Among atomically thin semiconductors, CrSBr stands out as both its bulk and monolayer forms host tightly bound, quasi-one-dimensional excitons in a magnetic environment. Despite its pivotal importance for solid-state research, the exciton lifetime has remained unknown. While terahertz polarization probing can directly trace all excitons, independently of interband selection rules, the corresponding large far-field foci substantially exceed the lateral sample dimensions. Here, we combine terahertz polarization spectroscopy with near-field microscopy to reveal a femtosecond decay of paramagnetic excitons in a monolayer of CrSBr, which is 30 times shorter than the bulk lifetime. We unveil low-energy fingerprints of bound and unbound electron-hole pairs in bulk CrSBr and extract the nonequilibrium dielectric function of the monolayer in a model-free manner. Our results demonstrate the first direct access to the ultrafast dielectric response of quasi-one-dimensional excitons in CrSBr, potentially advancing the development of quantum devices based on ultrathin van der Waals magnets.

Department of Electrical Engineering and Computer Science University of Michigan Ann Arbor Michigan 48109 United States

Department of Inorganic Chemistry University of Chemistry and Technology Prague 166 28 Prague 6 Czech Republic

Department of Physics and Regensburg Center for Ultrafast Nanoscopy University of Regensburg 93040 Regensburg Germany

Institute of Applied Physics and Würzburg Dresden Cluster of Excellence Dresden University of Technology 01187 Dresden Germany

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Controlling Coulomb correlations and fine structure of quasi-one-dimensional excitons by magnetic order