Patrick Lorazo

Ablation of molecular solids under nanosecond laser pulses: The role of inertial confinement

The thermal routes to ablation in molecular solids having a long (micron scale) optical penetration depth are investigated under nanosecond laser pulses using a two-dimensional molecular-dynamics model. The authors demonstrate that the mechanisms of matter removal are mainly determined by the local degree of inertial confinement; by increasing level of confinement, these are (trivial) fragmentation, phase explosion, and heterogeneous nucleation of vapor bubbles at solid-liquid boundaries. The thermodynamic pathways to ablation are shown to be different from those predicted by the model of Miotello and Kelly [Appl. Phys. Lett. 67, 3535 (1995); Appl. Phys. A: Mater. Sci. Process. 69, S67 (1999)].

Simulation of picosecond pulsed laser ablation of silicon: the molecular-dynamics thermal-annealing model

A molecular-dynamics thermal-annealing model is proposed to study the mechanisms of ablation induced in crystalline silicon by picosecond pulses. In accordance with the thermal annealing model, a detailed description of the microscopic processes resulting from the interaction of a 308 nm, 10 ps, Gaussian pulse with a Si(100) substrate has been embedded into a molecular- dynamics scheme. This was accomplished by explicitly accounting for carrier-phonon scattering and carrier diffusion. Above the predicted threshold energy for ablation, Fth equals 0.25 J/cm2, ablation is driven by subsurface superheating effects: intense heating by the pulse leads to the thermal confinement of the laser-deposited energy. As a result, the material is overheated up to its critical (spinodal) point and a strong pressure gradient builds up within the absorbing volume. At the same time, diffusion of the carriers in the bulk leads to the development of a steep temperature gradient below the surface. Matter removal is subsequently triggered by the relaxation the pressure gradient as a large--few tens of nm thick--piece of material is expelled from the surface.

Femtosecond x-ray diffuse scattering measurements of semiconductor ablation dynamics

Femtosecond time-resolved small and wide-angle x-ray diffuse scattering techniques are applied to investigate the ultrafast nucleation processes that occur during the ablation process in semiconducting materials. Following intense optical excitation, a transient liquid state of high compressibility characterized by large-amplitude density fluctuations is observed and the build-up of these fluctuations is measured in real-time. Small-angle scattering measurements reveal the first steps in the nucleation of nanoscale voids below the surface of the semiconductor and support MD simulations of the ablation process.