(color online) (a) Schematic of the optical stark effect. The dashed black and yellow lines denote the unperturbed ground and exciton states, respectively. The pump photon energy is lower than the exciton resonance energy. (b) Absorbance of monolayer WS₂ with (red dash) and without (black) pump excitation that simulates the stark effect. (c) The simulated change of absorption induced by the pump pulses. (d) The transient reflection spectrum of monolayer WSe₂ at zero delay of probe. Panels (a) and (d) are reproduced from Ref. [34]; and panels (b) and (c) are reproduced from Ref. [33].