Effect of tensile stress amplitude and temporal characteristics on threshold of cavitation-driven ablation

Understanding of laser tissue ablation mechanisms may help to optimize laser ablation parameters in order to achieve minimal thermal and mechanical damage to surrounding tissues. One of the possibility to avoid unwanted thermal and mechanical effects is the use of cavitation-driven ablation. This regime provides ablation of aqueous media at temperatures substantially below 100°C. Cavitation-induced ablation can be achieved at stress- confined irradiation of absorbing media with free surface. Under these irradiation conditions relatively powerful thermoelastic tensile pressure waves can be generated. The tensile waves can produce cavitation inside irradiated medium and cavitation-induced ablation. Temporal profile of the tensile wave depends on absorption coefficient of the medium while amplitude depends on both absorption coefficient and fluence of laser pulses. In our study effect of the amplitude and duration of the tensile waves on threshold of cavitation-induced ablation is investigated. Absorbing aqueous solutions of potassium chromate with various concentrations were irradiated by third harmonic of Q-switched Nd:YAG laser. On the basis of the experimental results, one can conclude that ablation threshold under this irradiation conditions is effected by both amplitude and temporal characteristics of the tensile pressure waves. Effect of duration of tensile pulse is caused by resonance and cooperative phenomena upon cavitation and coalescence of cavitation bubbles in the irradiated volume. Other effects (initial distribution of cavitation bubbles, etc.) are also discussed.