Disentangling Many-Body Effects in the Coherent Optical Response of 2D Semiconductors*

In monolayer transition metal dichalcogenides (1L-TMDs), the reduced Coulomb screening results in strongly bound excitons which dominate the linear and the nonlinear optical response. Despite the large number of studies, a clear understanding on how many-body and Coulomb correlation effects affect the excitonic resonances on a femtosecond time scale is still lacking. Here, we use ultrashort laser pulses to measure the transient optical response of 1L-WS₂. From the pump-probe spectra we retrieve the absorption spectrum as a function of time using Kramers−Kronig constrained variational analysis within the thin-film model approximation. In order to disentangle many-body effects, we perform exciton lineshape analysis on the out-of-equilibrium absorption spectrum and we systematically study its dependence on pump photon energy and intensity [C. Trovatello et al., Nano Letters, 22, 5322–5329 (2022)].

We find that resonant photoexcitation produces a blue shift of the A exciton, which originates from bandgap renormalization, while for above-resonance photoexcitation the transient response at the optical bandgap is largely determined by a reduction of the exciton oscillator strength. Microscopic calculations quantitatively reproduce the nonlinear absorption and its dependence on excitation conditions.

Our results provide a more refined understanding of the transient response of TMDs and give important insights into the complex interplay between many-body effects and excitonic interactions.