Shock Wave Emission by Laser Generated Bubbles
The phenomena occurring when short pulses of laser light are focused into a liquid are reviewed from the first findings after the invention of the laser to the present state of knowledge. Dielectric breakdown with plasma and bubble formation, the breakdown shock wave, bubble dynamics with expansion and collapse, and the bubble collapse shock wave or waves are addressed. Breakdown plasma lengths as a function of laser pulse energy are given. The propagation speed of the breakdown shock wave and the related shock peak pressure as determined by high speed streak recordings for nano pico and femtosecond laser pulses focused into water are discussed. Breakdown shock wave velocities up to 5000 m/s and peak pressures up to 100 kbar are reported for these monopolar acoustic pulses for laser pulse energies up to some 10 mJ. In tissue bipolar pressure pulses are observed due to the elasticity of the medium. The widths of the shock waves reach values in the range of tens of nano seconds to beyond 100 ns. The simultaneously generated breakdown bubble gets about half of the energy of the shock wave. Bubble energy rises linearly with the laser pulse energy with different slopes depending on the laser pulse duration. Equations for bubble dynamics are given and compared with laser induced bubble dynamics. Strength and width of bubble collapse shock waves measured with PVDF and fiber optic hydrophones are presented together with the breakdown shock waves. Similar values are obtained for both collapse and breakdown in the bulk of the liquid. Shock wave emission from bubbles collapsing near boundaries (solid and elastic) is discussed together with applications in cleaning and erosion or cell destruction.