Probing high-energy and band-edge exciton dynamics in monolayer WS₂ using transient absorption spectroscopy under near-resonant and high-energy excitations

Insight into exciton dynamics of two-dimensional (2D) transition metal dichalcogenides (TMDs) is critical for the optimization of their performance in photonic and optoelectronic devices. Although current researches have primarily concentrated on the near-resonant excitation scenario in 2D TMDs, the case of excitation energies resonating with high-energy excitons or higher energies has yet to be fully elucidated. Here, a comparative analysis is conducted between high-energy excitation (360 nm) and near-resonant excitation (515 nm) utilizing transient absorption spectroscopy to achieve a comprehensive understanding of the exciton dynamics within monolayer WS₂. It is observed that the high-energy C-exciton can be generated via an up-conversion process under 515 nm excitation, even the energy of which is less than that of the C-exciton. Furthermore, the capacity to efficiently occupy band-edge A-exciton states leads to longer lifetimes for both the C-excitons and the A-excitons under conditions of near-resonant excitation, accompanied by an augmented rate of radiative recombination. This study provides a paradigm for optimizing the performance of 2D TMDs-based devices by offering valuable insights into their exciton dynamics.