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In recent years, 2D materials attracted substantial scientific interest from a fundamental perspective, and for their potential technological applications. In this context, transient changes of the optical properties, in particular the A exciton resonance (AX) were investigated, which displays complex dynamics (linewidth broadening, blue, and red shifts) depending on substrate, excitation density, and photon energy. However, the particular interplay of the dielectric screening of the substrate and the dynamic screening of quasi-free carriers and excitons on exciton binding energy and free carrier band gap remains elusive.
We access the time-dependent dielectric function of the A exciton in monolayer WS2 on different substrates using reflectivity and transmittivity contrast experiments and applying the 2D linearized [1] and the Fresnel transfer matrix model, tracking peak shift and broadening of the resonance upon photoexcitation [2]. These are then described by a simple two/three-level model based on the time evolution of the photo-excited carrier and exciton populations. We determine exciton formation and recombination times, the Auger recombination rate, broadening parameters of quasiparticle scattering events, the free carrier band gap, and its renormalization parameter in good agreement with literature values where available. This allows the disentanglement of the thermal, exciton, and free carrier contribution to the XA peak shift and broadening as well as to conclude that (i) linewidth broadening is dominated by exciton scattering, (ii) exciton screening acts mainly on the exciton binding energy, while (iii) quasi-free carriers are largely responsible for band gap renormalization. These findings constitute a comprehensive picture of the non-equilibrium dynamics in monolayer WS2 and will pave the path towards the application and control also of other 2D materials’ transient properties.
[1] Y. Li et al. 2D Mater. 5, 025021 (2018)
[2] S. Calati et al. PCCP 23, 22640 (2021)
[3] S. Calati, Q. Li, X.-Y. Zhu, J. Stähler, submitted (2022)
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