Time-resolved investigations of laser-induced shock waves in water by use of polyvinylidenefluoride hydrophones

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Abstract
Laser light from a Q-switched Nd:yttrium-aluminum-garnet laser (λ=1064 nm; pulse duration=20 ns; pulse energies up to 150 mJ) focused into water creates shock waves by rapidly expanding microplasmas. Using piezoelectric, thin-film polyvinylidenefluoride (PVDF) as a transducer, a broadband hydrophone (100-MHz bandwidth) was developed to investigate underwater shock waves. The electrical signal is analyzed with respect to reflections of the shock wave within the transducer and the input impedance of the measuring device. The shock waveform is determined, its peak pressure ranging to kbars (108 Pa), decreasing with r−1.12 and increases by the square root of the laser pulse energy. The time resolution of the hydrophone (4 ns) is sufficient to determine the plasma dimensions and the number of shock waves generated by a single laserpulse. Both vary statistically, primarily because of contaminations in the fluid. Because of the length of the region containing plasmas, different peak pressures are found in the direction of the laser beam and perpendicular to it.