Ultrafast Dynamics Study Of Atoms,Molecules,and Crystal Materials Interacting With Strong Laser Fields

As an effective method to explore the internal structure and dynamic processes in materials,the interaction between ultra-intense ultrashort laser field and materials has been one of the most active fields and frontier of science after decades of rapid development.This thesis theoretically studied ultrafast dynamics phenomena of matter interacting with strong laser fields.We extended the research object from traditional gas-state atoms and molecules to solid-state materials.In chapter one,we briefly summarized the development of laser technology and its importance in strong field science.Then,we introduced some possible physical processes when gaseous atoms and molecules interact with intense laser pulses,focusing on the mechanism,significance,and unsovled problems of atomic and molecular high-order harmonic generation(HHG).We also detailed some ultrafast dynamic imaging method for molecular structures,and subsequently we presented the potential applications of interaction between strong laser fields and solid materials as well as the research progress of solid HHG in experiments and theory.In chapter two,as for the question how to improve the conversion efficiency of HHG,we studied the HHG from H2+ and HeH2+ in intense laser fields using the time-dependent quantum wave packet method.We found molecular and atomic plateaus of harmonic spectra can be effectively distinguished at large internuclear distances,where the harmonic efficiency of the molecular plateau is several order of magnitudes higher than that of the latter.We put forward a physical model on the origin of the molecular supercontinuum and revealed that this plateau results directly from the interference of intramolecular electronic wave packet localized in two potential wells following the laser field.When the macroscopic propagation effects are included,we theoretically discovered that the molecular harmonic plateau can be greatly enhanced to generate intense isolated attosecond pulses by optimizing gas density of target.In chapter three,aiming at how to extend the cutoff energy of HHG,we performed a theoretical investigation of HHG from He atom in spatially nonhomogeneous laser fields by solving the one-dimensional and three-dimensional time-dependent Schrodinger equations.We determined that the harmonic cutoff is extended in both symmetric and asymmetric nonhomogeneous laser fields,and the spatial symmetry of the nonhomogeneous field greatly influences the movement of electronic wave packet,which subsequently affects harmonic emission.In combination with the two-color field technique,multi-cycle laser pulses with asymmetric spatially nonhomogeneous characteristics are capable of generating a coherent extreme ultraviolet supercontinuum,which can produce isolated few-attosecond pulses.In addition,the HHG from H2+ in spatially nonhomogeneous laser fields was studied.At large internuclear distances,minima were clearly observed in high energy part of harmonic spectra,which can be attributed to the two-center interference in diatomic molecule.In chapter four,based on laser-induced electron diffraction and partial one-dimensional molecular alignment,here two effective methods for reconstructing two-dimensional structures of polyatomic molecules CF4 and CICF3 by electron diffraction pattern extracted from above-threshold ionization spectra were provided.We demonstrated that electron diffraction images in both scattering angle and broadband energy can be utilized to retrieve structure information,and they complement each other.In chapter five,we extended HHG study from the traditional gas-phase systems to crystal materials.In particular,we theoretically investigated the HHG from alpha-quartz SiO2 under a strong laser field by solving the extended semiconductor Bloch equations.The accurate band structures and dipole moments between different bands were obtained from state-of-the-art first-principles calculations.We found that the shape of k-space-dependent dipole moment plays an important role in harmonic generation.Base on that dependence on dipole moment,we unveiled that symmetry group of crystal greatly affects the harmonic spectrum from the solid materials.Moreover,two-color synthesized field was used to achieve a supercontinuum near cutoff region in harmonic spectrum,and isolated attosecond pulses can be obtained directly by filtering out the solid harmonics with low energy.