Fig. 1: Accessing the internal structure of anisotropic excitons in CrSBr.

a, Schematic of the coupling of excitons (blue surfaces) to the magnetic order (red arrows) in CrSBr (dark spheres, Cr; yellow spheres, S; brown spheres, Br). In the AFM phase (T < T_N, large red arrows), the quasi-1D excitons stretched along the b axis are confined to individual layers (bottom) because interlayer hopping is spin-forbidden (crossed-out blue double arrow). The paramagnetic phase (T > T_N, large red double arrows) allows excitons to be delocalized (blue double arrow) across neighbouring layers (top). b, Schematic of the band structure of CrSBr. For clarity, the bandgap is substantially reduced. The projections of the elliptic paraboloids onto the a (Γ–X) and b (Γ–Y) directions highlight the large difference in the effective mass of the lowest conduction band (m^e) and the highest valence band (m^h). c, Anisotropic Coulomb potential (grey surface) of the excitons. The isoenergy contours (white ellipses) and the projections (black lines) onto the two crystallographic directions highlight the large in-plane anisotropy. Although the a and b axes share a common 1s level (light grey line), the degeneracy of the 2p states is lifted. Consequently, the 1s–2p_a and 1s–2p_b transitions (vertical red lines) feature different energies that are interrogated by MIR transients (red waveforms) of orthogonal polarization (E_MIR||a and E_MIR||b, respectively). d, Schematic of the time-resolved NIR pump–MIR probe spectroscopy experiment. After a variable delay time t_pp, the electric field of the MIR waveform is transmitted through the CrSBr crystal and the diamond substrate. The transmitted MIR transient, E_MIR, and changes to its electric field, ΔE_MIR, induced by the presence of the photogenerated electron–hole pairs are electro-optically recorded.