Theoretical Investigation Of The Electron Collision Dynamics In Crystal High-order Harmonic Generation

In recent years,ultrafast laser technology has made great development,in which the generation and application of high-order harmonics have opened the door of attosecond science and attracted much attention.Scientists have extensively researched the HHG of atoms and molecules in the gas phase,but the yield of the HHG is still a bottleneck that is difficult to break through.Due to the natural high density,regular alignment,and strict periodicity of the crystal materials,which can overcome the difficulty of the low efficiency of the gaseous medium to induce higher harmonic yields.Therefore,the HHG in crystals has become an area of hot research interest,and scientific researchers have also made a series of progress,such as the generation of the ultrashort laser pulses and the extreme ultraviolet light sources.It makes the study of solid material gradually advance to the smaller spatial scale and the shorter time scale.The study of the internal dynamic process of matter can be carried out by extracting the dynamics of the electrons.The realization of real-time measurement of electron dynamics has great significance for understanding the microstructure of the matter,and it is also the basis for us to understand the HHG process in crystals.The high-order harmonics in the crystal are mainly contributed by the intraband harmonics with energy below the band gap and the interband harmonics with energy above the band gap.Therefore,controlling the Bloch oscillation of the electron can directly regulate the harmonics below the band gap.On the other hand,controlling the transition process of the electron can realize the manipulation of the harmonics in the above-band-gap regime.The real-time control of the electron dynamics process is the fundamental method to modulate the high-order harmonic radiation process.In this thesis,we combine the semiconductor Bloch equations with the semiclassical model to theoretically study the modulation of the HHG in crystals by controlling the laserinduced electron dynamics.The main work is listed as follows:(1)A scheme of manipulating the radiation of the HHG in crystals by using the orthogonally polarized two-color(OTC)laser fields is proposed.It is found that the yield of the harmonics is sensitive to the relative phase and intensity of the laser field.In single-active-electron approximation,the generation of high-order harmonic induced by the OTC laser fields from ZnO crystal is theoretically investigated.The relative phase and field strength of the OTC fields can effectively control the ionization and recollision process of electron hole pairs,and then regulate the key parameters such as the yield,phase and maximum photon energy of the radiated high-order harmonics.By extracting the information of the semiclassical recollision trajectories,a method for fastly estimating the spectral intensity of the HHG is proposed.(2)A multiple collision model in real space is proposed,and it is found that the multiple collision trajectories can help us reveal the harmonic suppression phenomenon and combined with the semiclassical recollision trajectories can comprehensively explain the dynamics process of electrons and holes.Firstly,we discuss the multiple collisions,in which the electrons and holes are produced at different times,and the collisions are classified according to the interval between the birth times of electrons and holes.The multiple collision trajectories overlap in some energy intervals,which leads to the suppression of the specific harmonic.The overlaps of the multiple collision trajectories give rise to the suppressions of the harmonics.The collisions between the electrons and holes which are born at the same time but at different positions in real space are also studied,and these collision trajectories even perform better in the higher-order harmonic region.The multiple collision model plays an important role in analyzing the HHG spectra,which is also a helpful supplement to the recollision model.The real-time probe and control of electron dynamics are studied theoretically,which can further promote the understanding of the HHG in crystals,and provide theoretical support for the design of relevant experimental schemes.