Interaction of short and intense light pulses with matter: visible versus VUV

Abstract

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 (h&#957;<Eg) and VUV light (h&#957;>Eg). We discuss here two important aspects that must taken into account to understand the energy balance of the interaction. The first is the <i>energy distribution</i> 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 <i>excitation density</i> 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.