Streak-photographic investigation of shock wave emission after laser-induced plasma formation in water

The shock wave emission after dielectric breakdown in water was investigated to assess potential shock wave effects in plasma mediated tissue ablation and intraocular photodisruption. Of particular interest was the dependence of shock wave pressure as a function of distance from the plasma for different laser pulse energies. We have generated plasmas in water with a Nd:YAG laser system delivering pulses of 6 ns duration. The pulses, with energies between 0.4 and 36 mJ (≈ 180 times threshold), were focused into a cuvette containing distilled water. The shock wave was visualised with streak photography combined with a schlieren technique. An important advantage of this technique is that the shock position as a function of time can directly be obtained from a single streak and hence a single event. Other methods (e.g. flash photography or passage time measurements between fixed locations) in contrast rely on reproducible events. Using the shock wave speed obtained from the streak images, shock wave peak pressures were calculated providing detailed information on the propagation of the shock: The shock peak pressure as a function of distance r from the optical axis was found to decrease faster than 1/r² in regions up to distances of 100 - 150 μm. For larger distances it was found to be roughly proportional to hr. The scaling law for maximum shock pressure Ps at a given distance was found to be proportional to the square root of the laser pulse energy E for distances of 50 - 200μm from the optical axis.