We investigate the mechanisms involved in the modification of dielectric materials by ultrashort laser pulses. We show that the use of a double pulse (fundamental and second harmonic of a Ti–Sa laser) excitation scheme allows getting new insight in the fundamental processes that occur during the interaction. We first measure the optical breakdown (OB) threshold map (intensity of first pulse versus intensity of second pulse) in various materials (Al2O3, MgO, α-SiO2). Using a simple model that includes multiphoton excitation followed by carrier heating in the conduction band, and assuming that OB occurs when a critical amount of energy is deposited in the material, we can satisfactorily reproduce this evolution of optical breakdown thresholds. The results demonstrate the dominant role of carrier heating in the energy transfer from the laser pulse to the solid. This important phenomenon is also highlighted by the kinetic energy distribution of photoelectrons observed in a photoemission experiment performed under similar conditions of double pulse excitation. Furthermore, we show, in the case of α-SiO2, that the formation of self-trapped exciton is in competition with the heating mechanism and thus play an important role especially when the pulse duration exceeds a few 100 fs. Finally, also in quartz or silica, we observe that the initial electronic excitation plays a key role in the formation of surface ripples and that their characteristics are determined by the first pulse, even at intensities well below OB threshold. The consequence of all these experimental results in the domain of UV or VUV induce damage will be discussed. In particular we demonstrate the possibility to dramatically increase the ablation efficiency by VUV light by using such double pulse scheme.
This investigation was conducted to study the tunability of the filamentation process using a near infrared laser source at wavelength ranging from 1.6 μm to 2 μm. A Krypton cell filled statically with 4 bar was employed as filamentation medium. A spectral broadening via filamentation was observed over the whole range of wavelengths employed and achieved broadening factor of 2-3. In best experimental conditions about 300nm of bandwidth where generated. The accumulated group velocity dispersion in the filament was compensated by fused silica since the dispersion of fused silica is negative at these wavelengths. It was possible to compress the pulses down to the few-cycle regime with 2-3 cycles for 1.7 μm, 1.8 μm and 1.9 μm. Theses pulses contained about 200 μJ pulse energy. Understanding the phenomenon of filamentaion at theses wavelengths bares significant potential for strong field physics applications such as attosecond science where longer wavelength and few-cycle pulses are of great advantage. As a proof of principle we used these few-cycle pulses to generate high harmonic spectra in several gases such as xenon, krypton, argon and nitrogen.
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