Line Plot from Scientific Research

Experimental (left) and theoretical (right) pump-induced changes in the MIR absorption (Δ α ; top) and the real part of the dielectric function (Δ ε 1 ; bottom) as a function of photon energy. The experimental data were recorded at a pump–probe delay time of t pp = 500 fs along the crystallographic a (orange dots; pump fluence, Φ NIR = 450 μJ cm –2 ) and b (blue dots; Φ NIR = 125 μJ cm –2 ) axes and at a lattice temperature of T = 5 K. The error bars were retrieved by quantifying the uncertainty in the pump-induced change to the electric field Δ E MIR via the standard deviation of a set of five measurements. For clarity, the experimental and theoretical data along the a axis have been scaled by a factor of 1.5 and 5, respectively. The rather large fluence needed to achieve a measurable MIR response along the a axis indicates a weaker intraexcitonic oscillator strength compared with that along the b axis. Our experiment–theory analysis is fully quantitative since Rydberg spectroscopy measures changes in the transmitted MIR field that have a one-to-one connection to a uniquely scaled susceptibility 32 , 33 ξ (right axis). This allows us to easily construct the intrinsic linear absorption α = 2Im[ ξ ] and Δ ε 1 ( Methods ). Inset: schematic of the internal quantum transitions of anisotropic photogenerated excitons with non-degenerate 2 p states triggered by MIR pulses polarized along the crystallographic a or b axis ( E MIR || a or E MIR || b , respectively). The dispersions are plotted as a function of centre-of-mass momentum K .

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