An effective method to control the half-cycle harmonic emission process through the combination of multiple laser beams is proposed. The control process can be separated into three steps. In the first step, by properly choosing the chirp and the chirp delay of the fundamental pulse the relevant negative and positive half-cycle waveform for extending the harmonic cutoff can be obtained. In the second step, using either an asymmetric inhomogeneous field or a half-cycle unipolar pulse, the harmonic cutoff from the negative or the positive half-cycle waveform can be further extended. In the third step, by properly adding an ultraviolet pulse, the harmonic yield can be enhanced due to ultraviolet resonance ionization. Finally, by superposing some harmonics on the harmonic spectra, a number of isolated attosecond pulses shorter than 70 as can be obtained.

No abstract

The lower harmonic intensity is still the reason that limits the application of attosecond pulse. Thus, in this paper, on the basis of resonance ionization schemes, we propose the effective methods to improve the harmonic intensity of atomic and molecular systems. For atomic system (i.e. He atom), with the combination of chirped-UV pulse, not only the harmonic cutoff can be extended, but also the harmonic intensity can be enhanced, caused by the UV resonance ionization between ground state and excited state. As a result, the single harmonic energy peak (HEP) with the higher intensity and higher emitted photon energy can be obtained, which can support the attosecond pulses shorter than 40 as. For the molecular system (i.e. [Formula: see text] and [Formula: see text]), by properly choosing the pulse duration of the fundamental pulse, the time region of charge resonance enhanced ionization (CREI) can be well controlled. Therefore, the maximum HEP with the higher intensity can be obtained. Second, by properly adding a half-cycle unipolar pulse, the selected maximum HEP in the CREI region can be further extended. Thus, an intense and broad spectral region can be produced, which can generate the attosecond pulses with durations of 36[Formula: see text]as.