<title>Effects of laser pulse shape on jetlike plasma formation in laser ablation of metals under atmosphere at high fluence: observation by nanosecond imaging technique</title>

Abstract

We have developed a nanosecond imaging tecluüque. where fundamental radiation of a Nd:YAG laser was used as ablating beam while the second harmonic radiation from the same laser was used as illuminating light pulse. to study surface phenomena during and immediately after the ablating laser pulse. Using this system. we observed the ablation phenomena of metals in air and found that. at fluence higher than about 10 J/cm2. there appeared jet-like plasma growing towards incident laser beam at velocities of as high as 1O ms' in addition to the laser induced plume. The jet grew during the laser pulse and when the pulse terminated, its rapid growth stopped. In this paper. we investigated the effects of laser pulse shape on this jet-like plasma. We found that appearance of the jet became later and its growth continued longer as pulse width increased. Growth speed of the jet depended on the pulse shape, even though the jet grew up to similar height at the end of the pulse. Our results showed that, even though a laser parameter in laser ablation was usually represented as the fluence. the controlling parameter for the jet growth was temporal change of laser power.