Molecular Dynamics Simulation Of Femtosecond Laser Ablation Of Titanium Film Based On Two-Temperature Model

Femtosecond lasers can process nano-sized metal materials by virtue of their own advantages in breaking through the diffraction limit.In order for the femtosecond laser to perform micro-nano processing on the surface of the titanium metal film,it is necessary to understand the interaction between the femtosecond laser and the titanium metal film.The two-temperature model(TTM)is a theoretical model for studying the femtosecond laser heating metal and non-equilibrium transmission of energy inside the metal,and molecular dynamics(MD)simulation can observe the microstructure evolution during the femtosecond laser ablation of metal from the atomic scale.Based on the two-temperature model,the molecular dynamics simulation was used to study the process of femtosecond laser ablation of titanium film.The stress propagation and the temperature evolution of the titanium film absorption layer in the process can further help understanding ablation mechanism.In addition,studying the influences of laser parameters and multi-pulse femtosecond laser on the ablation results of titanium film can be used in practical applications to achieve different processing purposes by adjusting laser parameters.The main simulation method and conclusions of this paper are as follows:(1)First of all,the 200 nm titanium film was used as the research object,the titanium film absorption layer was layered,and MATLAB was used to solve the TTM to obtain the equilibrium lattice temperature at seven different positions of the titanium film absorption layer.Secondly,the FS-EAM potential function was selected to build a titanium film model with a thickness of about 200 nm in the LAMMPS software.The seven parts of the titanium film absorption layer were heated to the specified lattice temperature,and the rest was kept at300 K.Finally,the internal thermal conduction of the titanium film was simulated in the NVE ensemble.(2)Through the above simulation method,the ablation threshold of a single-pulse femtosecond laser with a wavelength of 800 nm and a pulse width of 300 fs to ablate a 200nm titanium film is 0.053 J/cm~2.The reason for the fracture of the surface layer of the titanium film under the ablation threshold is the spallation caused by the tensile stress.The single-pulse femtosecond lasers with pulse widths of 100 fs,300 fs and 500 fs were simulated for ablation of titanium films.It is found that when the pulse width is 100 fs,the electron peak temperature of the titanium film is the highest,the coupling time is the shortest,and the ablation time is the shortest.The single-pulse femtosecond lasers with energy densities of0.010 J/cm~2,0.015 J/cm~2 and 0.020 J/cm~2 were also simulated for ablation of titanium films.The results show that the maximum electron temperature,coupling time,and electron-lattice equilibrium temperature of the titanium film all increase with the increase of energy density.It is also found that the ablation of the titanium film at higher energy density is caused by both spallation and phase explosive.In addition,based on the two-temperature model,molecular dynamics simulation method was used to determine the ablation threshold of a femtosecond laser with a wavelength of 400 nm and a pulse width of 40 fs to ablate a titanium film with a thickness of 700 nm is 0.048 J/cm~2.This result is in full agreement with the experimental results,and verifies the feasibility of the simulation model.(3)Based on TTM,the double-pulse and three-pulse heat sources were introduced,and the MD simulation method was used to study the ablation threshold of the double-pulse femtosecond laser and three-pulse femtosecond laser ablating the titanium film.For a titanium film with a thickness of about 200 nm,under the loading of the double-pulse and three-pulse femtosecond lasers with wavelength of 800 nm,pulse width of 300 fs,and pulse delay time of1 ps,and the ablation thresholds of the titanium film are 0.028 J/cm~2 and 0.017 J/cm~2respectively.Meanwhile,the optimal pulse delay time interval to promote ablation was determined to be[0.3 ps,1 ps],and it was also found that changing the position of different pulse delay times would result in sighificantly different evolution processes of the surface microstructure of the titanium film.In addition,by studying the relationship between the ablation depth and the number of pulses,it is found that the ablation depth of the titanium film increases with the increase of the number of pulses.