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With a classical three-dimensional ensemble model, we investigate the correlation behavior of the electrons from nonsequential double ionization of helium at intensities below the recollision threshold. At these laser intensities well below the recollision threshold, the two-electron momentum spectra pairs along the laser polarization show anticorrelated behaviors. This anticorrelated behavior becomes stronger when the laser intensity decreases, and then becomes weaker as the laser intensity further decreases. The responsible microscopic dynamics for the anticorrelated behavior and its intensity dependence is explored by back analyzing the double ionization trajectories.

We theoretically investigate the generation of ultra broadband supercontinuum from helium atoms exposed to a linearly polarized mid-infrared field. By adopting a UV trigger pulse to the mid-infrared field, the continuous harmonic yields are significantly enhanced by 3.5 orders, and a supercontinuum with the width of 230 eV is observed. The spectrum can support a sub-20 as pulse, which is below one atomic unit of time (24 as). The short quantum path is selected by adjusting the time delay between the UV pulse and the mid-infrared pulse, then broadband single 70 as pulses with tunable central wavelengths are obtained, which can be extended to the ‘water window’ region (284–543 eV).