inaccessible view of the complete exciton relaxation cascade, which is of paramount importance for the future characterization of non-equilibrium excitonic phases and the efficient design of optoelectronic devices based on two-dimensional materials.

    https://arxiv.org/abs/2505.06074v1