Instantaneous measurement of the intensity distribution of a focused high power laser pulse

A digitalized TV system controlled by a microprocessor allows the focused intensity distribution of laser pulses to be determined instantaneously. The use of silicon mosaic vidicons at 1.06 μm is discussed and their performance is compared with that obtained using photographic methods. The linearity response of the vidicon is very good provided a preillumination system is used. Dynamic ranges of 64, and 255 x 255 pixels are analysed. This system allows the minimum cross-section Smin of the focused laser intensity distribution to be measured five seconds after each laser shot, with an accuracy of ± 4 % for absolute values, and ± 2 % for relative variations of Smin . The usefulness of this system is amply demonstrated in a graphical plot of the variation in the number of ions formed, as a function of laser intensity for the four-photon ionization of Cs atoms.Un microprocesseur associé à une mémoire digitale contenant une image TV numérisée, permet de déterminer en temps réel la distribution de l'éclairement d'une impulsion laser focalisée. L'emploi d'un vidicon à mosaïque de silicium à 1,06 μm ainsi que ses performances sont discutés et comparés aux résultats obtenus par des méthodes photographiques. Un système de préillumination de la cible permet de rendre parfaitement linéaire la réponse du vidicon en fonction de l'intensité incidente. La résolution de l'image numérisée est de 255 x 255 pixels et la dynamique du système est de 64 niveaux résolus. Cinq secondes suffisent pour calculer la section minimum Smin de la distribution de l'éclairement au foyer d'une lentille. En valeurs relatives, les variations de la section calculée n'excèdent pas ± 2 %, et en valeur absolue ± 4 %. Un exemple d'application de ce système est donné dans le cas de l'ionisation à 4 photons d'atomes de césium, mettant en évidence la nécessité de corriger chaque point expérimental

FUNDAMENTALS OF PULSED LASER IRRADIATION OF SILICON

Most of the experimental work on pulsed laser processing of semiconductors is consistent with an optical heating model. Thermal equilibrium between the dense electron-hole plasma and the lattice of amorphous or crystalline silicon is established on a time scale of 2 ps or less, during the laser pulse. Nevertheless, a series of speculations on the nonthermal nature of laser annealing of ion-implanted silicon surfaces has persisted. These invoke a hot, dense carrier plasma, inducing a phase transformation in a much cooler silicon lattice. According to the thermal model the observed phase changes involve melting. Picosecond irradiation experiments provide a stringent test of the time scale on which the thermal model remains valid. In this paper we present our recent time-resolved measurements of refractive index changes of silicon on a picosecond time scale. During and after the irradiation with a single picosecond laser pulse, the reflectivity and transmission of thin silicon films exhibit characteristic transients, which allow a detailed insight into the plasma kinetics, energy transfer to the lattice and lattice heating, and subsequent melting of the surface. In agreement with recently-published results, obtained with subpicosecond resolution, the energy stored in the electron-hole plasma is found to be transferred to the lattice in a few picoseconds