Bilayer graphene in strong ultrafast laser fields.

We theoretically investigate the interaction of an ultrastrong femtosecond-long linearly polarized optical pulse with AB-stacked bilayer graphene (BLG). The pulse excite electrons from the valence into the conduction band (CB), resulting in finite CB population. Such a redistribution of electrons results in the generation of current which can be manipulated by the angle of incidence of the pulse. For the normal incidence, the current along a direction transverse to the polarization of the optical pulse is zero. However, the interlayer symmetry is broken up by a finite angle of incidence due to which BLG possesses a single axis of symmetry. Thus, for an oblique incidence, if the pulse is polarized normal to the symmetry axis then there is an induction of electric current in the direction perpendicular to the polarization of the pulse. We show that the magnitude and the direction of such a current as well as charge transfer along this direction can be manipulated by tuning the angle of incidence of the laser pulse. Further, the symmetry of the system prohibits the generation of transverse current if the pulse is polarized along the axis of symmetry of BLG.

Energy spectra and optical transitions in germanene quantum dots.

The band gap of buckled graphene-like materials, such as silicene and germanene, depends on external perpendicular electric field. Then a specially design profile of electric field can produce trapping potential for electrons. We study theoretically the energy spectrum and optical transitions for such designed quantum dots (QDs) in graphene-like materials. The energy spectra depend on the size of the QD and applied electric field in the region of the QD. The number of the states in the QD increases with increasing the size of the dot and the energies of the states have almost linear dependence on the applied electric field with the slope which increases with increasing the dot size. The optical properties of the QDs are characterized by two types of absorption spectra: interband (optical transitions between the states of the valence and conduction bands) and intraband (transitions between the states of conduction/valence band). The interband absorption spectra have triple-peak structure with peak separation around 10 meV, while intraband absorption spectra, which depend on the number of electrons in the dot, have double-peak structure.

A quantum dot in topological insulator nanofilm.

We introduce a quantum dot in topological insulator nanofilm as a bump at the surface of the nanofilm. Such a quantum dot can localize an electron if the size of the dot is large enough, ≳5 nm. The quantum dot in topological insulator nanofilm has states of two types, which belong to two ('conduction' and 'valence') bands of the topological insulator nanofilm. We study the energy spectra of such defined quantum dots. We also consider intraband and interband optical transitions within the dot. The optical transitions of the two types have the same selection rules. While the interband absorption spectra have multi-peak structure, each of the intraband spectra has one strong peak and a few weak high frequency satellites.