Diffusion dynamics of valley excitons by transient grating spectroscopy in monolayer WSe₂

The transient grating spectroscopy is widely used to determine the diffusion coefficients of valley excitons or spins in low-dimensional semiconductor materials. Here, we present the investigation on the diffusion dynamics of the valley excitons in a high-quality large-scale mechanically exfoliated tungsten diselenide (WSe₂) monolayer by this technique at room temperature. Collinearly polarized laser excitation (at a photon energy of 1.66 eV resonant to the energy of valley A-excitons) was used to introduce a spatially periodic density of valley excitons. Through probing the spatial and temporal evolution of the initial density of valley excitons, we find that the signals of transient grating exhibit an nonexponential decay, and its decay rate is independent of the period of optical grating Λ. Combined with the transient reflection measurements, we show that the exciton-exciton annihilation plays a key role in decay processes of the transient grating spectroscopy, which results in the distortion of sinusoidal gratings. Based on Einstein relationship, we estimate the diffusion coefficient of valley exciton D_X = 0.7 cm²/s.