Microcavity dynamics during laser-induced spallation of liquids and gels

Photomechanical fracture induced by thermoelastic stress waves is an important mechanism of tissue ablation by short laser pulses. In this study, we present experimental investigations of the fracture process in ductile, water-containing materials and compare the results with a theoretical calculation. The model describes cavitation caused by the negative part of a bipolar thermoelastic stress wave. Pulses from a Q-switched, frequency-doubled Nd:YAG laser with 8 ns duration were used to irradiate dyed water and gelatine with variable absorption coefficient. Cavitation and ablation were observed with various time-resolved methods such as stress detection, video imaging and an optical pump-probe technique for the detection of individual cavities. Quantitative agreement between experiment and simulation could be achieved in the case of cavity lifetimes, especially at low laser fluence where the bubble density is low and no coalescence takes place. An increase of the threshold energy density for ablation with rising absorption coefficient and a distortion of the thermoelastic wave in the presence of cavitation were experimentally observed and could be qualitatively explained by use of the simulation. The results obtained in this study should facilitate the choice of the optimal laser parameters for photomechanical tissue ablation.