Theoretical Study Of Ultrafast Electron Dynamics In Strong-field Radiation From Periodic Structures

The interactions of the strong femtosecond laser with atoms,molecules and con-densed matters contain many new physical phenomena,which are the frontiers of physical research.The research of strong field physics can be used to probe and control the ultra-fast dynamics of electrons in matters,which is meaningful and valuable.Recently,the target materials are changing from atoms and molecules to solids.As the development of the laser technology,high-order harmonic generation?HHG?from solids broaden to vacuum ultraviolet and extreme ultraviolet has opened the door of strong field physics in solids.HHG from solid can not only be used to generate the isolated attosecond pulse,but also can be used to probe the ultrafast electron dynamics and the band structure of the target materials.Although there are a lot of experimental and theoretical research,the microscopic mechanism underlying HHG from solids is still controversially debated,due to the high density and the periodic structure of solid.Based on the study of diatom-ic molecular HHG,we propose a mechanism of HHG from solids in real space,and the instantaneous electron dynamics in solids has also been investigated.Then we study the synchronized radiation of HHG and terahertz wave?THz?from solids driven by the two-color laser field.We establish the connection between the ultrafast electron evolution and the coherent radiation spectra,which is crucial for understanding the mechanism of the strong-field quantum coherent radiation from solids.Firstly,HHG from local-microfield enhancement is investigated using the simple diatomic model H₂⁺,which is initially prepared in large internuclear distance.The HHG spectroscopy is calculated by solving the time-dependent Schr?dinger equation?TDSE?using the split-operator technique in a large internuclear distance.Numerically we use asymmetric-spatial mask functions,multi-trajectories contributions are revealed in the harmonic spectra,which contribute to different frequencies of harmonic spectroscopy.Also we numerically solve the Newton equation to get the classical analysis,the maximum return energies show great agreement with the cut-off energies of HHG from different trajectories.Due to the tunneling ionization has a sensitive response to the instantaneous electric field,all the trajectories can be controlled by the carrier-envelop phase?CEP?of intense few-cycle laser field.It is shown that electrons migrating directly from one nucleus to the neighboring one without typical tunneling ionization contribute dominantly to the lower order harmonics,and the intensities are dramatically enhanced along with the enhancement of the local-microfield.The migration of electrons between nuclei in multi-nuclei model will mimic the dynamics of electrons in solids,which will shed light on the mechanism of HHG from solids.Secondly,we theoretically investigate the HHG from periodical structure driven by an intense laser pulse.Including the full bands,the single electron TDSE is numerically solved in the velocity gauge using Bloch states to obtain the emission spectra.The con-tributions from different crystal sites are identified using the localized Wannier functions and time-frequency analysis.It is found that the cutoff energy of HHG is depending on the migration distance of the electron and the instantaneous laser field strength when the electron transports into the related sites.We show that the coherence among different sites during electron propagation is crucial for HHG from solids which can be taken ad-vantage to control the individual site contribution to the certain frequency range of the total harmonic spectra.Thirdly,the HHG from periodical structure driven by an intense laser pulse is inves-tigated using the semiconductor Bloch equations?SBEs?.Under the lower field strength,the cut-off energy of HHG can be estimated by the recollision model.While increasing the field strength,the harmonic spectrum shows an extra plateau.Based on the temporal population of electron and the time-frequency analysis,the HHG in extra plateau is gen-erated by the Bragg-reflection electron at the edge of the first Brillouin zone.Due to the time response of the Bragg-reflection electron is much shorter than that of the recollision electron,the harmonics from Bragg-reflection electron is a potential source of the shorter isolated attosecond pulse.Fourthly,the HHG and THz from ZnO driven by two-color intense laser pulse are investigated using the two-band SBEs.Because of the weak second harmonics break the symmetry of the temporal electron wave packet,the even order harmonics emerge,and the higher THz yield than that in single-color field is also emitted.The macroscopic pho-tocurrent model of THz from ZnO is proposed,and the unified physical image of HHG and THz based on the rescattering process is illustrated.The coherent dynamics of the electrons can be controlled by the relative time delay between the two color fields,which can be used to improve the THz yield.When we increase the field strength,the THz yield increases firstly,then decreases and a radiation minimum occurs at the certain field strength of dynamic localization.The field strength is a root of the ordinary Bessel func-tion of order zero.This characteristic can provide theoretical guidance for the realization of strong terahertz radiation in the solids.