In this study, we investigate the conversion of femtosecond laser energy deposition from plasma into a shockwave in ambient air. The experiments are carried out using a 380fs pulsed laser at 1.03μm, with laser intensities below the filamentation threshold. The measurements of this dynamic phenomenon are carried out with the help of a time-resolved transmission microscope, and the pressure and temperature space-time evolution are evaluated using a theoretical model. In our conditions we generate shockwaves with initial pressure loading in the range of GPa and maximum propagation velocity in the order of a few km/s.
We are addressing the need for better understanding of the mechanical effects involved in the femtosecond laser processing of transparent materials. By focusing infrared (1.03μm) laser pulses with a duration of 360fs and an energy of ~1μJ inside the bulk of a fused silica cube, we generate a plasma that absorbs laser energy. We then use a time-resolved microscope-polariscope based on a pump-and-probe scheme to measure optical transmission and stress-induced birefringence. Our methodology provides a link between initial ionization stages and the hydrodynamic response of the excited material thus allowing a full description of the interaction until the nanosecond timescale.
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.
Large third-order nonlinearity and transparency in the mid-infrared region are the basic motivations for prospective applications of chalcogenide glasses in nonlinear photonics and laser technologies. We present the state-of-the-art and our recent results of measurement and evaluation of the nonlinear optical constants, plasma dynamics, and thermal regimes upon irradiation of As-S-Se samples using 40-fs pulses at 790-nm wavelength.
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