Study On The Generation Of Low-order Harmonic Radiation By He Atom

The interaction between light and matter is the one of the leading and central subjects of physics disciplines as well as the main way for human beings to understand the world.With the rapid development of laser technology during the last several decades,strong field physics as a brand-new field has brought people to recognise a series of interesting and strange phenomena in the process of the interaction of light and matter,such as multiphoton ionization(MPI),tunnel ionization(TI),above-threshold ionization(ATI),nonsequential double ionization(NSDI),and high-order harmonic generation(HHG),etc.In the past thirty years,HHG has been paid more attention to and achieved good research results,but researchers are interests in the characteristics of harmonics and their applications near the highorder harmonic plateau region and cutoff locations.It is not known for most people that the harmonics near the ionization threshold,Recently,some researchers focus on the low-order harmonic generation(LOHG)near the ionization threshould of atoms because its applications on a high-repetition-rate and high-intensity light source.However,the LOHG process can not be explained in the semiclasscal three-step model,because LOHG radiation involves the transition between the bound states of atoms and continuum states.In this case,both the continuum states and the bound states of the target atom play a important role in producion of HHG radiation,which makes the HHG process becomes more complex.Therefore,it is necessary to study the LOHG in more detail.This paper mainly focuses on The LOGH below ionization threshould of He atom in a laser pulse is investigated by using solving the three-dimensional(3D)timedependent Schrodinger equation with the single-active-electron approximation.In the calculation,an angular momentum-dependent exact model potential of He atom was used.The side-peak structures of the low-order harmonics appeared in wavelength335-470 nm.By detail analysis of a synchrosqueezing transform technique,we find two parts in some side-peak of LOHG,one includes the transition of the dressed states of the excited states 2p,3p 4p with Stark shift,the other is field-free transition between the excited states and the ground state.And one part in some side-peaks onlyinclude one part with the dress state of atom.By introducing an energy relationship,we can explain the locations of the side peaks of LOGH,which is in a good agreement with the population of the excited states.Our results show that the energy difference between some side and main peak of the LOGH reflect the largest ACStark shift of the excited states of He atom in the laser field.For 2p state,the Stark shift is 0.9Up,and for 4p state,it is about Up.At the same time,the red shift of the main peaks is caused by the contribution of dressed state.