| The realization of the ultra-short attosecond pulses generation(1as=10-18s) provides a powerful tool to control the ultrafast electronic processes. As we know, an isolated attosecond pulse is very important to observe and probe the electronic dynamical behaviors in the atoms and molecules. To produce the single attosecond pulses, many methods are proposed, such as the amplitude gating, the ionization gating, and the polarization gating. In recent years,as one of the most promising routes of the generation of attosecond pulse, the high-order harmonic generation(HHG) in strong laser fields has been a subject of much interest. According to the previous studies, the HHG spectrum is characterized by a rapid drop at low orders followed by a broad plateau where all the harmonics have a similar amplitude, and finally a sharp cutoff beyond which no further harmonic emission is seen. To obtain the ultra-short attosecond pulses, the extension of the HHG cutoff and broadband supercontinuum harmonic spectrum is required.We presented a numerical study of the HHG and attosecond pulse generation from helium atom driven by the combined field of two laser pulses with the same color and a mid-infrared laser pulse in nano-structure plasmon by means of solving the one-dimensional time-dependent Schr?dinger equation. The results show that the cut-off position of the harmonics is obviously extended in the combined field, and the HHG shows the supercontinuum distribution in plateau area. Furthermore, the absorbing effects determined by the structures of the plasmon for the electrons in the continuum states have the strong effects on the cut-off position of the harmonics. We find that the contributions from several trajectories of the electrons are enhanced, and other trajectories are suppressed by changing the plasmon position. As a result, an isolated 28 as pulse is produced. |
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