Abstract

In atomically thin semiconductors based on transition metal dichalcogenides, photoexcitation can be used to generate high densities of electron-hole pairs. Due to optical nonlinearities, which originate from Pauli blocking and many-body effects of the excited carriers, the generated carrier density will deviate from a linear increase in pump fluence. In this paper, we describe nonlinear absorption properties and excited carrier dynamics using a theoretical approach that combines results from ab initio electronic-state calculations with a many-body treatment of optical excitation. We determine the validity range of a linear approximation for the excited carrier density vs pump power and identify the role and magnitude of optical nonlinearities at elevated excitation carrier densities for ${\mathrm{MoS}}_{2}, {\mathrm{MoSe}}_{2}, {\mathrm{WS}}_{2}$, and ${\mathrm{WSe}}_{2}$ considering various excitation conditions. For photoexcitation at the bandgap, we find that the use of a linear absorption coefficient of the unexcited system can strongly underestimate the achievable carrier density in ${\mathrm{MoS}}_{2}$ due to many-body renormalizations of the two-particle density of states. The same holds for excitation of the high-energy band continuum in W-based materials.

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