First-Principle Simulations Of Ultrafast Dynamics In Condensed-Matter Systems Under Strong Laser Pulses

Under strong field irradiation,different degrees of freedom(e.g.photons,electrons,phonons and plasmons)are coupled in condensed-matter systems.The complex interactions will lead to highly nonlinear electronic and photonic behaviors and sometimes induce the emergence of new quantum states,which have basic scientific significance and broad application prospects.With the continuous development of experimental techniques,various kinds of novel strong-field phenomena are observed in solid-state materials.Deep understanding of the underlying excited-state dynamics has become a research hot topic in the field of optics and material scienceIn this thesis,based on ab initio time-dependent density functional theory(TDDFT),we focus on the ultrafast processes driven by strong-field in condensed-matter materials.The systematic studies on high-harmonic generation(HHG),photoemission and topological phase transitions aim at clarifying the inherent linking among the structure and electronic properties and dynamics upon excitation.The main contents of this thesis are listed as below.1.In MoS2 monolayer,we studied the dependence of HHG yields on electronic properties and the ultrafast control of the underlying dynamics.Firstly,we reveal that flatter band dispersion and Berry curvature enhance the harmonic yields due to the cooperative effect of intraband and interband transitions.Band structure,including band gap and energy dispersion,can be retrieved with high reliability by monitoring the strain-induced evolution of HHG spectra.Secondly,we demonstrate that the sub-cycle electron dynamics in MoS2 can be controlled via varying the relative phase of the two-color laser components.Two-dimensional materials provide a unique platform where both bulk-like and atomic-like electron dynamic behaviors can be achieved.These findings will help design two-dimensional-material based optoelectronic devices.2.We investigated the photoemission dynamics from single-walled carbon nanotubes(SWCNs).In experiment,extreme strong-field photoemission with astounding nonlinearity was reported in semiconducting SWCNs,whereas metallic SWCNs behave similar with conventional metallic nanostructures.Based on first principle simulations,we reveal that the distinct photoemission behaviors between metallic and semiconducting SWCNs are closely related to their unique electronic properties near the Fermi level.In metallic nanotubes,the excitation of high-energy occupied states is forbidden due to the linear energy dispersion,which is necessary for the emergence of the ultra-high nonlinearity.Furthermore,dynamic field emission patterns in the early stage of the photoemission are provided,which are determined by both the atomic and electronic structures of SWCNs,showing real-space pictures of the field emission characteristics.3.Using TDDFT molecular dynamic simulations,we demonstrated that the topological phase transition direction of type-II Weyl semimetal WTe2 can be controlled by tuning the parameters of linearly polarized laser pulses.We demonstrated that the carrier excitations pathways around the Weyl points not only depend on the chirality selection rule,but also determined by the symmetry features of atomic orbitals comprising Weyl bands.We provided phase diagram of laser-driven WTe2 topological phase transition on the dependence of photon energy and incident angle.Our work provides a new insight into controlling Berry flux field singularity around the Weyl nodes.We explored excited-state dynamics in different kinds of solid state materials,which involve complex interactions under strong field.These works will make a contribution to the further application in ultrafast optoelectronic devices.