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Etude des mécanismes d'excitation électronique associés au claquage des diélectriques induit par un champ laser intense

Abstract : What are the conduction electrons injection mechanisms involved in laser-induced breakdown of dielectric solids ? This question, which has been debated since the late sixties, has been recently revived in the case where pico- or femtosecond pulses are used to destroy the solid. For this duration range, it has been proposed that most of the conduction electrons are injected through a very strong electronic avalanche, which would be initiated by a weak density of free carriers excited by multiphoton absorption. So far, the experimental evidences for this scenario only consists in measurements of the breakdown threshold as a function of pulse duration. We have tried to adopt a more direct approach to this problem, based on two pump-probe experiments. Using time-resolved interferometry in the frequency domain, we have measured the total conduction electron density excited by an ultra-short laser pulse, as a function of its duration and intensity, both below and above the breakdown threshold. These measurements directly show that multiphoton excitation of valence electrons is the dominant injection mechanism : clearly, no electronic avalanche occurs with laser pulses shorter than a few ps. Impact ionization of valence electrons by conduction electrons is a crucial step in avalanche models. Therefore, we have carried out a time-resolved photoemission experiment to evaluate the impact ionization rates in dielectrics. Valence electrons were injected in the conduction band, above the impact ionization threshold, thanks to a 60 fs, 40 eV pump beam. The ensuing relaxation of these ?high? energy electrons was then probed with an intense infrared probe pulse. By measuring the energy distribution of the electrons as a function of the pump-probe delay, we have showed that the typical time scale for the impact ionization process in SiO2 is a few tens of ps : this low impact ionization rate could explain why no avalanche occurs with ultra-short laser pulses, as we have demonstrated by the frequencydomain interferometry technique.
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Submitted on : Monday, November 17, 2008 - 2:33:44 PM
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  • HAL Id : tel-00339273, version 1

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Fabien Quere. Etude des mécanismes d'excitation électronique associés au claquage des diélectriques induit par un champ laser intense. Physique [physics]. Université Paris-Sorbonne - Paris IV, 2000. Français. ⟨tel-00339273⟩

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