High-order Harmonic Generation And Attosecond Pulses Generation From Different Combined Laser Field

The interaction between an intense laser pulse and an atomic ormolecular system leads to a series of nonlinear nonperturbativephenomena, such as above threshold ionization (ATI), Coulombexplosion, high-order harmonic generation (HHG), etc. The HHG is thebest known and the most studied, due to its important applications ingenerating extreme-ultraviolet radiation and isolated attosecondpulses. The realization of isolated attosecond pulse enables us totrace ultrafast processes of electrons in atoms and molecules. In thisarticle, we investigate the atomic and molecular HHG and attosecondpulse generation from different combined field by solving thetime-dependent Schr dinger equation.The main contents in this paper are as follows:(1) the HHG andisolated attosecond pulse generation of He atoms from a two-colorcombined laser field. The results show that the harmonic efficiencyfor the case of the coherent superposition state is noticeably higherthan the case of the ground state. And by optimizing the intensities ofthe two pulses, the spectrum with broader plateau region and higherefficiency is obtained. When the relative phase is properly selected,the plateau of the spectrum is more slippery, which is beneficial toachieving an isolated attosecond pulse.(2) the HHG and isolated attosecond pulse generation of H2+molecules from the combined fieldof the trapezoidal laser field and static field. The results show that theHHG cutoff is obviously extended and the long path is seriouslysuppressed for the case of the combined field. Additionally, based onthe time-frequency distributions, the time-dependent electronic wavepacket probability densities, and the combined Coulomb and laserfield potentials, the physical mechanism of the quantum selection isdiscussed in detail. Finally, a90-as isolated pulse is generated bysuperposing a proper range of the HHG spectrum.(3) the HHG andisolated attosecond pulse generation of HeH2+molecules from thecombined field of the trapezoidal laser field and half-cycle-like field(HCLF). By performing the time-frequency distributions and theelectronic wave packet probability densities, we found that theoptimizing combined field is not only useful to extend the HHG cutoff,but also to simplify the recombination channels through controllingthe electron localization. In addition, by adjusting the intensity of theHCLF, a dominant short quantum path is selected to contribute theHHG spectrum. As a result, a75-as isolated attosecond pulse isobtained by superposing a proper range of the harmonics.