An ultrashort pulse exhibits an instantaneous high irradiance, which typically induces nonlinear interactions within various materials. In comparison to a short pulse, an ultrashort pulse has an exceedingly brief interaction duration, resulting in a relatively limited energy redistribution induced by thermal transfer. Therefore, ultrashort lasers possess the inherent potential to effectively mitigate the processing constraints encountered in the production of components with superior quality, exceptional precision, elevated hardness, and arduous machinability, surpassing the capabilities of conventional processing methodologies. In the domain of ultrashort laser micro/nano manufacturing, the paramount significance of theoretical inquiries cannot be overemphasized because they establish the fundamental basis for achieving precise control and manipulation. In addition, the intricate nature of the interaction between ultrashort lasers and materials is a subject of profound interest in the field of optical physics.
In stark contrast to the conventional paradigm of laser thermal processing, the interaction between ultrashort lasers and materials manifests a myriad of complex phenomena unfolding across various temporal and spatial scales. When an ultrashort laser interacts with a material, the photons are primarily absorbed by charge carriers. Simultaneously, the excitation and motion of the electrons induce a modification in the potential of the atoms, facilitating the transfer of electron energy from the optical phonon wave to the acoustic phonon wave within a time frame measured in picoseconds (10-12-10-10 s). The duration of the plasma motion, material ablation, and sputtering can range from nanoseconds to microseconds. Therefore, it is imperative to develop an all-encompassing framework that incorporates the intricate dynamics of laser beam propagation, electron ionization and energy transfer, plasma motion, thermal and non-thermal phase transitions, and laser ablation. This holistic model will be indispensable in unraveling the underlying principles governing the intricate interplay between ultrashort laser pulses and materials. Nevertheless, the advancement of such a theoretical framework poses a significant impediment in the ultrashort laser field.
Considering the inherent disparities in the properties of metals, semiconductors, and dielectric materials, this scholarly article commences by elucidating the intricacies of the electron dynamics and intricate interplay between photons, electrons, and ions during ultrashort laser irradiation at the atomic level. This paper first introduces the computation of the electron dynamics of materials under ultrashort pulses. This manuscript initially presents the utilization of time-dependent density function theory (TDDFT) to scrutinize the impact of laser parameters on the rate of electron excitation. Furthermore, it introduces the concept of employing TDDFT to compute the optical properties. Subsequently, taking into account the adherence of a metallic system to the Fermi-Dirac distribution, a streamlined approach that employs density functional theory (DFT) is introduced to derive the electron excitation parameters. Then, the utilization of real-time TDDFT in conjunction with molecular dynamic simulation is introduced to explore the intricate mechanisms underlying the coupling between photons, electrons, and ions. Additionally, a streamlined approach known as ab initio molecular dynamics is presented as a means to investigate the non-thermal phase transition phenomena exhibited by crystalline materials. Finally, the paper highlights that the utilization of an atomic scale model is inherently constrained when investigating phenomena occurring within a few picoseconds or even femtoseconds.
Then, this paper provides a comprehensive overview of the current cross-scale multi-physics coupling models employed in the simulation of ultrashort laser machining. The two-temperature equation is introduced as a means to explore the intricate dynamics of the energy exchange between electrons and ions within the context of metallic systems. Furthermore, we introduce a methodology that combines the two-temperature equation and electron excitation rate equation to analyze the energy transfer dynamics between electrons and ions within semiconductor and dielectric materials. This paper elucidates the two methods capable of manifesting the electron-ion energy in the realm of macroscopic scales.
Based on the principles of energy transfer, this paper presents a comprehensive overview of the contemporary cross-scale multi-physics coupling models utilized in the simulation of ultrashort laser ablation. This manuscript presents a novel approach that synergistically merges the principles of two-temperature equation and molecular dynamics simulations. This combined methodology enables a comprehensive description of material ablation phenomena, encompassing non-equilibrium phase transition thresholds and intricate chemical reactions. It is important to note, however, that the applicability of this method is primarily confined to the realm of nanoscale laser ablation. This paper also presents a novel approach that integrates the lattice temperature with fluid mechanics and heat and mass transfer models, employing the framework of the two-temperature equation (or the two-temperature model-coupled electron excitation rate equation). This methodology facilitates the visualization of laser-induced ablation phenomena at the microscale level.
In addition, it suggests potential avenues of research that could be pursued in the future within this field.
In recent years, researchers have undertaken extensive investigations into the intricate interaction between ultrashort laser pulses and materials. By utilizing DFT and ab initio molecular dynamics, these scholars have simulated the intricate processes of electron excitation, energy transfer, and atomic motion within materials. Furthermore, they have successfully constructed a comprehensive theoretical framework that encompasses the cross-scale coupling of electron excitation, the two-temperature equation, molecular dynamics, and fluid mechanics. The interconnection of these models facilitates the comprehensive characterization of the intricate phenomena occurring in the realm of ultrashort laser processing. This successful integration allows for the accurate simulation of material ablation, nano-ripple generation, and microstructure evolution.
Nevertheless, the intricate examination and formulation of models for ultrashort laser processing at various scales present a series of challenges that will undoubtedly shape the trajectory of future advancements in theoretical modeling. First and foremost, it is imperative to propose a precise and straightforward methodology for the calculation of the photo-induced alteration of material properties. This methodology must take into account the intricate crystal modifications that occur within materials when subjected to multiple ultrashort pulses. Furthermore, it is imperative to develop a novel theoretical framework that can seamlessly integrate the precision of molecular dynamics with the efficiency of fluid dynamics. Finally, novel approaches for coupling and spatial-temporal resolution optimization are being actively pursued in order to maintain the computational precision while enhancing the computational efficacy, enabling the simulation of ultrashort laser processing involving tens of millions of pulses.
