A Single Short Pulse Produced By The Excited State Of Na Atom Exposed To An Mid-infrared Laser

Recently some intense laser field within range of certain strength are widely used,one of the important reasons is that the interaction between intense laser field and atoms or molecules can produce high-order harmonic generation(HHG),which is a major source of synthesis of attosecond pulse and single pulse.Based on this theory,this paper selects alkali metal atom Na as the target atom and different states as initial wave functions,HHG caused by the target atom interacting with laser field that around threshold can be used to synthesize single pulse with range of center frequency in high frequency band of visible and middle ultraviolet parts of the spectrum,the specific contents are listed as follows.Firstly,the time-dependent Schr?dinger equation(TDSE)of alkali metal atom Na in mid-infrared laser field is solved with pseudo-spectral method and used precise model potential function in this theory.Because bound states energies corresponding to experimental results obtained by using this model potential function,it is possible to study characteristics of high-order harmonic when atom's initial state is in the excited state and atom experienced the over-the-barrier ionization(OTBI).Secondly,the atomic excited states,4s and 5s,are set as the initial wave function to obtain high-order harmonic,respectively.Studies show that the high-order harmonic near the threshold has the characteristic of super straight when the initial state is in the excited states,and a single pulse with center frequency in high frequency band of visible and middle ultraviolet parts of the spectrum can be obtained by superposing certain harmonic orders near threshold.Finally,it shows that the differences of target atomic low order harmonic generation between over-the-barrier and tunneling process by analyzing the target atom ionization probability and the time-frequency characteristics of high-order harmonic.