Optical and Acoustical Techniques for the Characterization of Material Ablation by Q-Switched ND:YAG and Excimer Laser Radiation

Characterization of the effects of the interaction between short laser pulses and a material has been made in the conditions of surface cleaning and surface micro-machining. Fast heating of the target surface results in a pressure wave that propagates inside the sample and a shock wave in the surrounding gas medium. By monitoring the energy of the shock wave using a microphone and a piezoelectric transducer, it is possible to relate the ablation rate to the observed signals. Experiments have been made using Nd:YAG and KrF lasers. At high fluences, in particular at 1,06 µm, gas breakdown drastically alters the process conditions and disperses the recorded data. By using an additionnal optical beam deflection technique, we have determined the range of validity for monitoring of the process using simple detection devices. Experimental observations and theoritical modellings show that the ablation mechanism is of photolytic type during irradiation by excimer laser and is based on explosive boiling with Q-switched Nd:YAG laser.