The invention of ultrashort laser pulses marked the beginning of a new era in science and technology, providing unprecedented tools for studying and manipulating materials. Under such extreme non-equilibrium conditions, materials exhibit ultrafast phase transitions, surface pattern formation, and complex energy redistributions that are both scientifically intriguing and technologically promising. LIPSS (Laser Induced Periodic Surface Structures) are particularly interesting due to their unique properties, universality and potential applications in various fields, including micro- and nano-fabrication, optical devices, and biosensors. Despite significant advancements in numerical simulation methodologies, developing a unified picture to understand the processes involved in these ultrafast laser-matter interactions remains a complex and ambitious task to achieve. In this work, we aim to deepen our understanding of these ultrafast phenomena and develop a novel framework that integrates the interaction of incident laser irradiation with materials. Through a combination of theoretical modeling and numerical simulations, we explore the mechanisms driving ultrafast phase transitions, energy redistribution, and material responses under extreme non-equilibrium conditions. In discussing electromagnetic phenomena, we introduce a theory for modeling Surface Plasmons Polaritons (SPPs), referred to as the TTM+P model (Two-Temperature Model and Plasmons), relying on hydrodynamic theory describing free electron. We find that these collective oscillations play a significant role in redistributing energy within the system, since the interference between SPPs and the incident laser field creates a periodic lattice temperature profile, particularly above the ablation threshold, facilitating the formation of particular types of LIPSS. Additionally, we present self-consistent FDTD (Finite Differences Time Domain) simulations that combine numerical solutions of Maxwell's equations with the two-temperature model. This was done by taking into account the thermal dependence of the dielectric function in the calculation of the source term absorbed by the electron system. Our results demonstrate that the increase of the electron temperature heavily modifies the optical properties of the material via the temperature-dependent dielectric function. As a consequence, changes in the dynamics of the electromagnetic fields, lead to an interdependent feedback loop between the material's optical and thermal response to the laser. Furthermore, we also investigate ultrafast lattice dynamics of thin gold film, uncovering the complex interplay of different macroscopic parameters in driving phase transitions and structural changes under intense laser irradiation. The simulations show that the oscillations of the pressure waves, induced by internal stresses from the laser pulse, play a critical role in driving structural instabilities of the lattice, resulting in ultrafast melting and recrystallization. Our results are in good agreement with the experimental data and provide a new insight into the underlying mechanisms of ultrafast phase transitions in gold. Finally, we bring together electromagnetic radiation, energy absorption and redistribution, and atomic-level lattice dynamics in a newly developed model, MMD-TTM (Maxwell–Molecular Dynamics–Two-Temperature Model) that is presented and tested in this work. This unified framework paves the way for more realistic simulations of ultrafast laser–matter interactions, helping to bridge the gap between theoretical predictions and experimental observations.
Othmane Benhayoun (Wed,) studied this question.