This opposite phase relationship between ground state bleach and stimulated emission provides advantages in analyzing the measured spectral dynamics. A typical monolayer MoS 2 photoexcitation and relaxation process is described in Figure . The electron is photoexcited from the valence band to the conduction band and excitons are formed. An exciton and an extra electron can further combine to form trions at a lower energy state (with a rate constant k 1 ), which occurs at 10-20 ps according to literatures . Lastly, trion recombination occurs with a rate constant of k 2 . Based on this kinetic model, the transient population of each state can be described as,