|
Light sources capable to deliver intense and ultrashort pulses in the VUV domain, based on free electron lasers or on the
high order harmonic generation have appeared recently. They bring the possibility to explore a new domain in the
field of laser matter interaction. Such sources are available in the visible or near IR range -specially at 800 nm, thanks to
Ti-Sa lasers - since more than ten years, and the interaction of femtosecond pulses with solids has been studied in great
details. In this paper we will discuss how the knowledge which has been acquired in the visible domain can be used for
the VUV studies. I will concentrate on the case of wide band gap dielectric materials (SiO2, MgO, Al2O3), and on the
intensity domain around breakdown and ablation threshold. This type of material is interesting not only because they are
involved in numerous applications, but above all because their band gap (Eg) lying in the range 6 to 10 eV, a clear
distinction can be made for what concern their interaction with visible (hνEg). We discuss here
two important aspects that must taken into account to understand the energy balance of the interaction. The first is the
energy distribution of photoexcited carriers, which are clearly different in the case of visible or VUV light.
Photoemission spectroscopy demonstrate that the distribution highly depends upon the incident intensity in the visible
and near IR, and can be "warmer" than the one observed by irradiation with VUV photon, despite their much larger
energies. The second important parameter is the excitation density achieved during the excitation. Experiments carried
out in the IR using the technique of time resolved interferometry allow to measure the density of electrons excited in the
conduction band at intensities above and below the optical breakdown threshold. The results show that in the process of
laser breakdown multiphoton excitation dominates the avalanche process for picosecond and subpicosecond pulses. The
simulations performed to interpret these measurements can be used to predict the damaging mechanism of wide band gap
dielectrics submitted to ultra intense VUV pulses.
|
