Shock wave energy and acoustic energy dissipation after laser-induced breakdown

We investigated the spatial distribution of energy dissipation during propagation of the shock front arising from optical breakdown in water, because it is related to the stress-induced cellular changes in plasma-mediated laser surgery. The dissipation can be calculated from the shock wave velocity u_s by a relation derived from the Rankine-Hugoniot equation, u_s was measured as a function of time and space for various laser parameters. With a 1 mJ/ 6-ns pulse, 64 % of the absorbed light energy are converted into acoustic energy, but the largest part of this energy are converted into heat already within the first 200 μm of shock from propagation. Afterwards, the dissipation occurs at a much slower rate. Only ≈ 10% of the acoustic energy reaches a distance of 10 mm. Far-field measurements can thus be very misleading for an energy balance. The energy dissipation at the shock front leads to a temperature rise of the medium. At 10 mJ pulse energy, the temperature close to the plasma exceeds the critical point of water. This means that the shock wave passage goes along with an enlargement of the cavitation bubble. High-pressure-induced bubble formation can also occur at locations further away from the laser plasma where shock waves from adjacent plasmas interfere. We have thus demonstrated a mechanism of stress wave induced cavitation which does not rely on tensile stress, but on very high overpressures. Since most of the dissipation takes place within the first 200 μm, the shock wave effects are mostly covered by the effects of the cavitation bubble which reaches a radius of 800 μm in water at the same laser parameters. Acoustic tissue effects are, nevertheless important, because the bubble is smaller in tissue than in water, the weakening of the tissue structure by the shock wave passage probably contributes to the cavitation-induced damage, and the range for acoustic damage is larger in nonspherical geometries.