Features of the electronic spectrum and optical absorption of ultrathin Bi$_2$Se$_3$ films

The electronic spectra and relative permittivity of ultrathin (1-3 QL) films of Bi$_2$Se$_3$ topological insulator have been calculated by the density functional theory. The calculated spectra exhibit a characteristic feature: the range of 0.0-0.9 eV below the Fermi level contains two doubly degenerate valence bands (“U-bands”), which are geometrically congruent to low-lying spectral branches in the conduction band. It has been shown that the saturation of optical absorption can result in a significant rearrangement of the electronic structure and properties in the near infrared spectral range in the considered film. In particular, the semiconductor (in the absence of interaction with light) type of conductivity of the film can be changed to the metallic type of conductivity strongly nonlinear in the intensity of light.