When a short, high peak-power laser pulse is focused on a material surface, vigorous material removal called ablation occurs. The ablation process is one of the major processes for laser microfabrication. We have observed a dynamics of infrared laser ablation process of copper, zinc and aluminum by a stroboscopic imaging technique with a few nanoseconds time resolution and found a growth of jet-like plasma during the laser pulse duration.

Output of Q-switched Nd:YAG lasers passed through a doubling crystal contains the fundamental and doubled radiation; the former was used as ablation pulse and the latter as illumination light. Metal targets were irradiated in air, argon or helium environment. Image was photographed by a gated CCD camera with a band pass filter at 532 nm from a tilted direction while the ablation laser hit perpendicular to the surface and the illuminating light hit at 60 degrees to the normal. Interval between ablating and illuminating laser pulses was varied by optical delay line.

When the laser fluence is high, a jet-like plasma column growing toward the incoming laser beam direction is observed. Its growth speed during the laser pulse duration is of 100 km/s, which is significantly faster than shock wave propagation under atmospheric pressure. The speed slows down to about 10 km/s when the pulse is terminated. The growth speed dependence on laser fluence and the effects of metal properties and gas environment are examined.

A possible mechanism of the jet formation would be laser-induced breakdown of environment gas triggered by electrons ejected from the sample surface at very beginning of the laser pulse.

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