Particle emission from solids under the action of ultrashort and temporally tailored laser pulses
Résumé
Particle emission from ultrafast laser irradiated solids is a longstanding topic of research. The ejection efficiency is commonly viewed as a probe of energy deposition into the material, allowing extensive insights into the fundamental ablation processes. This is strongly facilitated by the decoupling between the exciting pulse and the subsequent material emission. Non-thermal and thermal processes taking place on short temporal scales compete to define the energetic and kinetic characteristics of the ablated products. Mechanistic arguments will be given to distinguish the role of electronic and thermally-induced mechanisms of emission in different classes of materials, metals, semiconductors, dielectrics and to discuss the consequences of ion and neutral ejection. For example dielectric materials show individualized ways to respond to laser radiation depending on the efficiency of free-electron generation, surface charging, and on the ability to release the energy into the lattice. Impulsive ejection of charged particle appears due to surface neutrality breakdown, followed then by lattice heating and subsequent thermodynamic phase transformations. In conductive materials, thermal ejection mechanism may dominate and complex hydrodynamic behaviours were observed up the gas-phase transformation. The recent development in light modulation methods allows programmable temporal design of ultrashort laser pulses. Thus, the possibility to synchronize the energy delivery rate with the sequence of material transformations appears. This introduces the possibility of controlling and better understanding of the energy flow and, in turn, has important consequences on the energy coupling. The programmable shaping of laser pulses enables thus to tailor to a certain extent the kinetic properties of the emitted species within self-improving, adaptive feedback loops. Controlling the heating rate allows to regulate thermodynamic trajectories and to make accessible states around the critical point. Several examples will be given concerning adaptive optimization of ablation products (ions, neutrals, clusters, irradiation-affected regions) and the subsequent consequences for precise material processing.