Effects of pulse numbers in laser ablation of metals in air observed by photoacoustic and imaging techniques

Abstract

Laser ablation process has been applied in precision micromachining of various materials. The machining is usually carried out in air or in gas environment by irradiating multiple number of pulses. In this paper, effects of irradiating number of pulses of Nd:YAG laser in laser ablation of aluminum and copper in air have been studied by both photoacoustic and fast-imaging techniques. Nanosecond imaging technique, where the second harmonic radiation from the same laser was used as illuminating light, showed surface phenomena during and immediately after the ablating laser pulse. Photoacoustic detection technique using piezoelectric polymer film revealed the change of coupling between laser radiation and the target. Photoacoustic signal intensity as a function of laser fluence, measured at constant pulse energy, was constant at low fluence, started to increase with fluence at a certain threshold, reached a peak and then decreased gradually with increasing fluence. Shapes of the functions were similar but the threshold fluence and the fluence at the peak changed with irradiating pulse number in different fashion for aluminum and copper. Imaging observation revealed that a surface layer was ablated in initial few pulses for aluminum but laser induced oxidation seemed dominant for polished copper surface.Laser ablation process has been applied in precision micromachining of various materials. The machining is usually carried out in air or in gas environment by irradiating multiple number of pulses. In this paper, effects of irradiating number of pulses of Nd:YAG laser in laser ablation of aluminum and copper in air have been studied by both photoacoustic and fast-imaging techniques. Nanosecond imaging technique, where the second harmonic radiation from the same laser was used as illuminating light, showed surface phenomena during and immediately after the ablating laser pulse. Photoacoustic detection technique using piezoelectric polymer film revealed the change of coupling between laser radiation and the target. Photoacoustic signal intensity as a function of laser fluence, measured at constant pulse energy, was constant at low fluence, started to increase with fluence at a certain threshold, reached a peak and then decreased gradually with increasing fluence. Shapes of the functions were similar but the...