Energy absorbed monitoring in laser ablation by photoacoustic

RINALDI, CARLOS A.; C. FERRERO, J.; M. VILLAGRÁN-MUNIZ,

El Cairo, Egipto

Congreso; 14th INTERNATIONAL CONFERENCE ON PHOTOACOUSTIC AND PHOTOTHERMAL; 2007

Laser vaporization of solid targets by pulsed lasers has received a great deal of attention due to its potential applications in material coating, removal, and processing. Despite its versatility and wide applicability, however, many aspects of the detailed chemical physics underlying the ablation process are still far from completely understood. The absorbed energy by the sample is a truly good parameter to be taken into account for theoretical models and practical applications, but is hard to measure. The plume generated is a plasma-like substance consisting of molecular fragments, neutral particles, free electrons and ions, and chemical reaction products that can absorb or scatter part of the delivery pulse energy. In general, the incident energy is spent in absorbed, scattered and energy consumed by the plasma. Recently we proposed a method to measure simultaneously all of them and were possible to make an energy balance for metal samples [1]. For high fluences, in the ablative regime, part of the laser energy is used to induce the plasma. The magnitude of the breakdown was monitored inducing the discharge between the plates of a capacitor as was already described in [2]. Pulses from a Nd:YAG laser (1.06 mm, 7 ns) generates  photoacoustic signal detected by a PZT ultrasonic sensor (240 kHz) attached to the sample; this signal is proportional to the absorbed energy. Simultaneously the scattered energy was monitored through an energy meter. In this work we applied the same procedure to study halogenated earth alkaline metals crystal (NaCl, BaF2 and CaF2) to relate both, the nature of the crystal structure and the possibility to measure the absorbed energy; this last could be determined by using the techniques mentioned above. Simultaneously time resolved optical emission spectroscopy was performed to analyze the ion emission intensity and was plotted against the energy absorbed by the samples. This plot shows the typical sigmoid behavior for the ablation process: vaporization, screening region and ablation region. An heuristic equation has been used to relate the nature of the crystal structure that clearly influences the ablation process. From this equation the threshold ablation energy for each of the studied samples could also be determined, in agreement with previous values obtained with an ultraviolet (248 nm, 14ns) laser in a single pulse experiments [3]. The ultimate goal is to obtain a predictive model and to find the best material for practical applications in laser ablation applications.   Work supported by CONACYT and DGAPA from México; CONICET, ANPCyT, ACC and Fundación Antorchas from Argentina. [1]. H. Sobral, F. Bredice, M. Villagrán-Muniz, ?Energy Balance in Laser Ablation of Metal Targets?, Journal of Applied Physics, vol.98, no.8, pp. 83305-1-5, 2005. [2]. F. Bredice, D. Orzi, D. Schinca, H. Sobral, M. Villagrán-Muniz, ?Characterization of pulsed laser generated plasma through its perturbation in an electric field?, IEEE Transactions on Plasma Science, Vol.30, No.6, pp. 2139-43, 2002. [3]. M. Reichling, J. Sils ,H. Johansen, E. Matthias, ?Nanosecond UV laser damage and ablation from fluoride crystals polished by different techniques?, Applied. Physics A, 69, S743-S747, 1999.