ABSTRACT
Because the sequence of events that leads to laser ablation, which
may be defined as the collective ejection of matter (ions, atoms, clus-
ters, nanoparticles, etc.) following irradiation by ultrashort, intense
bursts of light, is formidably complex, the usual analytical tools of
theoretical physics are unable to account for the whole spectrum
of relevant processes taking place in the target, and thus cannot
provide a thorough understanding of the physical mechanisms
that underlie the phenomenon, as well as the physical nature of
the structural modifications inflicted to the system following the
absorption of energetic photons, notably in the so-called heat-
affected zone. To make the problem even more difficult, the process
takes place on an unusually wide range of length and timescales.
In view of these difficulties, computer simulations are, in spite of
their limitations, an excellent route to understanding the physics
of ablation [1-11], very nicely complementing experiment [12-14].
In particular, the numerical models developed by our group-
Perez et al. [5, 7-9] and Lorazo et al. [6, 10, 11]—have provided a comprehensive picture of the mechanisms that underlie ablation
in the thermal regime (as opposed to the non-thermal regime, where the physics is dominated by complex electronic effects
such as plasma formation and Coulomb explosion). It has been
demonstrated, in particular, that different routes are available for
ablation to occur, viz. spallation (ejection of fragments of material
following the passage of a tensile stress wave), phase explosion
(decomposition of a thermodynamically metastable homogeneous
liquid into a mixture of liquid droplets and gas), fragmentation
(disintegration of a homogeneous material into clusters under the
action of large strain rates) and vaporization (passage from the
solid or liquid to the gas phase), as a function of increasing fluence
[7]. Further, it has been demonstrated that, in the case of very
long (nanosecond) pulses in molecular solids, the way to ablation
is largely determined by the degree of local confinement, notably
depth into the target [9]. In contrast, the corresponding problem in
the presence of a liquid layer has remained largely unexplored, likely
because of the increased complexity of incorporating an additional
variable into an already extremely difficult problem. Nevertheless,
some progress has been realized using our simple two-dimensional
Lennard-Jones model [15].
