Opposite behavior of ultrafast dynamics of exciton shift and linewidth broadening in bilayer Re S2

The optical and optoelectronic properties of two-dimensional (2D) semiconductors are dominated by excitons, which usually appear as well-defined peaks in the spectral domain. Thus, detailed behaviors of excitons can be understood by tracking transient changes of the fundamental spectral observables, i.e., the resonance energy and the spectral linewidth. Rhenium disulfide (ReS2) is a 2D semiconductor that has recently attracted attention due to its excellent exciton properties, such as light-polarization selectivity and anisotropic coherent effects. However, an understanding of exciton dynamics and spectral behavior of excitons in ReS2 is lacking. Here, we used time- and spectrally resolved pump-probe spectroscopy to investigate ultrafast exciton dynamics in bilayer ReS2. Upon photoexcitation, the exciton resonance undergoes linewidth broadening and redshift, but they exhibit different dynamics, and an opposite pump fluence dependence on the approximately hundreds of picoseconds timescale. This is because the spectral broadening and the red-shift both have different origins (the exciton-carrier scattering and exciton-exciton attractive interaction, respectively) and different decay mechanisms (the trapping of carriers and exciton-exciton annihilation, respectively). On a longer timescale of ∼100 ps, both the spectral broadening and the redshift are well explained by the indirect recombination of carriers and lattice heating. This work provides in-depth insight into exciton dynamics in 2D rhenium dichalcogenides.