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When an intense ultrashort light pulse – in the visible domain- interacts with a wide band gap dielectric, a plasma can be generated by non-linear photoexcitation of carrier from the valence band to the initially empty conduction band. These carriers can be further excited in the conduction band, leading to an increase of their energy distribution, and thus of the amount of energy transferred to the material. If this deposited energy exceeds some critical threshold, permanent modification like damage or ablation may take place. The two key parameters determining the energy deposition are the density and the temperature of the plasma. In this presentation, we wish to demonstrate that a sequence of double pulse can be used to better control these two parameters, and thus to optimize energy deposition and facilitate for instance ablation of insulators and semi-conductors in the VUV domain.
First, in the visible domain, using the second harmonic and the fundamental of a Ti-Sa laser, we show that time resolved double pump- and probe experiment allows to directly observe the sequence of events carrier excitation/ carrier heating, provided the parameters (energy, duration, delay) are appropriately chosen. Then, the ablation threshold (due to the first pulse) is dramatically reduced by the presence of the second pulse, while the characteristic of ablation are still determined by the first pulse [1]. Finally, new information regarding the excitation mechanisms, in particular impact ionization and avalanche are obtained [2].
In the second part, we show that this double pulse technique can be extended in the VUV domain. Using high order harmonics of a Ti-Sa laser (harmonic 25, ie. wavelength of 32 nm), whose intensity of far too low to damage any material, we could observe direct ablation of a dielectric, namely quartz, a-SiO2, when the VUV pulse if followed by an IR pulse, whereas no effect is observed with each of these pulses.
[1] Guizard, S.; Klimentov S.; Mouskeftaras A. ; Fedorov N., Geoffroy G.; Vilmart G., Applied Surface Science 336, p. 206, 2015.
[2] A Mouskeftaras, S Guizard, N Fedorov, S Klimentov Applied Physics A 110 (3), 709-715, 2013.
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