Compression of extreme ultraviolet pulse for atom with resonant structure exposed to an infrared laser field

Abstract

The short attosecond (as) pulse is a basic tool for probing the ultra-fast electronic dynamics in matter. High-order harmonic generation (HHG) of atoms exposed to intense laser field is the most promising method of producing the short attosecond pulses. Therefore, the generation of ultra-short attosecond pulses through HHG has been of great interest. How to obtain the ultra-short pulse from HHG has been a hot research subject in recent years. In the present paper, we investigate the characteristic of HHG from atoms with both resonant and non-resonant structure (for short, the general atom) by using numerically solving a one-dimensional time-dependent Schrodinger equation of atom driven by two-color field (infrared (IR) laser + extreme ultraviolet (XUV)). We find that the HHG spectra from resonant atom are obviously different from those of the general atom. For a resonant atom, besides the great increase of the intensity of HHG at some energy (resonant energy + ionized energy), the intensity of HHG at the central frequency of XUV pulse is sensitive to the intensity of XUV pulse. Even the intensity of XUV pulse is weak, the enhancement of HHG spectra from resonant atom is still significant, while the general atom does not has this feature. Only the strength of the XUV pulse is much stronger than that in the case of resonant atom, the spectra of HHG near the center frequency of XUV from atom with non-resonant structure can significantly be enhanced. More importantly, adjusting the time delay of two-color laser pulse makes the width of input XUV pulse compressed obviously in the case of the resonant atom. By performing the time-frequency analysis of Morlet transform, we explain the compression of the attosecond pulse. The reason is that the relation of the input XUV pulse frequency to the resonant frequency of HHG for resonant atom makes the bandwidth of HHG in the region of the center frequency of XUV wider than that of the input attosecond pulse during the emission. Thus, we can obtain shorter pulse by superposing several orders HHG among the enhanced regions. Finally, we propose a way to compress the width of the input XUV pulse by using filter-multi-feedback method. Based on our scheme, the width of the input XUV pulse can be compressed from 200 as to 120 as, thereby offering a new method of obtaining shorter attosecond pulse in experiment.