Modeling and experimental observation of photomechanical effects in tissue-like media

Abstract

To investigate the contribution of photomechanical fracture to tissue ablation with short laser pulses, we performed a study in which experimental phenomena were compared to results of a theoretical model. In this mode the generation and one-dimensional propagation of thermoelastic stress waves caused by a laser pulse of finite duration is simulated and the dynamics of cavitation induced by the negative components of these stress waves are calculated. To experimentally observe the cavitation dynamics in water and gelatine, an optical pump- probe technique together with time-resolved imaging and stress detection methods was employed. With the pump-probe technique the lifetimes of individual cavitation bubbles until the first collapse could be measured. At low fluence values a good agreement between experiment and simulation was observed. At higher fluences, where the bubbles do not move independently from each other as it is assumed in the model, the experimental lifetimes were longer than the calculated ones. Additional properties of photomechanical damage such as the dependence of the cavitation process on the time-integral over the stress wave, the spatial occurrence of cavitation bubbles and the influence of viscosity in gelatine are demonstrated in the simulation.