Hauptinhalt
Preprints
237. R. Rosati, M. A. A. Nouh, R. Schmidt, R. Bratschitsch, E. Malic, "Strain engineering of transient exciton diffusion in WSe2 monolayers at cryogenic temperatures", arXiv: 2608.02513 (2026)
Tungsten-based transition metal dichalcogenides exhibit dark excitons as the energetically lowest states. These are crucial for exciton thermalization and propagation and they dominate low-temperature photoluminescence via the emergence of pronounced phonon sidebands. After a resonant excitation, highly mobile hot dark excitons are formed, which quickly thermalize into an equilibrium distribution. The application of strain modifies the exciton energy landscape and, in particular, the relative energy separation between bright and dark exciton states. The impact of strain on the transient photoluminescence and diffusion of non-equilibrium excitons has remained largely unexplored so far. In this work, we investigate the spatiotemporal exciton dynamics in strained hBN-encapsulated WSe2 monolayers at cryogenic temperatures. We demonstrate that tensile strain abruptly increases the excess energy of hot excitons, thereby accelerating their transient diffusion. We trace this back to suppressed phonon-mediated scattering from bright to dark excitons. Furthermore, we predict a periodic modulation of the transient exciton diffusion in the presence of a compressive strain resulting from strain-driven emission of M phonons. The gained microscopic insights illustrate how strain can be used to engineer transient photoluminescence and exciton diffusion in technologically promising 2D semiconductors.
arXiv: 2608.02513236. E. P. Kraus, J. M. Fitzgerald, C. Maciel-Escudero, E. Malic, „Perfect Absorption in the Strong Coupling Regime via Degenerate Critical Coupling”, arXiv: 2606.26708
Perfect absorption (PA) represents a fundamental limit of light-matter interaction and a means to maximize nanoscale energy conversion. While PA is now a well-established phenomenon, both the theoretical feasibility and a practical mechanism for achieving it under single-beam excitation within the strong coupling regime is unknown. Through rigorous solution of Maxwells equations for a compact photonic crystal (PhC) architecture incorporating a two-dimensional semiconductor, we present a general method based on degenerate critical coupling for single-port PA of exciton-polaritons. At the crossing of two polariton branches, we achieve near-unity absorption exceeding 99.8 \% in a structure thinner than 100nm. This effect is robust under realistic Gaussian beam excitation and can be realized across different temperatures and excitonic materials by tailoring the PhC geometry. Our results establish a strategy for enabling efficient light-matter coupling, with direct implications for the development of metal-free, ultra-compact polaritonic logic devices, sensors, and energy-harvesting platforms.
arXiv: 2606.26708235. K. Sonowal, D. Erkensten, E. Malic, R. Rosati “Coulomb-mediated interactions of charge-transfer excitons in TMD lateral heterostructures”, arXiv: 2606.04865
Lateral heterostructures of transition-metal dichalcogenides (TMDs) host spatially separated charge-transfer (CT) excitons. While analogous to interlayer excitons in vertical TMD heterostructures, these interfacial excitons possess much larger in-plane dipoles of several nanometers and an additional center-of-mass quantization. Here, we study the mutual interactions between these highly dipolar CT excitons on a microscopic footing. Accounting for the dipolar and quantum exchange interactions, we evaluate the experimentally accessible density-dependent energy renormalization and predict a net energy blueshift of a few meV for bound CT excitons. Interestingly, for small dipole moments, the energy renormalization displays a quadratic dependence with respect to the dipole moment, in contrast to the linear dependence found in vertical TMD heterostructures. We show that spatial energy offset and temperature are the key tuning knobs for controlling the density-dependent excitonic response. Overall, our results contribute to a better microscopic understanding of CT excitons and their interactions in lateral TMD heterostructures.
arXiv: 2606.04865234. S. Saris, R. Rosati, V. Bruevich, T. J. Sheehan, M. Roman, V. Podzorov, E. Malic, W. A. Tisdale, “Nonequilibrium transport in epitaxial CsPbBr3 single crystals”, arXiv: 2606.02460 (2026)
Transport of optically excited carriers in semiconductors is typically described within a quasi-equilibrium picture, where energy is carried by a single thermalized quasiparticle population characterized by well-defined transport coefficients. Here, we demonstrate that in epitaxial CsPbBr3 perovskite single crystals, this picture holds near room temperature - but breaks down dramatically at low temperature. Using transient microscopy, we show that optically measured carrier mobilities match device-scale Hall-effect and field-effect transistor measurements across a broad temperature range, resolving reported discrepancies and validating the equilibrium framework in the free-carrier regime. Below ~60 K, however, when excitonic effects become significant, equilibrium models begin to fail. We observe two coupled populations: a transient (<100 ps) hot-exciton gas with a diffusivity ~25-30 cm<sup>2</sup>/s - greatly exceeding the diffusivity expected for thermalized excitons - and a quasi-localized state that is fed by the cooling of the hot-exciton gas. These results reveal that in CsPbBr3, transport and thermalization are not separable processes: carriers move while still redistributing among internal degrees of freedom, breaking the timescale separation that underpins equilibrium transport theory in conventional semiconductors. By resolving transport at the population level, we can directly access the competing kinetics of exciton formation, interconversion, and cooling, offering a new space for controlling energy flow in perovskite materials and their photonic applications.
arXiV: 2606.02460233. 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)