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Preprints
231. J. Gradl, N. Hofmann, L. Weigl, S. Forti, N. Mishra, C. Coletti, R. Perea-Causin, E. Malic, I. Gierz, “Influence of sulphur vacancies on ultrafast charge separation in WS2-graphene heterostructures”, arXiv: 2603.16247 (2026)
Understanding how defects influence charge separation in WS2 -graphene heterostructures is crucial for future applications in light harvesting and detection. Previous studies have reported widely varying lifetimes for the charge-separated state, all supposedly linked to electron trapping at sulphur vacancies. The exact impact of these defects, however, has remained unclear. Here, we deliberately introduce sulphur vacancies by annealing the heterostructures at high temperatures in ultrahigh vacuum. Angle-resolved photoemission spectroscopy (ARPES) reveals that these vacancies modify both the band alignment and doping level of the heterostructure. Time-resolved ARPES (trARPES) further shows that increasing the sulphur vacancy concentration prolongs the lifetime of electrons in the WS2 conduction band but shortens the lifetime of the charge-separated state. Guided by model calculations, we attribute this behaviour to shifts in the energy alignment between sulphur vacancy states and graphene's Dirac point, combined with a reduced excitonic absorption. The model also yields a transfer time for electrons tunneling from sulphur vacancies into graphene's Dirac cone of 4ps, consistent with our trARPES measurements. Our study clarifies the role of sulphur vacancies in WS2-graphene heterostructures, further improving our microscopic understanding of charge dynamics for future optoelectronic applications.
arXiv: 2603.16247 (2026)230. E. P. Kraus, J. M. Fitzgerald, C. Maciel-Escudero, E. Malic, “Engineering strong coupling in ultra-compact photonic crystal/2D material platforms”, arXiv: 2604.12779 (2026)
Sub-wavelength thick photonic crystal (PhC) slabs coupled to 2D excitonic materials, such as transition metal dichalcogenides (TMDs), are a promising platform for highly tunable, room-temperature, on-chip optoelectronic devices. Unlike conventional Fabry-Perot microcavities, these compact open cavities exhibit non-trivial electric field profiles, leading to spatially distinct regions of weak and strong coupling with excitons within the PhC unit cell. Using coupled mode theory and rigorous solutions to Maxwell's equations, we investigate how the PhC geometry can be used to control these coexisting exciton/polariton contributions and tailor the resulting optical spectra. For large filling factors, i.e., small air gaps, we show that PhC polaritons can be modeled as dark waveguide modes brightened via the periodicity of the PhC slab. Furthermore, by spatially patterning the TMD monolayer based on the local field intensity, we reveal the simultaneous presence of excitons in both the weak and strong coupling regimes. Overall, this work provides fundamental insights into the strong light-matter coupling regime in structured photonic environments, offering a pathway to design and optimize metal-free, ultra-compact polaritonic devices.
arXiv: 2604.12779 (2026)229. J. K. König, J. M. Fitzgerald, D. Erkensten, E. Malic,“Exciton Polariton-Polariton Interactions in Transition-Metal Dichalcogenides”, arXiv: 2603.28409 (2026)
Microscopic insights into nonlinear interactions are essential for advancing polaritonic devices. Existing studies often rely on phenomenological models that overlook important many-body processes. Based on a material-specific and predictive approach, we investigate monolayer and homobilayer MoS2 embedded in a Fabry-Pérot cavity to characterize the exchange, saturation, and dipole-dipole contributions to polariton-polariton interactions in these technologically promising materials. A key finding is that the exchange interaction induces asymmetric energy shifts of the lower and upper polariton branches in a detuned cavity, a behavior driven by the difference in their excitonic character. Furthermore, we demonstrate that temperature and electron-photon coupling determine the energy renormalization through the equilibrium polariton distribution. In homobilayers, the dipole-dipole interaction is mediated by the interlayer character, enabling electrical control and facilitating the electric-field-induced closing of anti-crossings due to dipolar-interaction shifts. The gained insights on polariton-polariton interactions are important for the development of ultra-compact polaritonic circuitry.
arXiV: 2603.28409