Novel non-thermal Ablation Mechanics in the Laser Ablation of Silicon

We investigate the non-thermal material dynamics of strongly excited silicon during ultra-fast laser ablation. In contrast to metals, silicon shows strongly excitation-dependent interatomic bonding strengths, which gives rise to a number of unique material dynamics like non-thermal melting, Coulomb explosions and altered carrier heat conduction due to charge carrier confinement. In this study, we report novel non-thermal ablation mechanisms in the ultra-fast single shot laser ablation of silicon and perform large scale massive multi-parallel simulations on experimentally achievable length scales with atomistic resolution. For this, we model the ultra-fast carrier dynamics utilizing the Thermal-Spike-Model coupled to Molecular Dynamics simulations and include the accompanied excitation-dependent nonthermal bonding strength manipulation by application of the excitation-dependent modified Tersoff Potential. Further, we present first results on the systematic construction of the excitation-dependent phase diagram of silicon by thermodynamic integration.

💡 Research Summary

The manuscript presents a comprehensive investigation of non‑thermal material dynamics in strongly excited silicon during ultra‑fast single‑shot laser ablation. Recognizing that covalently bonded semiconductors behave fundamentally differently from metals under intense laser irradiation, the authors focus on the excitation‑dependent weakening of interatomic bonds, which can turn attractive potentials into repulsive ones within a few hundred femtoseconds. To capture these effects, they develop a multiscale simulation framework that couples the Thermal‑Spike Model (TSM) for carrier dynamics with classical Molecular Dynamics (MD). The key innovation is the use of an excitation‑dependent modified Tersoff potential (referred to as MOD*), whose parameters are derived from finite‑temperature density‑functional theory calculations and explicitly depend on the electronic temperature (or carrier density). This allows the MD part to respond directly to the degree of electronic excitation.

The authors implement the TSM–MD coupling in their in‑house code IMD, solving carrier‑density and carrier‑temperature transport on a finite‑difference grid that is distributed across many CPUs. Each FD cell contains an MD sub‑cell where atomic positions and velocities evolve under the MOD* forces. Energy exchange between the electronic and lattice subsystems occurs via electron‑phonon coupling. Laser irradiation is modeled as a spatial‑temporal Gaussian pulse (500 nm spot, 100 fs FWHM, fluence range 0.07–0.63 J cm⁻², λ = 800 nm).

Two sets of simulations are performed: one with the conventional Tersoff potential (MOD) and one with the excitation‑dependent MOD*. The MOD simulations reproduce the classic “phase explosion” scenario typical for metals: a shock wave propagates at the speed of sound, a heterogeneous melting front follows, and an over‑critical fluid expands into a mixture of droplets and vapor. In contrast, the MOD* simulations reveal four distinct non‑thermal ablation mechanisms that dominate once the electronic excitation exceeds a threshold:

1. Non‑thermal evaporation – anti‑bonding occupation makes the interatomic potential repulsive, causing instantaneous vaporization on sub‑picosecond timescales.
2. Pre‑shock‑wave non‑thermal melting – a melting front propagates at velocities approaching the speed of light, far exceeding the conventional sound‑limited rate.
3. Non‑thermal void formation – rapid melting creates a dense amorphous phase accompanied by a sudden pressure drop; the material flow cannot fill the emerging vacuum, leaving voids.
4. Void‑induced liquid spallation – the newly formed void surfaces partially reflect the shock wave, and the surrounding liquid/gas material is ejected as droplets.

Quantitative comparison of ablation depth versus fluence shows that MOD* predictions match experimental data (single‑shot ablation measurements by Zhang et al.) almost one‑to‑one, whereas MOD underestimates depth and misrepresents the dominant mechanism. This validates the importance of incorporating non‑thermal effects for accurate modeling of silicon laser processing.

Beyond the mechanistic study, the authors exploit the upgraded Hawk supercomputer to perform truly atomistic 2‑D simulations with lateral dimensions exceeding one micrometer, thereby capturing the full crater profile. They also demonstrate a “composed” simulation approach: many quasi‑1‑D runs at different local fluences are stitched together to reconstruct the 2‑D density field. This composition reproduces crater dimensions and non‑thermal rim sizes while reducing computational cost by a factor of ~4.7 compared with a single full‑scale 2‑D run. The trade‑off is the loss of lateral dynamics such as nano‑bump formation or lateral shock‑induced redeposition, which are only captured in the full 2‑D simulations.

Finally, the paper presents excitation‑dependent phase diagrams obtained via thermodynamic integration (implemented in Calphy interfaced with LAMMPS). By varying the electronic temperature, the authors show systematic shifts of melting and solid–solid transition lines to lower lattice temperatures, reflecting bond weakening due to population of anti‑bonding states. The calculated zero‑pressure melting temperature (≈1688 K) agrees with experiment, confirming the reliability of the MOD* potential for thermodynamic studies.

In summary, the work delivers (i) a physically grounded, excitation‑dependent interatomic potential for silicon, (ii) a multiscale TSM‑MD simulation platform capable of multi‑million‑atom, multi‑nanosecond runs, (iii) the identification and quantitative validation of four novel non‑thermal ablation mechanisms, (iv) a practical composition strategy for large‑scale laser‑matter interaction modeling, and (v) excitation‑temperature phase diagrams that link electronic excitation to structural transitions. These contributions significantly advance the predictive capability for ultra‑fast laser processing of covalent semiconductors and provide a solid foundation for future experimental and theoretical studies.