Ultrafast Processes in Phosphorene and Weyl semimetals

In this dissertation, we study theoretically the nonlinear response of phosphorene and Weyl semimetals to an ultrafast laser pulse. We apply a femtosecond pulse and investigate the electron dynamics of the system in terms of the conduction band population. The optical pulse induces a finite conduction band (CB) population in the reciprocal space. In case of phosphorene, which is a semiconductor with a band gap ~ 2 eV, the electron dynamics is highly irreversible which means that the residual electron CB population after the pulse is large and is comparable to the maximum conduction band population during the pulse. The large CB population appears near the Γpoint where the dipole matrix elements between the valence band and the conduction band is strong. Also, the optical pulse causes both interband and intraband electron dynamics during the pulse which a combination of both produces a net current through the system. The electron dynamics of three-dimensional topological Weyl semimetals in an ultrafast linearly polarized pulse is coherent and highly anisotropic. For some directions of pulse polarization, the electron dynamics is irreversible, while for other directions of polarization, the electron dynamics is highly reversible. Such high anisotropy in electron dynamics is related to anisotropy in interband dipole matrix elements. The optical pulse also causes net charge transfer through the system. The transferred charge has highly anisotropic dependence on polarization direction with almost zero transferred charge for some directions. Furthermore, we use the ultrafast pulse to illustrate the topological properties of Weyl semimetals such as chirality and topological resonance. The femtosecond pulse induces the topological resonance in Weyl semimetals. The topological resonance manifests itself in the distribution of the CB population. Such distribution in the conduction band is highly structured and is determined by the interference of the topological phase and the dynamic phase. The topological phase originates from the dipole. The topological resonance causes the Weyl points to be populated selectively, and this could be useful in applications such as optoelectronic devices