Figure 3

Illustration of the dynamic processes of the carriers in the rate-equation model.
The bulk bands and surface state are shown by parabolas and inclined lines, respectively. Three stages of the carrier dynamics are shown in time sequence: (a) carrier distribution to the bulk bands and surface state (oblique arrows) just after photoexcitation (vertical arrow), (b) carrier cooling via phonon emission (oblique arrows) and carrier escape from the bulk bands (dashed arrows), (c) the subsequent carrier relaxation near the bulk band minimum or maximum and the Dirac point.