(color online) (a) Band structure at K point in absence (left) and in presence (right) of pump pulse absorption. The red lines mark the optical excitation. Due to the presence of photo-excited carriers, there is reduction of bandgap (ΔE_g) and exciton binding energy (ΔE_b), the overall shift of the excitonic absorption resonances: . (b) Transient absorption spectra of 1L-MoS₂, recorded at fixed pump–probe delay for three pump photon energies that are in resonance with A (1.88 eV) and B (2.06 eV) exciton and out of resonance with C exciton (3.1 eV) respectively. (c) Calculated transient absorption spectra of 1L-MoS₂ for three pump photo energies by including renormalization of electronic gap and excitonic binding energy. (d) Transition from the low density excitonic regime to a dense electron–hole plasma. (e) Reflectance contrast spectra of WS₂ bilayer without pump excitation and 0.4 ps after excitation by a pump pulse at 70 K. (f) Schematic representation of the optical absorption at low and high carrier density regimes as predicted by theory. (g) Subtraction of a linear background from panel (d). Panels (a)–(c) are reproduced from Ref. [55]; and panels (e)–(g) are reproduced from Ref. [32].