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 enhanced light-matter interaction, strongly bound excitons, exciton Rydberg states and many-body effects[1]. Transient absorption spectroscopy has been extensively used to study exciton scattering processes on an ultrafast timescale. While it has been shown that on a ten- to hundred-ps timescale, the exciton decay dynamics is dominated by thermal effects[2], <b>the physical origin of exciton dynamics on a ps and sub-ps timescale is still under debate</b>. In this temporal window, many-body effects lead to a renormalization of the bands, inducing a transient energy shift of the excitonic resonance. Simultaneously, the increase of the electronic temperature after photo-excitation, due to multiple electronic scattering events, broadens the excitonic linewidth. Broadening and shift of the excitonic peak overlap in time with an abrupt absorption reduction due to phase-space filling effect[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. Transient exciton energy shifts have been roughly estimated from pump-probe measurements with contrasting results, e.g., blue vs red shift, and values from meV to tens of meV.<br/><br/>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. In order to capture the origin of the different transient signal shapes for both excitations, 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.<br/>When the sample is photo-excited the pump induces a dramatic change of the A exciton, resulting from several concomitant effects: a quenching of the exciton oscillator strength, a blue energy shift and an asymmetric lineshape broadening. 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.<br/><br/>Microscopic calculations based on excitonic Heisenberg equations of motion quantitatively reproduce the non-linear absorbance spectra of the material. All the lorentzian parameters show a linear dependence with rising excitation power for both the pump photon energies, and many-body effects are strongly enhanced for resonant excitation, resulting in a transient blue shift of the A exciton. The shift progressively decreases as the pump is detuned from the resonance and turns into a small red shift when the energy of the pump is close to the B excitonic resonance. The energy shift originates from Coulomb-induced bandgap renormalization while the asymmetric broadening is related to excitation induced dephasing mechanism[4].<br/><br/>In conclusion, we provide a complete picture of the transient optical response of 1L-WS₂ which can finally explain the strong differences observed in the pump-probe spectra following on- and off-resonant excitation [4]. 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.<br/><br/>[1] Chernikov, A. et al. <i>Phys. Rev. Lett</i>. <b>113</b>, 076802 (2014).<br/>[2] Moody, G. et al., <i>J. Opt. Soc. Am. B</i> <b>33</b>, C39–C49 (2016).<br/>[3] Katsch, F., et al., <i>Phys. Rev. Lett.</i> <b>124</b>, 257402 (2020).<br/>[4] Trovatello, C. et al., <i>Nano Letters</i>, <b>22</b>, 5322-5329 (2022).