Neural Network Molecular Dynamics Study of Ultrafast Laser-Induced Melting of Copper Nanofilms

TY Gao and QY Zeng and B Chen and DD Kang and JY Dai, ACTA METALLURGICA SINICA, 60, 1439-1450 (2024).

DOI: 10.11900/0412.1961.2024.00143

Exploring ultrafast structural transitions in materials triggered by femtosecond laser pulses- from their condensed states to high- temperature, high-pressure conditions, and potentially to ideal plasmas- is a crucial scientific endeavor with profound implications for fields such as inertial confinement fusion, metal additive manufacturing, and laser processing. These extreme conditions, which are challenging to replicate and directly observe in experiments due to temporal and spatial resolution limitations, require theoretical models and simulations to decode the underlying microscopic mechanisms. Molecular dynamics (MD) simulations, especially when paired with advanced potential energy surfaces, are effective tools for addressing these challenges. However, maintaining a balance between computational efficiency and physical accuracy, particularly when simulating excited states induced by laser interactions, remains a formidable task. In this context, neural network potential energy surfaces (NNPES) have demonstrated exceptional capability for capturing the complex interactions and properties of materials under extreme conditions, providing vital links between quantum mechanics and macroscale phenomena. Using Cu as a prototypical example, the ability of NNPES to accurately depict lattice vibrations, thermophysical properties, and complex dynamics during laser- matter interactions has been demonstrated. By seamlessly integrating NNPES with a two-temperature MD model, this study directly simulates the atomic-scale dynamics of Cu thin films subjected to intense pulsed laser irradiation. This innovative approach, which combines quantum-level accuracy with large- scale thermodynamics and detailed microstructural evolution, provides unprecedented insights into the fundamental mechanisms of laser-induced melting. Our findings reveal two distinct melting behaviors in Cu, dependent on laser fluence. At fluences near the melting threshold, a heterogeneous melting process initiated at the film surface because of the lower free energy barrier was observed. The solid- liquid interface then moves inward at velocities of tens of meters per second, requiring hundreds of picoseconds for melting to complete. Conversely, at fluences well above the threshold, Cu films experience rapid and homogeneous melting, markedly different from conventional heating-induced melting. Here, the lattice temperature almost instantaneously exceeds the thermal stability limit, leading to uniform liquid nucleation and rapid growth throughout the film, culminating in complete melting within just tens of picoseconds. This study not only illuminates the atomic-scale dynamics of laser- induced melting but also underscores the transition from heterogeneous to homogeneous melting mechanisms as a function of laser fluence. This study serves as an invaluable research tool for enhancing our understanding of laser- matter interactions and their potential applications in optimizing laser-based manufacturing processes and predicting material behavior under extreme conditions. Moreover, the reliability and versatility of NNPES set the stage for extending the research to more complex systems, including alloys and amorphous materials. This expansion fosters robust connections between microscopic theories and macroscale applications, deepening our understanding of material responses to intense laser irradiation. Future studies employing this framework could explore complex physical phenomena such as explosive boiling and material disintegration during laser ablation, offering unique atomic-scale insights that could pave the way for groundbreaking discoveries and technological advancements.

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