Our aim is to better understand and observe the many events occurring during the interaction of an intense and ultrashort pulse with a dielectric material. It is a challenging task, due to the competition between many different elementary physical mechanism, all occurring at sub-picosecond or femtosecond time scale: electron-phonon interaction, elastic and inelastic electron-electron scattering - including impact ionization, formation of transient or permanent defect states, exciton self-trapping, exciton-exciton interaction, etc. The direct observation of these processes is beyond the capacity of traditional time resolved femtosecond experiment due to a lack of temporal resolution.
To encompass this intrinsic difficulty, we have carried out experiments with a double exciting pulse scheme. Under appropriate conditions, we can control independently the two key parameters: plasma density and temperature. Then, using time resolved interferometry as a probe, we could directly observe for the first time an electronic avalanche induced by a laser pulse in a solid, namely crystalline SiO2 [1]. A complete modeling, using multiple rate equation and taking into account the laser propagation, allows to fully describe the experimental results. In materials where exciton self-trapping does not occur, no evidence of electronic avalanche is found, and the lifetime of excited carrier is much longer [2]. Our investigations on a large set of different materials (NaCl, KBr, CaF2, etc) provide a strong indication that a link exists between two apparently opposite relaxation mechanisms, exciton self-trapping and laser induced impact ionization and avalanche.
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