Abstract
The generation of the high-order harmonic and the attosecond pulse from He atom driven by the near-infrared (NIR) field combined with the XUV pulse has been theoretically investigated by solving the three-dimensional time-dependent Schrödinger equation. The results show that by properly adding the XUV pulse into the NIR field, (i) not only the harmonic yield is enhanced, caused by the laser-induced excited state effect; but also the multi-cutoff extension of the harmonics can be found, caused by the absorption of the extra XUV photons during its recombination process. (ii) With the introduction of the XUV pulse, the frequency modulation of the high-order harmonic generation (HHG) can be found and controlled. (iii) By directly superposing the harmonics in the cutoff region, two isolated XUV pulses with the durations of 150 as can be obtained.
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