(color online) (a) Reflectance contrast spectra of a WS₂ layer at different time delays after excitation, vertically offset for clarity (n.p. denotes the spectrum without pump excitation), showing the evolution of the spectra. The initial bleaching of the exciton peak and inversion both develop instantaneously within the experimental resolution of 0.4 ps, indicating that initial relaxation processes occur on the 100 fs timescale. After ∼ 250 ps, the optical response fully returns to the initial condition. (b) Illustration of the transition from the low-density excitonic regime to a dense electron–hole plasma.^[78]