Figure 4

Schematic for the influence of the 2nd surface state (SS) and bulk bands (BBs) on the P-polarized THz generation from p-type Bi2Te3.
BCB: bulk conduction band. BVB: bulk valance band. The 2nd SS and BBs would be responsible for the energy loss of the photoexcited electrons. Thus, the surface field
induced by band-bending also results in the drift current
in the narrow bandgap materials.