Study On The Oscillation Structures And Dynamics Induced By Sub-femtosecond Laser Field In High-order Harmonic Generation Of Atoms

With the rapid development of laser technology,the laser-matter interaction produces a series of new nonlinear optical phenomena,such as high-order harmonic generation(HHG).So far,HHG has become an important way to produce isolated attosecond laser pulses.The emergence of attosecond laser pulses provides the possibility to observe and manipulate ultrafast processes in real time in recent years.At present,the use of attosecond transient absorption spectroscopy to probe electron dynamics has become an important research method.In this paper,sub-femtosecond laser is used to induce He atom,alkali metal Na atom and Cs atom to radiate high-order harmonics,and its transient absorption spectrum structure is explored.In our results,we observed sub-cycle oscillation structures and investigated the corresponding dynamics mechanism.The theoretical method is to solve the time-dependent Schr(?)dinger equation(TDSE)of atoms in strong laser field by means of generalized pseudospectral method(TDGPS).TDGPS can calculate the result effectively and accurately through non-uniform discretized grid points.Firstly,for He atom,the two-color laser field pulse composed of near-infrared laser field pulse(NIR)and sub-femtosecond ultraviolet pulse.By changing the time delay between the two laser fields,the transient absorption spectra of He atoms are obtained,and the subcycle oscillation structure is observed.Comparing the calculation results of this paper with the previous research results of He atoms,we can see that our theoretical method is more accurate and efficient.Secondly,since most of the current studies on subcycle oscillation dynamics remain on the interaction between rare gases and laser fields,this theoretical method is extended to the interaction between alkali metal atoms and strong laser fields in this paper.In this paper,we studied the interaction of alkali metal Na and Cs atom with laser field,and the selection of laser field parameters is also adjusted.The central energy of the ultraviolet laser pulse is just the ionization energy of the atom.After the above adjustment,the ground state electron of the atom can absorb the energy in the ultraviolet laser field pulse,and it is easier to transition to the excited or continuous state.To sum up,we studied the subcycle oscillation structure in the transient absorption spectra of alkali metal atoms and provide a new understanding for the interaction between strong laser field and alkali metal atoms.