CC BY
31LM-I-22
Laser dielectric interactions: new insight from double pulse
experiments
Stéphane Guizard1, Allan Bildé2, Sergey Klimentov3, Alexandros Mouskeftaras4
1. Laboratoire Interactions, Dynamiques et Lasers, UMR 9222 CEA, CNRS, Université Paris-Saclay, CEA
Saclay, F-91191 Gif-sur-Yvette, France.
2. Laboratoire des Solides Irradiés, CEA/CNRS, Université Paris-Saclay, Ecole Polytechnique, 91128
Palaiseau, France,
3. General Physics Institute of the Russian Academy of Sciences, Vavilova St 38, 11991 Moscow, Russia.
4 Laboratoire LP3, Aix-Marseille University, CNRS, UMR 7341, 13009 Marseille, France.
Corresponding author: stephane. guizard@pea.fr
Abstract : Ultrafast lasers are powerful tools to study the dynamics of the relaxation processes occurring in matter, and this domain of time resolved science has known a broad development since the spreading of femtosecond lasers. The temporal resolution currently accessible - typically 30 fs - is however not always sufficient to directly resolve and provide a clear picture of the different physical mechanisms involved. This is for instance the case when an intense pulse impinges on wide band-gap dielectrics. In that case, several processes are already competing during the laser pulse itself. First, obviously the different excitation channels: usually a non linear photo-excitation, via multiphoton or tunneling ionization from the valence band to the conduction band, followed by further laser heating of carrier in the conduction band by multiphoton or sequence of single photon absorption. Then the first relaxation events begin before the end of the pulse: carrier-carrier collisions like impact ionization, electron-phonon coupling, exciton formation, etc. The repeated combination of laser heating-impact ionization, leads to an increase of the excited carrier population, eventually giving rise to laser induced avalanche. Whatever the pulse duration, it is thus not possible to resolve these different processes. 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.
[1] S. Guizard, A. Bildé, S. Klimentov, A. Mouskeftaras, under press.
[2] A. Bildé et al., to be published in J. Phys: Condens Matter, 2021.
