Figure 1: Schematic of the exciton distribution dynamics. (a) at\(t=0\), excitons are in a highly non-equilibrium state after a pulse excitation. (b) at \(t=\tau_{\text{th}}\), the excitons reach thermalisation of themselves at\(T_{\text{exciton}}>T_{\text{lattice}}\) . (c) at\(=\tau_{ex-ph}\) , the exciton temperature cools down and achieves a thermal equilibrium with lattice. \(g(E)\) and \(f(E)\) represent the density of states and Boltzmann distribution respectively. Q is the center of mass momentum of excitons. The line-thickness of the exciton dispersion (in red) represents the effective occupation. (d) Zoom-in of the dashed area in (b) sketches acoustic phonon assisted exciton photoluminescence.
In this letter, we elaborate the contribution of hot excitons to optical properties of monolayer MoSe2. With the intensity-dependent, temperature-dependent PL and PLE experiments combined with the simulations, we experimentally distinguish the influences of the exciton temperature and the lattice temperature in the PL spectrum. It is concluded that the acoustic phonon assisted photoluminescence accounts for the non-Lorentzian high-energy tail in the PL spectrum and the hot exciton effect is significant to optical properties of TMDs. Besides, the contrasting linewidth broadening behaviors owing to exciton temperature increase or lattice temperature increase are discussed. It is experimentally demonstrated that the effective exciton temperature can be tuned by excitation energy.
Results: