Study of the physical mechanisms involved in the femtosecond laser optical breakdown of dielectric materials

Abstract : We have carried out detailed time resolved experimental studies of the mechanism of electron excitation-relaxation, when an ultrashort (60 fs -1 ps) laser (UV and IR) pulse interacts with a wide band gap dielectric material.The studies cover a range of different dielectric materials and the investigated regimes span from nondestructive ionization of the material at the low power end (~TW/cm²) to ablative domain at a higher laser power (~10 TW/cm²).This gives fundamental insight into the understanding of the laser damaging process taking place under our irradiation conditions. The usage of time-resolved spectral interferometry technique allows to directlymeasurethe electron density of the irradiated material under different excitation conditions and hence leads to quantification of the process. The measurements, carried out at the optical breakdown threshold utilizing different pulse durations, raisequestions regarding the usage of critical excitation density as a universal ablation criterion. A new criterion related to the exchanged energy is proposed. Additionally, the use of an experimental setup implementing a double pump pulse allows the identification of different excitation mechanisms taking place at time scales of the order of the pulse duration used. Electronic avalanche is observed in some materials (SiO2,NaCl) while this is not the case for others (Al2O3,MgO). These differences are discussed in detail. Next, we measure the energy spectrum of excited electrons with a complementary technique: the photoemission spectroscopy. These results allow us on one hand to show a crossed effect between the two "pump" pulses and on the other hand to measure electron relaxation characteristic times, as a function of their kinetic energy. Finally, a morphological study of craters resulting from ablation in the case of a single pulse has been carried out for different irradiation parameters: number of shots, energy and pulse duration.This work has been extended to the case of double pulsedamage, with the relative delay being the studied parameter.
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Alexandros Mouskeftaras. Study of the physical mechanisms involved in the femtosecond laser optical breakdown of dielectric materials. Plasma Physics [physics.plasm-ph]. Ecole Polytechnique X, 2013. English. ⟨pastel-01006041⟩

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