Extended Data Fig. 5: Polaritons renormalize the resonance energy.

a, Schematic illustration of the dispersion relation of exciton-polaritons in bulk CrSBr. Due to strong self-hybridization effects, the 1s state (solid grey line) splits into multiple lower polariton (LP, grey-scale intensity illustrates the exciton fraction) and upper polariton (UP, dotted grey line) states. The 2p_b state is indicated by the blue line. In the low fluence regime and at early delays, the MIR pulse probes cavity polariton states close to the bare 1s-2p_b transition (50 meV, light blue arrow). The subsequent relaxation of the polaritons towards energetically lower-lying states (curved arrow) results in a blueshift (pink and purple arrows) of the 1s-2p_b transition by ∆ (yellow arrow). Energy separations are not drawn to scale. b, Pump-induced change of the MIR absorption (∆α) and the real part of the dielectric function (Δε₁) as a function of the photon energy for different pump-probe delay times t_pp (pump fluence Φ_NIR = 75 μJ/cm²). The arrow indicates a blueshift of the maximum in ∆α by ∆ for later delay times. c, ∆α and Δε₁ for a 20-nm-thin CrSBr flake (t_pp = 1 ps: red symbols, red shading; t_pp = 0.1 ps: orange symbols, no shading) as well as a bulk crystal (t_pp = 1 ps: blue symbols and shading). The arrow indicates a blueshift of the maximum in ∆α for the bulk sample due to polaritonic corrections. d, Temperature-dependent internal structure of excitons in 20-nm-thin CrSBr. ∆α and Δε₁ as a function of the photon energy at a delay time of t_pp = 1 ps at different temperatures (Φ_NIR = 600 μJ/cm²).