Study of the fundamental mechanisms of interaction between ultrashort laser pulses and dielectric materials

The interaction between an ultrashort laser pulse and dielectric materials is a constantly evolving field motivated by the birth of various applications (laser micromachining, cornea surgery,...) as well as its fundamental aspects (high-order harmonic generation, electronics at optical-cycle regime,...). However, the underlying mechanisms of this interaction are still to be understood, especially from a quantitative point of view. Indeed, most phenomena occur during the pulse propagation through the material (i.e. in a few tens of femtoseconds). Moreover, the involved intensities are very high: a few tens of TW/cm². These two aspects make it difficult to study this interaction. To push the understanding forward, it is necessary to design new experiments and carry out an extensive work in modelling. Two complementary experimental techniques are presented in this manuscript: time resolved spectral interferometry and absorption spectroscopy. These two techniques are used to study elementary electronic excitation and relaxation processes in three different materials: quartz, sapphire and magnesium oxide. Almost all experimental results are analyzed using a multiple rate equations (MRE) model allowing us to discriminate the relative importance of each of these processes. Firstly, a direct experimental proof of the existence of impact ionization in quartz is presented. These results have been obtained by exposing the sample to a sequence of two laser pulses, allowing us to tune independently both density and temperature of the plasma. The experimental results are successfully reproduced by the MRE model including multiphoton ionization, inverse Bremsstrahlung and impact ionization as excitation processes. We then focus on another series of results on sapphire relaxation after laser-induced excitation. A new relaxation mechanism is proposed and tested by modelling to attempt to explain the dynamic of this process. This mechanism involves the formation of self-trapped excitons and their recombination. Finally, the MRE model is applied to the detection of the laser-induced ablation threshold in all three solids. The choice of the physical criterion defining this threshold, a subject that has caused intense debates over the past years, is discussed. (author)