Figure 1: Li et al. used short ($35$-fs) laser pulses to excite electrons and holes in doped graphene monolayers. The figure shows the distribution of electron and hole states in a graphene-like material (that is, one with a linear dispersion) for (left) the case where the optically excited charge carriers don’t thermalize within the time scale of the optical pulse, so that an optical “pump” source (grey) can only excite charges into a narrow band of states, and (right) for the case in graphene, where the thermalization time is much less than the pulse time, so the pump light source (blue) can set up a broadband, population-inverted quantum state of extremely dense fermions. Stimulated emission from this state (red arrow) offsets the absorption loss, which leads to optical gain.