Exploration Of Spectral And Temporal Structures Of High-order Harmonic Generation In An Intense Laser Field

High-order harmonic generation(HHG) has attracted much attention over several two decades. HHG provides a promising method to produce the coherent extreme ultraviolet(XUV) source in the attosecond time scale, leading to applications such the observation and control of the electronic dynamic behaviors.The HHG procedure can be understood by the semiclassical three-step model,the electron tunnels through the barrier formed by the coulomb potential and the laser field in the beginning, then it oscillates quasifreely driven by the laser field and acquires additional kinetic energy, and finally it can recombine with the parent ion and emit radiation.One of the most interesting subjects is concerned with the dynamical origin of the HHG. Although the semiclassical three-step model can obtain the simple physical picture of HHG, which can not reasonably explain complex and strongly non-linear phenomenon,it is very necessary to explore the mechanism of the HHG from an ab initio simulations. We employ a synchrosqueezing transform(SST)to analyse the time-frequency spectra of the HHG from hydrogen atom and explore the dynamic behaviors of the HHG.The TDSE is solved accurately and efficiently by means of the time-dependent generalized pseudospectral(TDGPS) method,the numerical method has the advantages of the most basic:(i) choose a nonuniform spatial grid. This discretization is characterized by denser grids near the nuclear origin and sparser grids for larger distances,which can obtain high calculation precision with a very small amount of grid points and can greatly improve the computational efficiency.(ii) Due to the calculations of atom and molecule physics in the strong field always involve the coulomb potential,when r?0,there will be a coulomb singularity problem. Using the method of TDGPS can change the scope of space [r_min,r_max] to [-1,+1]which can choose grid points in the of more closer nuclear and effectively sovle the problem of coulomb singularity when select traditionally and uniformly grid points.we have presented an ab initio study on the HHG of hydrogen atom in an intense800-nm infrared laser fields by solving accurately the TDSE by using the TDGPS method we obtain spectral and temporal structures of high-order harmonic generationvia a synchrosqueezing transform.Combining with the classic quantum path analysis,we explore the dynamic mechanism of below- and under- threshod HHG.Our results reveal that multirescattering trajectories contribute to near-threshold harmonic generation is more.In addition, The synchrosqueezing time-frequency analysis allows us to distinguish the contributions of the multi-rescattering trajectories and resonant trajectories in HHG and shows the details of the spectral and temporal fine structures of HHG, providing insights regarding HHG mechanisms in different energy regimes.