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14LM-P-6
LASER-MATTER INTERACTION
Delayed effects in laser ablation
A.A. Samokhin, N.N. Il'ichev, A.V. Sidorin, P.A. Pivovarov
Prokhorov General Physics Institute of the Russian Academy of Sciences, 119991, Moscow, Vavilov str., 38
e-mail: [email protected]
In a number of experimental works [1-4] on nano- and subnanosecond laser ablation of metals after direct exposure to a laser pulse an additional effect was also observed which was significantly delayed relative to the irradiation pulse. To date, this effect remains without a satisfactory explanation as well as some other aspects of non-equilibrium matter behavior near its critical points of liquid-gaz transition. The multi-microsecond delay is difficult to relate to the delay time of explosive boiling in a highly superheated metastable liquid, which corresponds to picosecond times. It was noted in [5] that, under certain conditions, a large delay can be can be explained as due to the occurrence of explosive boiling in the heating zone and the subsequent cavitation process, for the development of which such long times are usual. However the experimental conditions [1-4] differ significantly from those when cavitation is usually observed during laser action on various liquids [5-8]. The work reports on the influence of various conditions of laser action on the occurrence of cavitation during irradiation of the surface of an absorbing liquid.
The results of the pulse action from a holmium laser (X = 2.92 ^m, t = 100 ns, E = 0.1-1.5 J/cm2, d^ = 0.2 cm) on water were recorded by a piezoelectric sensor under various conditions on the irradiated surface. The sensor was located directly under a water layer 0.2 cm thick. The absorption coefficient for such radiation in water is about 104 cm-1. In this case only an ablation acoustic signal was observed on the free surface which appeared during the action of the laser pulse. In the presence of a solid transparent cover on the surface (sapphire or quartz with a thicknesses of about 1 cm) a second acoustic signal appeared with a delay of up to hundreds of microseconds with an amplitude comparable to the first signal. The use of a thin (50 ^m) fluoroplastic film as a coating led to a decrease in the amplitude and delay of the cavitation signal. These results show that cavitation can develop at shorter absorption lengths than in [6] and with less massive cover than was used in [7, 8]. Observation of such a dependence makes it more probable that under the experimental conditions [1-4] the delayed effect of laser action was caused by the cavitation process that developed after explosive boiling in the liquid layer with an absorption length increased to micrometer values due to metal-nonmetal transition.
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