Ultrasonic Characterization of Laser Ablation
When a pulsed laser beam strikes the surface of an absorbing material, ultrasonic waves are generated due to thermoelastic expansion and, at higher laser power densities, ablation of the material. These sound generation mechanisms have been the subject of numerous theoretical [1-3] and experimental [4-6] studies and are now fairly well understood; several reviews have also been published [7-9]. In particular, it has been established that at low power densities the thermoelastic mechanism is well described by a surface center of expansion . This mechanism produces a characteristic waveform whose amplitude is proportional to the energy absorbed from the laser pulse and also dependent on the thermal and elastic properties of the material [1-2]. At higher power densities the melting point of the material is reached, and eventually vaporization of the material takes place . Rapid vaporization leads to ablation of material. Significant ablation occurs only during the laser pulse at power densities near the ablation onset threshold, creating an ultrasonic excitation source with the same time dependence as the laser pulse. At higher laser power densities the ablation process continues after the laser pulse and eventually the ultrasonic source changes from pulse to step like in time dependence [5,9]. In this region plasma absorption also plays a significant role.
KeywordsLaser Pulse Power Density Laser Pulse Energy Laser Power Density Incident Laser Pulse
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