Research Progress in Femtosecond Laser Machining Mechanism and Simulation Analysis

Xuefeng Wu; Sanlin Mei

doi:10.3788/LOP202158.1900005

Journals >Laser & Optoelectronics Progress >Volume 58 >Issue 19 >Page 1900005 > Article

Laser & Optoelectronics Progress
Vol. 58, Issue 19, 1900005 (2021)

Research Progress in Femtosecond Laser Machining Mechanism and Simulation Analysis

Xuefeng Wu^* and Sanlin Mei

Author Affiliations

School of Mechanical Power Engineering, Harbin University of Science and Technology, Harbin , Heilongjiang 150080, China

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DOI: 10.3788/LOP202158.1900005 Cite this Article Set citation alerts

Xuefeng Wu, Sanlin Mei. Research Progress in Femtosecond Laser Machining Mechanism and Simulation Analysis[J]. Laser & Optoelectronics Progress, 2021, 58(19): 1900005 Copy Citation Text

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Fig. 1. Difference between long pulse laser processing and short pulse laser processing^[14]

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Fig. 2. Width of the slot obtained at different laser frequency values^[33]. (a) 50 kHz; (b) 150 kHz; (c) 350 kHz

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Fig. 3. Aggregate color center and astigmatic nanoparticles found in sodium fluoride crystal^[40]. (a) Aggregate color center; (b) astigmatic nanoparticles

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Fig. 4. Three dimensional microstructure and functional components in microchip^[45]

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Fig. 5. Microstructures fabricated by two-photon micromachining technology. (a) Nano cattle^[49]; (b) micro gear^[50]

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Fig. 6. SEM photos of holes drilled in copper by laser with pulse width of 120 fs^[55]

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Fig. 7. Molybdenum oxide nanorods were observed on the surface of molybdenum films by scanning electron microscopy (SEM)^[63]

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Fig. 8. Arrangement diagram of FCC atoms, HCP atoms, surface atoms, and other atoms as well as relation between atom percentage and temperature^[79]. (a) Atoms arrangement; (b) relation between atom percentage and temperature

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Fig. 9. Evolution of chromium atom structure in single crystal copper under different laser pulses^[85]

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Fig. 10. Time evolution of energy deposition and diffusion for an SHI laser irradiation^[95]

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Fig. 11. Schematic of MPM method^[97]

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Fig. 12. Electron and lattice temperature distributions and stress distribution of aluminum film under different pulse widths^[96]. (a) Electron temperature; (b) lattice temperature; (c) stress distribution

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Processing mechanism	Characteristic	Brief introduction of mechanism	Application range /material
Coulomb explosion	The Coulomb explosion of molecular ions in solid membrane can be used to study the interaction between fast molecular ions，atomic ions，and solid	The high power density of femtosecond laser can cause electrons escaping from the surface layer of the processing area，while the ions keep relatively low temperature and produce extremely high net charge density and electrostatic field. When the electrostatic force in the material is greater than the inter lattice force，the chemical bond will break and cause lattice damage. This mechanism is called Coulomb explosion	It is suitable for the interaction between ultrashort pulse and medium，which is not observed in semiconductors
Micro blasting model	The results show that the micro explosion process takes place in a constant volume，the thermal diffusion of metal is significantly reduced，and no molten material is produced，which provides conditions for precision micro machining	The electron number density and average kinetic energy of the conduction band in the center of the laser focusing spot are very high，and they will diffuse to the surrounding in an instant. When the laser pulse is stopped，the net charge density is large，and the huge repulsion force between ions will lead to micro explosion	It is used to describe ultrafast deformation in metals and micromachining inside transparent materials
Color center model	In the process of color center formation，the processing energy of femtosecond laser is lower than the ablation threshold of material，so it will not cause permanent damage to the material. This kind of optical coloring will return to its original state after heat treatment	The nonlinear effect in the interaction between femtosecond laser and material causes the formation of color center，which results in the optical coloring of the processing area	It is mainly suitable for glass and other transparent nonmetallic materials
Two-photon ionization	The laser intensity must be larger than the ablation threshold of the material，and the multiphoton ionization rate is proportional to the K power of the laser intensity. The free electrons produced by two-photon ionization can provide seed electrons for avalanche ionization	Under the irradiation of femtosecond laser，when the photon density is high，the ground state electron can absorb two photons at the same time，and then release energy in the form of photons. The process of relaxation to the ground state is called two-photon excitation process	It is suitable for femtosecond laser pulse processing，optical storage，and photonic crystal preparation in transparent materials，such as quartz，glass，optical fiber，and so on

Table 1. Mechanism and application range of femtosecond laser processing

Model	A brief introduction to the model	Characteristic	Application
Two-temperature model	Based on the Boltzmann transport equation，the model is used to describe the mathematical model of femtosecond laser interaction with metal materials，and two different interaction processes of photon and electron，electron and lattice are considered. It can describe the temperature evolution process of electron system and lattice system	In the two-temperature model，electrons and lattices are considered as two independent systems. The interaction processes of photons and electrons，electrons and electrons，and electrons and lattices are analyzed，and the differential equations of temperature changes between electrons and lattices are given	This model is often used to predict the damage and melting threshold of materials，and it is the main model to describe the temporal and spatial evolution of electron and lattice temperature in metal materials irradiated by short pulse laser
Molecular dynamics model	Molecular dynamics simulation regards continuous medium as a particle system composed of many atoms or molecules. The force between particles can be obtained by deriving the potential energy function. The mathematical model of particle motion in the system is established by using classical Newtonian mechanics. Under the given boundary conditions and initial conditions，the trajectories of particles in phase space are obtained by numerical solution，and then the corresponding macroscopic dynamic and static characteristics of the system are obtained by the principle of statistical physics	This method only needs to give the model hypothesis of the interaction between atoms，and does not need any other assumptions. Moreover，it can combine macro phenomena with micro molecular dynamics，which has the advantages of communicating macro characteristics and micro structure，emphasizing the details of micro interaction，and not making any assumptions about the process under study	Molecular dynamics method is suitable for the simulation of fast non-equilibrium process，and can describe the micro process of the interaction between ultrashort pulse laser and metal
Two-temperature model（TTM）-molecular dynamics（MD）	In the process of femtosecond laser ablation of materials，not only the evolution of temperature field is involved，but also the movement of atoms，the change of stress and pressure in materials. The combination of TTM and MD can effectively predict the melting，gasification and ablation threshold of materials，and obtain the understanding of ablation mechanism and the optimization of parameters	The two-temperature model can describe the deposition of light energy，the heat transfer between electrons，the energy transfer between electrons and lattices，and the heat transfer between electrons. Molecular dynamics can directly describe the non-uniform processes such as overheating，ablation，and melting of materials	The combination of two-temperature model and molecular dynamics model is usually used to analyze the external and internal properties of metal or nonmetal materials from macroscopic and microscopic perspectives
Molecular dynamics（TTM）-material point method（MPM）	The combination of molecular dynamics method and material point method can not only describe the dynamic process of material such as pressure and transmission，but also simulate the discrete and continuous behavior of material，maintain the conservation of mechanical energy in the contact process，and reduce the amount of calculation in the simulation process	In the MD-MPM coupling method，the MPM method is used to solve the two-temperature model（TTM）electron energy equation describing the non Fourier heat transfer in the metal electronic subsystem. The MD method describes the dynamics of the metal lattice system，including the additional damping force considering the energy transfer between the electron and the lattice	It is used to simulate the interaction between ultra fast laser and metal. It can also be used to simulate the numerical simulation of bulk materials，and simulate the extrusion，sliding，separation，and other contact behaviors between particles

Table 2. Research progress of femtosecond laser in simulation

Xuefeng Wu, Sanlin Mei. Research Progress in Femtosecond Laser Machining Mechanism and Simulation Analysis[J]. Laser & Optoelectronics Progress, 2021, 58(19): 1900005

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