Laser ablation of some metals in gases induced by nanosecond Nd:YAG laser has been studied by both photoacoustic and fast imaging techniques. Photoacoustic technique using piezoelectric polymer film revealed the change of coupling among laser radiation, ablated matter, plasma and target as a function of the laser fluence. Nanosecond imaging technique showed surface phenomena during and immediately after the ablating laser pulse.Photoacoustic signal intensity as a function of laser fluence was measured at constant pulse energy. It is constant at low fluence, starts to increase with fluence at certain threshold, reaches the maximum and then decreases gradually with increasing fluence. At fluence higher than about 7 J/cm2, there appeared jet-like plasma growing toward incident laser beam at velocities of as high as 1 o ms' in addition to the laser induced plume. The jet grew during the laser pulse and when the pulse terminated, its rapid growth stopped. At lower fluence, laser induced plasma expanding at about 1 O ms' was observed. The growth speed of the jet-like plasma depended on laser fluence and gas atmosphere but did not change for different metals.
Laser ablation of transparent materials is induced by non-linear absorption and some laser-induced damages are introduced in the bulk as well as on the surface. This process is used in laser marking and other applications such as refractive index modification of optical materials and 3-D data storage. We have observed the laser ablation dynamics in inside of bulk transparent materials by nanosecond time-resolved imaging technique. Output of fundamental radiation (1064nm) from a Q-switched Nd:YAG laser was focused at inside of bulk PMMA and soda glass. Second harmonic radiation (532nm) from the same laser was used as illuminating light and images were taken by a CCD camera with a band-pass filter at 532 nm. Series of images were taken at different intervals between the fundamental and the second harmonic light, which was controlled by optical delay line. In observation at longer intervals than 5Ons, another laser was used as illuminating source. When the laser was focused at inside of the bulk PMMA, damages occurred simultaneously at several independent points without the ablation at the surface. They located along laser incident axis. Propagation of shock waves, which started from these points, was clearly observed in the bulk. In the glass, absorbing point in the bulk formed a continuous line and its end-point moved from inside outward to the surface along the laser beam. Laser induced damages (cracks) continued to develop until some microseconds after laser pulse in PMMA.
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