Theoretical Research On The Structural Characteristics Of High-order Harmonic Spectra From He And HeH²⁺

High order harmonic generation,one of the nonlinear phenomena which are produced by an intense laser field on atoms or molecules,have attracted much attention owing to their wide application.Based on the solution of the time-dependent Schr?dinger equation,this paper studies the structural characteristics of high-order harmonic spectra from He and HeH²⁺.The details are as follows:?1?The modulation of high order harmonic spectra from He at long wavelength and its suppression are theoretically investigated.It is found that a quantum path interference and a multiple rescatterings interference as two fundamental factors cause obvious spectral modulation.Numerical results show that the coherent superposition state in combination with plasmon-enhanced laser fields,is robust for effective suppression of spectral modulation and broadband supercontinuum generation.Then an isolated 78-as pulse can be directly generated.?2?The generation mechanism of below-threshold harmonics with spectral shift from HeH²⁺is numerically investigated.It is found that the below-threshold harmonics with spectral shift can be observed when the laser pulse with appropriate intensity is used.Furthermore,combined the time-dependent ionization probabilities and electronic wave packets'probability densities show that the generation mechanism of below-threshold harmonics with spectral shift is derived from quantum transition between electronic states of HeH²⁺.Time-frequency distributions are adopted to further provide that the generation mechanism of below-threshold harmonics with spectral shift is attributed to the transition between the ground and first excited state of HeH²⁺.?3?The time-delay dependent harmonic spectra from He is investigated.Changing the time delay in high-order-harmonic generation from helium atom driven by intense two-color laser pulses is studied to analyze the structures in the high-energy region of the time-delay harmonic spectra.Depending on the high-energy harmonic,the yield of the harmonic reveals three kinds of oscillations with respect to the time-delay.Based upon numerical calculations of the time-dependent Schr?dinger equation and the classical equation of motion,both the physical picture of each structure and corresponding physical mechanism are investigated and discussed in detail.