Molecular Dynamics Simulation Study on Ablation Threshold of Aluminum Film under Femtosecond Laser Irradiation
Objective Since the introduction of femtosecond laser technology in 1976,femtosecond laser processing has occupied a significant position in manufacturing technology.One of its primary advantages is its ability to create an extremely small heat-affected zone(HAZ),ensuring minimal heat conduction to the surrounding materials during laser irradiation,thereby reducing the risk of material deformation and damage.Moreover,femtosecond lasers possess exceptional focusing capabilities,allowing them to concentrate energy within a small area and granting them a unique competitive edge in the fields of microprocessing and nanomachining.Metallic materials typically exhibit good thermal conductivity,allowing them to rapidly conduct heat,potentially leading to larger HAZs during traditional laser processing.However,femtosecond lasers emit ultrashort pulses,release energy within femtoseconds,and cause minimal heat diffusion.This feature makes femtosecond lasers highly promising candidates for metal processing.Aluminum has a wide range of outstanding properties and is widely used in both industry and research.Its high strength,corrosion resistance,high-temperature mechanical performance,and fatigue resistance make it crucial in aerospace,maritime,and chemical engineering.To achieve the precise machining of aluminum with a femtosecond laser,a comprehensive understanding of the interaction between the femtosecond laser and aluminum is essential.This encompasses crucial parameters such as the ablation threshold and the selection of appropriate laser parameters for desired processing outcomes in practical applications.Research in this domain not only aids in optimizing femtosecond laser processing but also provides substantial support for technological applications in related fields.Methods Based on the two-temperature model and molecular dynamics,we construct an aluminum film model with dimensions of 405.00 nm×4.05 nm×4.05 nm and divide it into 400 individual grids,each with its own separate electron and lattice temperatures.Energy transfer between electron and atomic systems occurred within each surrounding grid.After establishing the model,the aluminum film is heated to 300 K and relaxed until equilibrium is achieved.After relaxation,simulations of the interaction between femtosecond laser pulses and aluminum film are conducted.First,we perform parameterized scans of laser fluence in single-pulse processing mode,obtaining temperature profiles of the surface electrons and the lattice as functions of the laser fluence.We also examine atomic motion on the surface of the aluminum film near and below the ablation threshold.We then study the changes in internal temperature and pressure at the ablation threshold.Furthermore,we investigate the ablation threshold of the aluminum film in burst mode,compare it to that in single-pulse mode,and explore the reasons for the reduced ablation threshold and input energy in burst processing mode.Results and Discussions Using the initial model we constructed,simulations of the interaction between the aluminum film and the femtosecond laser are conducted.The variation curves of surface electron and lattice temperatures of the aluminum film under laser irradiation at 0.66,0.68,and 0.70 J/cm2 are obtained(Fig.3).The ablation threshold of aluminum exhibits a step-boundary phenomenon(Fig.4).The changes in the internal temperature and pressure of the aluminum film at the ablation threshold are shown(Fig.5).Additionally,an investigation into the burst mode processing of the aluminum film shows that the ablation threshold and energy needed to ablate the aluminum film gradually decrease as the number of subpulses increases(Table 2).A comparison between a single pulse and four subpulses is performed to uncover the reason for this(Fig.7).Meanwhile,the surface motion of atoms differs between the single-pulse and four-subpulse conditions(Fig.10),and the void generation and ablation time of the four subpulses are delayed compared to the single-pulse laser.Additionally,owing to the reduced total input energy,the size of HAZ decreases(Fig.8),which is beneficial for laser processing.Conclusions This study is based on the TTM-MD model and investigates the interaction between an aluminum film and femtosecond laser pulses.The proposed method successfully determines the ablation threshold of the aluminum film in the single-pulse processing mode,and the results show good agreement with the experimental data,confirming the feasibility of the model.This study provides curves depicting the changes in the surface electron and lattice temperatures of the aluminum films.The motion of the surface atoms of the aluminum film is examined at energy densities near the ablation threshold and compared with the behavior of the surface atoms at the ablation threshold.This study demonstrates the step boundary phenomenon when femtosecond lasers ablate metals at low energy densities and identifies the ablation type of aluminum films at low energy densities.Furthermore,the internal temperature and pressure variations of the aluminum film at the ablation threshold are examined,indicating that the atomic clusters detached from the top surface retain considerably high temperatures,whereas the internal stress dissipates.Based on these findings,the interaction between the aluminum film and femtosecond laser pulses in burst mode is further explored.This highlights the fact that burst-mode processing significantly reduces the total energy required for material ablation and the ablation threshold.Moreover,it is observed that the ablation threshold of the aluminum film in burst mode decreases with an increasing number of subpulses because photons can absorb energy at a higher efficiency compared to a single-pulse laser.Additionally,owing to the lower total input energy,the HAZ formed by the femtosecond laser ablation of the aluminum film is smaller in burst mode.
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