Disentangling many-body effects in the coherent optical response of a 2D Semiconductor

Monolayer transition metal dichalcogenides (1L-TMDs) have received increasing attention because of their strongly bound excitons[1]. Transient absorption spectroscopy has been extensively used to study exciton scattering processes on an ultrafast timescale[2], however the physical origin of exciton dynamics on a ps and sub-ps timescale is still under debate. In this temporal window, many-body effects lead to a renormalization of the bands, inducing a transient energy shift of the excitonic resonance, and the increase of the electronic temperature broadens the exciton linewidth[3]. All these processes are difficult to disentangle, and their dynamical interplay determines the complex shape of the transient absorption spectra of TMDs across the bandgap at early pump-probe delays.

In this work, we measure the transient optical response of 1L-WS₂ on SiO₂ across the A and B excitonic resonances. The sample is photoexcited on- and out-of-resonance with the A exciton, and at variable pump fluences below the exciton-Mott transition. We disentangle absorption reduction, energy shift and broadening of the excitonic peak from the transient optical response, using Kramers-Kronig constrained variational analysis. From the measured transient reflectivity (ΔR/R) of 1L-WS₂ we retrieve the absorbance spectrum as a function of pump-probe delay. All the spectra are well reproduced by a fitting function made by the sum of two Lorentz oscillators on top of a polynomial background. The transient energy shift persists over a timescale much longer than the temporal overlap of pump and probe pulses, excluding the possibility that it originates from optical Stark effect.

Microscopic calculations based on excitonic Heisenberg equations of motion quantitatively reproduce the non-linear absorbance spectra of the material[4].

In conclusion, our combined experimental and theoretical studies give important insights into the complex interplay between many-body correlations and excitonic interactions determining the non-equilibrium response of 1L-TMDs[4].

[1] Chernikov, A. et al. Phys. Rev. Lett. 113, 076802 (2014).

[2] Moody, G. et al., J. Opt. Soc. Am. B 33, C39–C49 (2016).

[3] Katsch, F., et al., Phys. Rev. Lett. 124, 257402 (2020).

[4] Trovatello, C. et al., Nano Letters, 22, 5322-5329 (2022).