Figure 4.

Pump-induced Absorption. Pump-induced absorption as function of the probe energy for four different times. The colored spectra show the calculation at room temperature, whereas the black lines correspond to the same experiment at 77 K (scaled by a factor of 0.2). The displayed times correspond to the time axis of the last section. For early times the occupation of the higher excitonic states yields multiple features in the absorbtion for energies below 150 meV. At low temperatures, the population inversion between 2 s and 2p gives rise to optical gain for energies around 40 meV. For larger delay times the absorption spectrum is dominated by the response of the 1s-np (n = 2,3, etc.) transitions of excitons in the ground state.