In multi-pulse laser damage and ablation experiments, the laser-induced damage threshold (LIDT) usually changes with the number of pulses in the train, a phenomenon known as incubation. We introduce a general incubation model based on two physical mechanisms—pulse induced change of (i) absorption and (ii) critical energy that must be deposited to cause ablation. The model is applicable to a broad class of materials and we apply it to fit data for dielectrics and metals. It also explains observed changes of the LIDT as a function of the laser repetition rate. We discuss under which conditions the crater-size method to determine LIDTs can be applied in multi-pulse experiments.
The transient electron temperature in a weakly ionized femtosecond-laser-produced air plasma filament was determined from optical absorption and diffraction experiments. The electron temperature and plasma density decay on similar time scales of a few hundred picoseconds. Comparison with plasma theory reveals the importance of inelastic collisions that lead to energy transfer to vibrational degrees of freedom of air molecules during the plasma cooling.
Third harmonic generation by a weak femtosecond probe pulse intersecting a pump laser-induced plasma in air is investigated and a general model is developed to describe such signal, applicable to a wide range of focusing and plasma conditions. The effect of the surrounding air on the generated signal is discussed. The third-order nonlinear susceptibility of an air plasma with electron density N(e) is determined to be χ(p)((3)) = χ(a)((3)) + γ(p)N(e) with γ(p) = 2 ± 1 × 10(-49) m(5) V(-2) and χ(a)((3)) being the third-order susceptibility in air. Lateral scans of the probe through the plasma are used to determine electron density profiles and the effect of focusing and phase mismatch is discussed.
In the last two decades, there has been a strong research interest in producing radioisotopes with ultra-intense lasers, as an application of laser-driven accelerators in nuclear medicine. Encouraging progress has been obtained in both experiments and simulations. This Review presents the results of several intense studied radioisotopes in detail, i.e., 18F, 11C, 13N, 15O, 99mTc, 64Cu, and 62Cu. As for other less studied radioisotopes, the results are summarized in Sec. II G. The results are listed in Tables I–VII along with laser intensities, maximum ion/photon energies, number of ions/photons per shot, reactions, and laser repetition rates and facilities. For research based on high repetition rate lasers, both single-shot and multi-shot productions are provided for the purpose of comparison. With key technologies implemented in new commissioning ultra-intense lasers, further experiments will definitely help moving this area forward, which will bring the realization of laser-driven radioisotope production closer.
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