In this study the classical three-dimensional ensemble model is utilized for investigating the role of final state electron-electron repulsion in forming the ultimate correlated two-electron momentum distribution. For the first time, a comprehensive analysis has been made to thoroughly understand such repulsive force associating with each microscopic mechanism of nonsequential double ionization. The helium exposed to 800-nm laser with two representative intensities of 3.5×1014 W/cm2 and 4.5×1014 W/cm2 is used for illustration. The results indicate the dominance of electron-electron repulsion in direct and recollision-induced excitation with subsequential ionizations. While its contribution in case of exchanging-state mechanism gradually emerges as the laser intensity increases.
High-order harmonic generation (HHG) driven by two non-collinear beams including a fundamental and its weak second harmonic is numerically studied. The interference of harmonics from adjacent electron quantum paths is found to be dependent on the relative delay of the driving pulse, and the dependences are different for different harmonic orders. This frequency dependence of the interference is attributed to the spatial frequency chirp in the HHG beam resulting from the harmonic dipole phase, which in turn provides a potential way to gain an insight into the generation of high-order harmonics. As an example, the intensity dependent dipole phase coefficient α is retrieved from the interference fringe.
We demonstrated a femtosecond Ti:sapphire oscillator at 10?MHz repetition rate by introducing a specially designed multi-pass telescope to increase the cavity length. Stable mode locking laser with average power of 200 mW is obtained under 532?nm laser pump of 5?W power, corresponding to a single pulse energy of 20?nJ. Based on this laser, we further explored the characteristics of pulse duration and spectrum with the net intracavity group-delay dispersion (GDD). The result indicates that the near-bandwidth-limited pulse can be obtained when a little net negative GDD exists, the optimized pulse duration as short as 56?fs was generated in this case. The positive GDD will lead to a widened pulse with the increase of positive GDD, and the pulse duration can extend to more than 600?fs with the largest allowable positive GDD.
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