Theoretical Investigation Of Nonlinear Electron Dynamics Response In Materials Under Ultrafast Laser Irradiation

We investigate the interaction electron dynamics between ultrafast laser pulses andmaterials within the time-dependent density functional theory. Taking the CaF₂crystal asan example, we give a comparison of electron dynamics response between single anddouble pulses. Moreover, the nonlinear energy absorption and electron excitation processesare simulated by adjusting the polarization direction of the sub-pulse. Furthermore, we alsoinvestigate the energy absorption and electron excitation processes using two-colorattosecond pulses through adjusting the phase and separation time. Present resultsdemonstrate that the double pulses show lower electron excitation a nd energy absorptionthan the single pulse, which is in accordance with experimental higher ablation thresholdand smaller heat-affected zones of the double pulses. In addition, the curves of final excitedelectron number and energy absorption exhibit the quasi-symmetry about the axis of180°,which has not been reported yet. Plus, excited electron curve exhibits interesting oscillationbehavior with the increase of separation time between two pulses. Finally, the electrondensity distribution and electron occupation are displayed at the end of laser pulses.In addition, we simulate nonlinear electron dynamics in CaB6crystal under one-colorfemtosecond laser fields （400nm,800nm） and two-color cases （400nm+800nm） withdifferent relative phases of=0,π/4,π/2. The energy absorption and the electron excitationof CaB6crystal is observed in detail. Computational results indicate that for one-color case,the excitation behaviors are distinct due to the different frequencies. For two-color laser, weadjust the phase and obtain the asymmetric field, which causes the change of the dynamicsresponse comparing with the symmetric field.