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8LM-I-7
LASER-MATTER INTERACTION
Mechanisms of femtosecond ablation of optical crystals depending on free electron lifetime
Sergey Klimentov 1, Stéphane Guizard2, Nikita Fedorov3, Allan Bildé2, Alexandros Mouskeftraras4, Anton Popov1
1 National Research Nuclear University "MEPhI", Moscow, Russia
2 Laboratoire des Solides Irradiés, Ecole Polytechnique, Palaiseau, France,
3 Centre Lasers Intenses et Applications, Université Bordeaux I, Talence, France 4 Laboratoire LP3, Aix-Marseille University, CNRS, UMR 7341, 13009 Marseille, France.
Intense ultrashort laser pulses have proven to be a versatile tool for today technologies of wide band gap materials including precision modification, ablative micromachining and fabrication of nanomaterials for biomedical applications [1]. The effect of laser exposure and the particular sequence of stages, or mechanisms, resulting in modification and ablation are critically dependent not only on characteristics of the incident laser pulse but also on the nature of irradiated materials, primarily on the fast photo-electron kinetics playing the clue role in deposition of energy into the lattice of the dielectric. In our study, we focus in interaction of femtosecond IR, visible and UV pulses with the conventional optical materials, namely the crystalline quartz, sapphire, magnesium oxide and alkali halides, the model materials featured by the high initial transparency and the lifetime of free electrons covering the range form tens of femtoseconds to hundreds of picoseconds.
The pump-probe experimental approach employing three synchronized ultrashort laser pulses have been used in several kinds of complementary measurements, when the first pump pulse in the trio was tailored to generate free electrons via multiphoton absorption, the second pump aimed to boost their energy via the induced intraband absorption, while the third one was used to probe the transient absorption and refraction index modification, brought on by the first and the second pulses, aiming to observe and quantify possible multiplication of the charge carriers via impact ionization mechanism. The automated experimental setup allowed to choose pulse-width, wavelength and delay between the first two pulses which was then fixed during the measurement. The third one, usually the shortest, arrived at variable delay to trace the fast electron kinetics. Variety of ablation thresholds was measured in the same two pump experimental conditions with the reference to a particular electron concentration induced by the first pulse. The geometry of interaction was kept unchanged as much as possible in the different kinds of measurements. Energy of the electrons within the conduction band was estimated in similar configuration in two-pulse photo-electron spectroscopy experiments. This way, the complete set of direct measurements was performed for quantitative characterization of all stages of the kinetics ending up at the surface ablation in these materials.
The obtained experimental data and the results of theoretical modeling indicate the cascade intraband absorption, occurring simultaneously with electron-phonon coupling, to play the clue role in laser ablation of optical crystals known for long free electron lifetime (Al2O3 and MgO) which happens through the mechanism of thermal instability of the lattice, in spite of the short pulses involved in the excitation process. The evidences of free electron multiplication via impact ionization were revealed only in SiO2 and alkali halides known for the fast trapping of electrons with formation of self-trapped excitons [2]. In spite of popular opinion, electron multiplication or the avalanche was not the main cause of optical breakdown in any of the studied cases. The research received partial financial support from LASERLAB-EU-ROPE (the grant agreement 654148).
[1] A. Popov, G. Tikhonovski, P. Shakhov, E. Popova-Kuznetsova, G. Tselikov, R. Romanov, A. Makeev, S. Klimentov, A. Kabashin, Synthesis of Titanium Nitride Nanoparticles by Pulsed Laser Ablation in Different Aqueous and Organic Solutions, Nanomaterials, vol. 12(10), p. 1672 (2022)
[2] S. Guizard, S. Klimentov, A. Mouskeftaras, N. Fedorov, G. Geoffroy, C. Vilmart, Ultrafast Breakdown of dielectrics: Energy
absorption mechanisms investigated by double pulse experiments, Applied Surface Science, vol. 336, pp. 206-211 (2015)
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