Experimental observation of immediate focus of underwater shock wave by using concave emitter induced by nano-pulsed laser

Propagation of an underwater micro-shock wave induced by a single-pulsed laser was observed by the shadowgraph technique. To introduce the micro-shock wave into water, different types of shock wave emitters are designed using flat and concave glass coated with 100-nm-thick titanium metal. The experiment was carried out in low irradiation intensity range from 0 to 200 GW/m², and the shock wave was produced by the thermo-elastic effect of the metal film. In this study, we proposed an immediate focusing technique using a concave surface. The focusing effect was confirmed by optical observation and pressure measurement. The velocity and Mach number of shock wave were calculated using a shadowgraph imaging technique. The result revealed that the laser-induced micro-shock wave propagated approximately at just under the sound of speed in water. From the pressure measurement, it was confirmed that the shock wave strength from the concave emitter was 1.5 times stronger than the flat one. The effective area of the focus was determined by the radius of curvature of the concave surface and the irradiation beam diameter.

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