Hauptinhalt
Veröffentlichungen 2025
219. 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. Matthijs Jansen, E. Malic, S. Mathias and M. Reutzel, "Ultrafast nano-imaging of dark excitons", Nature Photonics 19, 187 (2025)
The role and impact of spatial heterogeneity in two-dimensional quantum materials represents one of the major research quests regarding the future application of these materials in optoelectronics and quantum information science. In the case of transition-metal dichalcogenide heterostructures, in particular, direct access to heterogeneities in the dark-exciton landscape with nanometer spatial and ultrafast time resolution is highly desired, but remains largely elusive. Here, we introduce ultrafast dark field momentum microscopy to spatio-temporally resolve dark exciton formation dynamics in a twisted WSe2/MoS2 heterostructure with 55 femtosecond time- and 500~nm spatial resolution. This allows us to directly map spatial heterogeneity in the electronic and excitonic structure, and to correlate these with the dark exciton formation and relaxation dynamics. The benefits of simultaneous ultrafast nanoscale dark-field momentum microscopy and spectroscopy is groundbreaking for the present study, and opens the door to new types of experiments with unprecedented spectroscopic and spatiotemporal capabilities.
Nature Photonics 19, 187 (2025)218. E. Lopriore, F. Tagarelli, J. M. Fitzgerald, J. Francisco Gonzalez Marin, K. Watanabe, T. Taniguchi, E. Malic and A. Kis, "Enhancing interlayer exciton dynamics by coupling with monolithic cavities via the field-induced Stark effect", Nature Nanotechnology (2025)
Optical microcavities provide a powerful and versatile framework for manipulating the dynamics of photonic emission from optically active materials through light recirculation. Spatially indirect interlayer excitons (IXs) exhibit broad tunability of their emission energy via the quantum-confined Stark effect. However, the electrical tunability of IXs has not been exploited in cavity-coupled systems until now. Here we modulate the detuning between the cavity resonance and the IX emission in a monolithic Fabry–Perot cavity using an applied vertical electric field. We reveal a simultaneous enhancement of both the emission intensity and lifetime of weakly coupled IXs when in resonance with the optical cavity owing to strong Purcell inhibition and cavity transparency effects. We further investigate the tunable momentum dispersion of coupled IXs through back-focal-plane imaging and explain our results by the cavity coupling of IX transition dipoles as supported by theoretical modelling. Our work demonstrates an integration effort enabling the versatile tuning of highly interacting IXs within monolithic cavities, revealing the attractiveness of electrically tunable IX cavity coupling for both fundamental studies towards exciton condensate manipulation and future integration of excitonic devices.
Nature Nanotechnology (2025)217. G. Meneghini, S. Brem, E. Malic,"Spatiotemporal dynamics of moiré excitons in van der Waals heterostructures", Nature Communications 16, 8557 (2025)
Heterostructures of transition metal dichalcogenides (TMDs) offer unique opportunities in optoelectronics due to their strong light-matter interaction and the formation of dipolar interlayer excitons. Introducing a twist angle or lattice mismatch between layers creates a periodic moiré potential that significantly reshapes the energy landscape and introduces a high-dimensional complexity absent in aligned bilayers. Recent experimental advances have enabled direct observation and control of interlayer excitons in such moiré-patterned systems, yet a microscopic theoretical framework capturing both their thermalization and spatiotemporal dynamics remains lacking. Here, we address this challenge by developing a predictive, material-specific many-body model that tracks exciton dynamics across time, space, and momentum, fully accounting for the moiré potential and the complex non-parabolic exciton band structure. Surprisingly, we reveal that flat bands, which typically trap excitons, can significantly enhance exciton propagation. This counterintuitive behavior emerges from the interplay between the flat-band structure giving rise to a bottleneck effect for exciton relaxation and thermal occupation dynamics creating hot excitons. Our work not only reveals the microscopic mechanisms behind the enhanced propagation but also enables the control of exciton transport via twist-angle engineering. These insights lay the foundation for next-generation moiré-based optoelectronic and quantum technologies.
Nature Communications 16, 8557 (2025)216. J. Jasiński, J. Hagel, S. Brem, E. Wietek, T. Taniguchi, K. Watanabe, A. Chernikov, N. Bruyant, M. Dyksik, A. Surrente, M. Baranowski, D. K. Maude, E. Malic, P. Płochocka, "Quadrupolar Excitons in MoSe2 Bilayers", Nature Communications 16, 1382 (2025)
The quest for platforms to generate and control exotic excitonic states has greatly benefited from the advent of transition metal dichalcogenide (TMD) monolayers and their heterostructures. Among the unconventional excitonic states, quadrupolar excitons - a hybridized combination of two dipolar excitons with anti-aligned dipole moments - are of great interest for applications in quantum simulations and for the investigation of many-body physics. Here, we unambiguously demonstrate for the first time in natural MoSe2 homobilayers the emergence of quadrupolar excitons, whose energy shifts quadratically in electric field. In contrast to, so far reported trilayer systems hosting quadrupolar excitons, MoSe2 homobilayers have many advantages, a stronger interlayer hybridization, cleaner potential landscapes and inherent stability with respect to moiré potentials or post-stacking reconstruction. Our experimental observations are complemented by many-particle theory calculations offering microscopic insights in the formation of quadrupole excitons. Our results suggest TMD homobilayers as ideal platform for the engineering of excitonic states and their interaction with light and thus candidate for carrying out on-chip simulations.
Nature Communications 16, 1382 (2025)215. E. Lopriore, C. Louca, A. Genco, I. Landa, D. Erkensten, C. J. Sayers, S. Brem, R. Perea-Causin, K. Watanabe, T. Taniguchi, C. Gadermaier, E. Malic, G. Cerullo, S. Dal Conte, A. Kis, “Electrically tunable ultrafast dynamics and interactions of hybrid excitons in a 2D semiconductor bilayer” accepted by Nature Communications (2025)
Extended efforts have been devoted to the study of strongly-interacting excitons and their dynamics, towards macroscopic quantum states of matter such as Bose-Einstein condensates of excitons and polaritons. Momentum-direct layer-hybridized excitons in transition metal dichalcogenides have attracted considerable attention due to their high oscillator strength and dipolar nature. However, the tunability of their interactions and dynamics remains unexplored. Here, we achieve an unprecedented control over the nonlinear properties of dipolar layer-hybridized excitons in an electrically gated van der Waals homobilayer monitored by transient optical spectroscopy. By applying a vertical electric field, we reveal strong Coulomb interactions of dipolar hybrid excitons, leading to opposite density-dependent energy shifts of the two main hybrid species based on their dipolar orientation, together with a strongly enhanced optical saturation of their absorption. Furthermore, by electrically tuning the interlayer tunneling between the hybridized carriers, we significantly extend the formation time of hybrid excitons, while simultaneously increasing their decay times. Our findings have implications for the search on quantum blockade and condensation of excitons and dipolaritons in two-dimensional materials.
accepted by Nature Communications (2025)214. G. Lo Gerfo Morganti, R. Rosati, G. D. Brinatti Vazquez, S. Varghese, D. Saleta Reig, E. Malic, N. F. van Hulst and K.-J. Tielrooij, "Transient ultrafast and negative diffusion of charge carriers in suspended MoSe2 from multilayer to monolayer", Nature Communications 16, 5184 (2025)
Understanding the ultrafast transport properties of charge carriers in transition metal dichalcogenides is essential for advancing technologies based on these materials. Here, we study MoSe2 crystals with thicknesses down to the monolayer, combining ultrafast spatiotemporal microscopy and quantitative microscopic modelling. Crucially, we obtain the intrinsic ultrafast transport dynamics by studying suspended crystals that do not suffer from detrimental substrate effects. In mono- and bilayer crystals, we identify four sequential transport regimes. The first two regimes involve high-energy non-thermalized and quasi-thermalized carriers that propagate rapidly with diffusivities up to 1000 cm2/s. After ~1.5 ps, a remarkable third regime occurs with apparent negative diffusion, finally followed by exciton propagation limited by trapping into defect states. Interestingly, for trilayer and thicker crystals, only the first and last regimes occur. This work underscores the role of traps and dielectric environment in electron transport, offering valuable insights for the development of (flexible) (opto)electronic applications.
Nature Communications 16, 5184 (2025)213. H. Shan, J. M. Fitzgerald, R. Rosati, G. Leibeling, K. Watanabe, T. Taniguchi, S. Ariel Tongay, F. Eilenberger, M. Esmann, S. Höfling, E. Malic and C. Schneider, "Tuning relaxation and nonlinear upconversion of valley-exciton-polaritons in a monolayer semiconductor", accepted by Nature Communications (2025)
Controlling exciton relaxation and energy conversion pathways via their coupling to photonic modes is a central task in cavity-mediated quantum materials research. In this context, the light-matter hybridization in optical cavities can lead to intriguing effects, such as modified carrier transport, enhancement of optical quantum yield, and control of chemical reaction pathways. Here, we investigate the impact of the strong light-matter coupling regime on energy conversion, both in relaxation and upconversion schemes, by utilizing a strongly charged MoSe2 monolayer embedded in a spectrally tunable open-access cavity. We find that the charge carrier gas yields a significantly modified photoluminescence response of cavity exciton-polaritons, dominated by an intra-cavity like pump scheme. In addition, upconversion luminescence emerges from a population transfer from fermionic trions to bosonic exciton-polaritons. Due to the availability of multiple optical modes in the tunable open cavity, it seamlessly meets the cavity-enhanced double resonance condition required for an efficient upconversion. The latter can be actively tuned via the cavity length in-situ, displaying nonlinear scaling in intensity and fingerprints of the valley polarization. This suggests mechanisms that include both trion-trion Auger scattering and phonon absorption as its underlying microscopic origin.
accepted by Nature Communications212. J. M. Fitzgerald, R. Rosati, E. Malic, “Polariton transport in 2D semiconductors: Phonon-mediated transitions between ballistic, superdiffusive and exciton-limited regimes”, accepted by Science Advances (2025)
Exciton transport in 2D semiconductors holds promise for room-temperature, ultra-compact optoelectronic devices, but it is limited by short propagation distances. Hybridization of excitons with cavity photons to form exciton-polaritons can enhance the propagation by orders of magnitude, enabling a coherent, ballistic transport. However, a microscopic understanding of the role of phonons is still lacking, particularly regarding their influence on the crossover from the ballistic to the diffusive polariton transport regime. Here, we investigate the spatiotemporal polariton dynamics in \ce{MoSe2} monolayers at moderate to high temperatures, explicitly including the phonon-mediated coupling to the intervalley exciton reservoir. We identify three distinct transport regimes: (i) an initial sub-ps ballistic-like regime characterized by a phonon-induced velocity renormalization, (ii) a transient, few-ps superdiffusive regime characterized by strongly enhanced diffusion, and (iii) a slower, exciton-limited diffusion following thermalization. The gained microscopic insights will trigger and guide future experimental studies on the phonon-mediated polariton transport in atomically thin semiconductors.
accepted by Science Advances (2025)211. M. Dyksik, M. Baranowski, J. J. P. Thompson, Z. Yang, M. Rivera Medina, M. Antonietta Loi, E. Malic, and P. Plochocka, "Steric Engineering of Exciton Fine Structure in 2D Perovskites", Advanced Energy Materials 15, 9 (2025)
A comprehensive study of excitonic properties of 2D layered perovskites is provided, with an emphasis on understanding and controlling the exciton fine structure. First, an overview of the optical properties is presented, discussing the challenges in determining the bandgap and exciton binding energies. Through magneto-optical spectroscopic measurements (up to B = 140 T), scaling laws are established for exciton binding energy as a function of the band gap and the diamagnetic coefficient. Using an in-plane magnetic field, the exciton fine structure for various 2D perovskites is examined to measure the energy splitting between the excitonic levels. The exciton fine structure and exchange interaction are correlated with structural parameters, employing an effective mass model, to highlight the role of steric effect on the exchange interaction. These findings reveal that lattice distortions, introduced by organic spacers, significantly influence the exchange interaction, driving a tunable energy spacing between dark and bright excitons. This unique feature of 2D perovskites, not present in other semiconductors, offers a novel tuning mechanism for exciton control, making these materials highly promising for efficient light emitters and advanced quantum technologies.
Advanced Energy Materials 15, 9 (2025)210. R. Rosati, S. Shradha, J. Picker, A. Turchanin, B. Urbaszek, E. Malic, "Impact of charge transfer excitons on unidirectional exciton transport in lateral TMD heterostructures", Nano Letters 25, 11319 (2025)
Lateral heterostructures built of monolayers of transition metal dichalcogenides (TMDs) are characterized by a thin 1D interface exhibiting a large energy offset. Recently, the formation of spatially separated charge-transfer (CT) excitons at the interface has been demonstrated. The technologically important exciton propagation across the interface and the impact of CT excitons has remained in the dark so far. In this work, we microscopically investigate the spatiotemporal exciton dynamics in the exemplary hBN-encapsulated WSe2-MoSe2 lateral heterostructure and reveal a highly interesting interplay of energy offset-driven unidirectional exciton drift across the interface and efficient capture into energetically lower CT excitons at the interface. This interplay triggers a counterintuitive thermal control of exciton transport with a less efficient propagation at lower temperatures - opposite to the behavior in conventional semiconductors. We predict clear signatures of this intriguing exciton propagation both in far- and near-field photoluminescence experiments. Our results present an important step toward a microscopic understanding of the technologically relevant unidirectional exciton transport in lateral heterostructures.
Nano Letters 25, 11319 (2025)209. J. K. König, J. M. Fitzgerald, E. Malic, "Magneto-Optics of Anisotropic Exciton Polaritons in Two-Dimensional Perovskites", Nano Letters 25, 21, 8443 (2025)
Layered 2D organic-inorganic perovskite semiconductors support strongly confined excitons that offer significant potential for ultrathin polaritonic devices due to their tunability and huge oscillator strength. The application of a magnetic field has proven to be an invaluable tool for investigating the exciton fine structure observed in these materials. Yet, the combination of an in-plane magnetic field and the strong coupling regime has remained largely unexplored. In this work, we combine microscopic theory with a rigorous solution of Maxwell's equations to model the magneto-optics of exciton polaritons in 2D perovskites. We predict that the brightened dark exciton state can enter the strong coupling regime. Furthermore, the magnetic-field-induced mixing of polarization selection rules and the breaking of in-plane symmetry lead to highly anisotropic polariton branches. This study contributes to a better understanding of the exciton fine structure in 2D perovskites and demonstrates the cavity control of highly anisotropic and polarization-sensitive exciton polaritons.
Nano Letters 25, 21, 8443 (2025)208. A. M. Kumar, D. J. Bock, D. Yagodkin, E. Wietek, B. Höfer, M. Sinner, P. Hernández López, S. Heeg, C. Gahl, F. Libisch, A. Chernikov, E. Malic, R. Rosati, K. I. Bolotin, "Strain engineering of valley-polarized hybrid excitons in a 2D semiconductor", Nano Letters (2025), selected for the cover page
Encoding and manipulating digital information in quantum degrees of freedom is one of the major challenges of today's science and technology. The valley indices of excitons in transition metal dichalcogenides (TMDs) are well-suited to address this challenge. Here, we demonstrate a new class of strain-tunable, valley-polarized hybrid excitons in monolayer TMDs, comprising a pair of energy-resonant intra- and intervalley excitons. These states combine the advantages of bright intravalley excitons, where the valley index directly couples to light polarization, and dark intervalley excitons, characterized by low depolarization rates. We demonstrate that the hybridized state of dark KK' intervalley and defect-localized excitons exhibits a degree of circular polarization of emitted photons that is three times higher than that of the constituent species. Moreover, a bright KK intravalley and a dark KQ exciton form a coherently coupled hybrid state under energetic resonance, with their valley depolarization dynamics slowed down a hundredfold. Overall, these valley-polarized hybrid excitons with strain-tunable valley character emerge as prime candidates for valleytronic applications in future quantum and information technology.
accepted by Nano Letters (2025)207. B. Han, J. M. Fitzgerald, L. Lackner, R. Rosati, M. Esmann, F. Eilenberger, T. Taniguchi, K. Watanabe, M. Syperek, E. Malic, C. Schneider, "Infrared magneto-polaritons in MoTe2 mono- and bilayers", Physical Review Letters 134, 076902, selected as Editor's suggestion (2025)
MoTe2 monolayers and bilayers are unique within the family of van-der-Waals materials since they pave the way towards atomically thin infrared light-matter quantum interfaces, potentially reaching the important telecommunication windows. Here, we report emergent exciton-polaritons based on MoTe2 monolayer and bilayer in a low-temperature open micro-cavity in a joint experiment-theory study. Our experiments clearly evidence both the enhanced oscillator strength and enhanced luminescence of MoTe2 bilayers, signified by a 38 % increase of the Rabi-splitting and a strongly enhanced relaxation of polaritons to low-energy states. The latter is distinct from polaritons in MoTe2 monolayers, which feature a bottleneck-like relaxation inhibition. Both the polaritonic spin-valley locking in monolayers and the spin-layer locking in bilayers are revealed via the Zeeman effect, which we map and control via the light-matter composition of our polaritonic resonances.
Physical Review Letters 134, 076902206. H. Pashaei Adl, C. Bennenhei, M. Struve, P. Peksa, M. Dyksik, M. Baranowski, K. Wee Song, M. Gittinger, C. Lienau, J. K. Konig, J. M. Fitzgerald, D. Cahen, P. Plochocka, E. Malic, O. Kyriienko, M. Esmann, C. Schneider, "Tunable room-temperature polaritons in the very strong coupling regime in quasi-2D Ruddlesden-Popper perovskites", accepted by Advanced Optical Materials (2025)
Layered perovskites are an emergent class of materials which feature extraordinarily large light-matter coupling, driven by excitons with binding energies significantly beyond the thermal energy at room temperature. In this work, we demonstrate widely tunable room-temperature cavity exciton polaritons at the cross-over from the strong coupling to the very strong coupling regime in mechanically exfoliated crystals of quasi-2D Ruddlesden-Popper iodide perovskite (BA)2(MA)2Pb3I10 within an open microcavity. The coupled excitoncavity system features a Rabi splitting up to ΩR ≃155 meV, which exceeds the experimentally determined exciton binding energy of Eb = 100 ± 10 meV and thus operates at the onset of the very strong coupling regime. Our combined experimental-theoretical effort provides a consistent microscopic picture for our findings, and can furthermore successfully describe our observed peculiar scaling of the Rabi-splitting with the increasing effective cavity length. Our results open exciting possibilities for future applications with tuneable polaritonic devices and photonic circuits based on strongl coupled perovskite-based systems.
accepted by Advanced Optical Materials205. P. Parzefall, N. Paulik, C. Serati de Britoa, J. Göser, J. Trapp, K. Watanabe, T. Taniguchi, D. Erkensten, G. Meneghini, Y. Galvão Gobato, E. Malic, A. Högele, and C. Schüller, "Programmable phonon-assisted resonant energy transfer between moiré cells in charge-tunable MoSe2-WS2 heterobilayers", njp 2D Materials 9, 84 (2025)
Moiré superlattices in van-der-Waals heterostructures offer a versatile platform for exploring emergent quantum phenomena. In type-I MoSe2-WS2 moiré superlattices, the large lattice mismatch ensures robustness of the moiré period against twist-angle disorder. The excitonic ground state is formed by moiré-trapped MoSe2 intralayer excitons. However, a key challenge is the controlled transfer of excitonic energy across moiré sites. This work investigates gate-controlled phonon-assisted resonant energy transfer (RET) as a means to transfer excitonic energy between moiré cells. By harnessing the interplay between resonantly excited moiré excitonic complexes and single or few phonons, energy transfer pathways can be modulated via the charging state of moiré cells. We discuss two potential RET mechanisms: phonon-assisted resonant tunneling and Försterlike dipole-dipole transfer. Our findings highlight the potential of this approach for excitonic circuits and nanoscale energy transport, paving the way for future applications in quantum technologies.
njp 2D Materials 9, 84 (2025)204. M. Cuccu, T. Venanzi, E. Wietek, X. Sun, R. Perea-Causin, T. Taniguchi, K. Watanabe, E. Malic, M. Helm, S. Winnerl, and A. Chernikov, “Terahertz-induced population transfer between exciton complexes in monolayer WSe2”, accepted by Physical Review B, Editor’s Suggestion (2025)
Two-dimensional van der Waals semiconductors feature a variety of stable Coulomb-bound electron-hole complexes, which determine the optical response of the materials and serve as primary carriers of energy and spin-valley encoded information. Importantly, transitions between different excitonic states are found in the terahertz spectral range, motivating the use of strong THz radiation for their manipulation on ultrafast timescales. In this work, we apply this technique to efficiently transfer populations within the manifold of excitonic complexes in monolayer WSe2, combining pulsed optical injection with a perturbation induced by a THz free-electron laser source. Monitoring time-resolved photoluminescence we show conversion between different Coulomb-bound species across biexcitonic and excitonic regimes. Depending on the lattice temperature, these processes involve both short-lived bright and long-lived dark states. Combining experimental findings with theory support, we outline possible dissociation and formation pathways of charged excitons and biexcitons induced by the THz radiation. Finally, we demonstrate the access to formation dynamics of charged biexcitons under controlled conditions of thermalized populations of their constituents, avoiding complications of excess energies that otherwise occur after non-resonant optical excitation.
accepted by Physical Review B, Editor's Suggestion203. R. Rosati, I. Paradisanos, E. Malic, and B. Urbaszek, "Two dimensional semiconductors: Optical and electronic properties", book chapter in "Comprehensive Semiconductor Science and Technology", Second Edition, vol. 1, pp. 312–351, Elsevier
In the last decade atomically thin 2D materials have emerged as a perfect platform for studying and tuning light-matter interaction and electronic properties in nanostructures. The optoelectronic properties in layered materials such as transition-metal-dichalcogenides (TMDs) are governed by excitons, Coulomb bound electron-hole pairs, even at room temperature. The energy, wave function extension, spin and valley properties of optically excited conduction electrons and valence holes are controllable via multiple experimentally accessible knobs, such as lattice strain, varying atomic registries, dielectric engineering as well as electric and magnetic fields. This results in a multitude of fascinating physical phenomena in optics and transport linked to excitons with very specific properties, such as bright and dark excitons, interlayer and charge transfer excitons as well as hybrid and moiré excitons. In this book chapter we introduce general optoelectronic properties of 2D materials and energy landscapes in TMD monolayers as well as their vertical and lateral heterostructures, including twisted TMD hetero- and homobilayer bilayers with moire excitons and lattice recombination effects. We review the recently gained insights and open questions on exciton diffusion, strain- and field-induced exciton drift. We discuss intriguing non-linear many-particle effects, such as exciton halo formation, negative and anomalous diffusion, the surprising anti-funneling of dark excitons.
Comprehensive Semiconductor Science and Technology202. P. G Steeneken, M. Soikkeli, S. Arpiainen, A. Rantala, R. Jaaniso, R. Pezone, S. Vollebregt, S. Lukas, S. Kataria, M. Houmes, R. Álvarez-Diduk, K. Lee, H. Suryo Wasisto, S. Anzinger, M. Fueldner, G. J. Verbiest, F. Alijani, D. Hoon Shin, E. Malic, R. van Rijn, T. Nevanen, A. Centeno, A. Zurutuza, H. S. J. van der Zant, A. Merkoci, G. S. Duesberg and M. Lemme, "Towards wafer-scale 2D material sensors", 2D Materials, 12, 2 (2025)
The unique properties of two-dimensional (2D) materials bring great promise to improve sensor performance and realise novel sensing principles. However, to enable their high-volume production, wafer-scale processes that allow integration with electronic readout circuits need to be developed. In this perspective, we review recent progress in on-chip 2D material sensors, and compare their performance to the state-of-the-art, with a focus on results achieved in the Graphene Flagship programme. We discuss transfer-based and transfer-free production flows and routes for complementary metal-oxide-semiconductor (CMOS) integration and prototype development. Finally, we give an outlook on the future of 2D material sensors, and sketch a roadmap towards realising their industrial and societal impact.
2D Materials 12, 2 (2025)