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.
The fundamental mechanisms of matter removal involved in the interaction of short laser pulses with absorbing solids have been investigated using molecular-dynamics/Monte~Carlo simulations. This is accomplished under the two following assumptions: (i) the elementary thermodynamic properties of targets (metals and semiconductors) are adequately described by empirical potentials; (ii) in the regime where ablation is thermal, the complete time evolution of the system can be followed in p-<i>T-P</i> space and the result mapped onto the equilibrium phase diagram of the material. We find remarkable similarities in the physical pathways to ablation in metals and semiconductors for pulse durations ranging from 200 fs to 400 ps: (i) under conditions of isochoric heating and rapid adiabatic cooling with femtosecond pulses, several mechanisms can simultaneously account for matter removal in the target: spallation, phase explosion, vaporization, and fragmentation; the latter is identified for the first time in the context of laser ablation. (ii) Under nonadiabatic cooling with picosecond pulses, ablation is driven by a "trivial" fragmentation process in the metallic, supercritical fluid; this suggests a pulse duration upper limit for phase explosion of ~ 10<sup>-11</sup> s.
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, F<SUB>th</SUB> equals 0.25 J/cm<SUP>2</SUP>, 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.
Molecular-dynamics simulations are used to investigate single- shot pulsed laser ablation and desorption of crystalline silicon. The motion of approximately 32000 atoms, contained in a 5 X 5 X 27 nm<SUP>3</SUP> surface rectangular box irradiated by a single 308 nm, 10 ps, Gaussian laser pulse is followed on the picosecond time scale. Because melting and, possibly, ablation or desorption of the target following absorption of the laser pulse are described within the thermal annealing model, care is taken not to exceed carrier densities of approximately 10<SUP>22</SUP> cm<SUP>-3</SUP>. More precisely, the interaction of photons with the target is thought to cause the generation of a dense gas of hot electrons and holes which thermalizes, at first, on a time scale of a few tens of femtoseconds through carrier-carrier scattering. These hot photocarriers then transfer their kinetic energy to the lattice by means of carrier-phonon interactions characterized by a very fast initial cooling rate. The result is the creation, above a characteristic threshold energy, of a plume containing single atoms and clusters leaving the target with high axial velocities. Preliminary results about the melting fluence threshold and mechanisms underlying ablation are presented. Carrier diffusion is found to be an essential mechanism for relaxation and is presented as a possible cause of subsurface boiling.