Sub-picosecond laser absorption and ablation in aluminum: Role of collision frequency

Abstract

We present a detailed theoretical investigation of sub-picosecond laser absorption and ablation in solid-density aluminum using a two-temperature hydrodynamic model. Our focus lies on accurately modeling the electron collision frequency, a key parameter influencing laser absorption and energy transport. To achieve a smooth interpolation between the solid-state and plasma regimes, we model the collision frequency as a harmonic mean of electron–phonon collisions (in cold solids) and electron–ion collisions (in plasmas), with the latter described using the Lee–More formalism. This approach captures the effects of electron degeneracy and naturally transitions to the classical Spitzer–Härm limit at high temperatures. We evaluated the fraction of laser energy absorbed over a broad intensity range (1011−1017W/cm2) and demonstrated that our model yields significantly improved agreement with experimental absorption data, especially in the warm dense matter regime. Furthermore, we compute critical ablation parameters, such as ablation depth, threshold fluence, and optical penetration depth. Coupling the collision frequency models to a two-temperature hydrodynamic code, rather than a purely two-temperature model framework, allows us to capture the ablation behavior of aluminum without invoking heuristic “phase-explosion” cutoffs. We have devised a novel approach to study the dynamics of the ablation process by capturing the temporal evolution of the liquid–vapor boundary, thus predicting threshold fluence and ablation depths with reasonable accuracy. Our study will provide firm theoretical backing to design experiments in the field of laser micromachining.