Main Content

2025

  • D. Schmitt, J. P. Bange, W. Bennecke, G. Meneghini, A. AlMutairi, M. Merboldt, J. Pöhls, K. Watanabe, T. Taniguchi, S. Steil, D. Steil, R. T. Weitz, S. Hofmann, S. Brem, G. S. M. Jansen, E. Malic, S. Mathias, and M. Reutzel, Ultrafast nano-imaging of dark excitons, Nature Photonics 19, 187 (2025).

    Understanding the impact of spatial heterogeneity on the behaviour of two-dimensional materials represents one of the grand challenges in applying these materials in optoelectronics and quantum information science. For transition metal dichalcogenide heterostructures in particular, direct access to heterogeneities in the dark-exciton landscape with nanometre spatial and ultrafast time resolution is highly desired but remains largely elusive. Here we report how ultrafast dark-field momentum microscopy can spatio-temporally resolve dark-exciton formation dynamics in a twisted WSe2/MoS2 heterostructure with a time resolution of 55 fs and a spatial resolution of 480 nm. This enables us to directly map spatial heterogeneity in the electronic and excitonic structure, and to correlate this with the dark-exciton formation and relaxation dynamics. The advantage of the simultaneous ultrafast nanoscale dark-field momentum microscopy and spectroscopy reported here is that it enables spatio-temporal imaging of the photoemission spectral function that carries energy- and momentum-resolved information on the single-particle band structure, many-body interactions and correlation phenomena.

    DOI: 10.1038/s41566-024-01568-y

  • F. Falorsi, S. Zhao, K. Liu, C. Eckel, J.F. Pöhls, W. Bennecke, M. Reutzel, S. Mathias, K. Watanabe, T. Tanigucchi, Z. Wang, M. Polozij, X. Feng, T. Heine, R.T. Weitz, Interlayer charge transfer in graphene 2D polyimide heterostructures, 2D Materials 12, 2 (2025).

    The vertical integration of multiple two-dimensional (2D) materials in heterostructures, held together by van der Waals forces, has opened unprecedented possibilities for modifying the (opto-)electronic properties of nanodevices. This not only allows for the exploration of new physical phenomena but also greatly broadens the application horizon of existing monolayer devices. Graphene, with its remarkable opto-electronic properties, is an ideal candidate for such applications. The other potential candidates are 2D polymers, crystalline polymeric materials with customizable structures and electronic properties, as they can be synthesized in all mathematically possible Bravais lattices. In this study, we investigated the optoelectronic properties of a heterostructure created by pristine graphene and a rectangular 2D polyimide (2DPI) film. This imprints a new superlattice on graphene in conjunction with a direct influence on its electronic properties. Theoretical and experimental analyses reveal that interlayer charge exchange between the 2D polymer and graphene induces hole doping in the graphene layer. We have also observed that the properties of the heterostructure are dependent on the substrate used in experiments, likely due to the porous character of the 2DPI allowing direct interaction of graphene with the support. Furthermore, we demonstrate a direct correlation between the thickness of the 2DPI layer and the extent of hole doping in graphene. These findings highlight the unique ability to tailor functionalities in 2D polymers-based heterostructures, opening avenues for the development of optoelectronic devices with precisely engineered properties and stimulating further exploration of the diverse phenomena accessible through tailored designs of the 2D polymers.

    DOI: 10.1088/2053-1583/adac6e

  • M. Aeschlimann, J.P. Bange, M. Bauer, U. Bovensiepen, H.-J. Elmers, T. Fauster, L. Gierster, U. Hoefer, R. Huber, A. Li, X. Li, S. Mathias, K. Morgenstern, H. Petek, M. Reutzel, K. Rossnagel, G. Schönhense, M. Scholz, B. Stadtmüller, J. Stähler, S. Tan, B. Wang, Z. Wang, M. Weinelt, Time-resolved photoelectron spectroscopy at surfaces, Surface Science 753, 122631 (2025).

    Light is a preeminent spectroscopic tool for investigating the electronic structure of surfaces. Time-resolved photoelectron spectroscopy has mainly been developed in the last 30 years. It is therefore not surprising that the topic was hardly mentioned in the issue on “The first thirty years” of surface science. In the second thirty years, however, we have seen tremendous progress in the development of time-resolved photoelectron spectroscopy on surfaces. Femtosecond light pulses and advanced photoelectron detection schemes are increasingly being used to study the electronic structure and dynamics of occupied and unoccupied electronic states and dynamic processes such as the energy and momentum relaxation of electrons, charge transfer at interfaces and collective processes such as plasmonic excitation and optical field screening. Using spin- and time-resolved photoelectron spectroscopy, we were able to study ultrafast spin dynamics, electron–magnon scattering and spin structures in magnetic and topological materials. Light also provides photon energy as well as electric and magnetic fields that can influence molecular surface processes to steer surface photochemistry and hot-electron-driven catalysis. In addition, we can consider light as a chemical reagent that can alter the properties of matter by creating non-equilibrium states and ultrafast phase transitions in correlated materials through the coupling of electrons, phonons and spins. Electric fields have also been used to temporarily change the electronic structure. This opened up new methods and areas such as high harmonic generation, light wave electronics and attosecond physics. This overview certainly cannot cover all these interesting topics. But also as a testimony to the cohesion and constructive exchange in our ultrafast community, a number of colleagues have come together to share their expertise and views on the very vital field of dynamics at surfaces. Following the introduction, the interested reader will find a list of contributions and a brief summary in Section 1.3.

    DOI: 10.1016/j.susc.2024.122631

  • M. Merboldt, M. Schüler, D. Schmitt, J. P. Bange, W. Bennecke, K. Gadge, K. Pierz, H. W. Schumacher, D. Momeni, D. Steil, S. R. Manmana, M. A. Sentef, M. Reutzel, S. Mathias, Observation of Floquet states in graphene. Nat. Phys. (2025).

    Floquet engineering—the coherent dressing of matter via time-periodic perturbations—is a mechanism to realize and control emergent phases in materials out of equilibrium. However, its applicability to metallic quantum materials and semimetals such as graphene is an open question. The report of light-induced anomalous Hall effect in graphene remains debated, and a time-resolved photoemission experiment has suggested that Floquet effects might not be realizable in graphene and other semimetals with relatively short decoherence times. Here we provide direct spectroscopic evidence of Floquet effects in graphene through electronic structure measurements. We observe light–matter-dressed Dirac bands by measuring the contribution of Floquet sidebands, Volkov sidebands and their quantum path interference to graphene’s photoemission spectrum. Our results demonstrate that Floquet engineering in graphene is possible, even though ultrafast decoherence processes occur on the timescale of a few tens of femtoseconds. Our approach offers a way to experimentally realize Floquet engineering strategies in metallic and semimetallic systems and for the coherent stabilization of light-induced states with potentially non-trivial topological properties.

    DOI: 10.1038/s41567-025-02889-7