Bubble dynamics and shock waves generated by laser absorption of a photoacoustic sphere

Theoretical analysis and extensive numerical simulations are performed on the system of a spherical absorber illuminated by a pulsed laser. Both bubble formation and shock wave generation are investigated. We find that a strong shock front forms just outside the absorber and attenuates quickly into an acoustic wave as it propagates away from the absorber. The formation and growth of the bubble is shown to have a gentle threshold dependence and to be a continuous function of the energy of the laser pulse with no explosive threshold. There is a weak dependence of the threshold fluence for bubble formation on the pulse duration of the laser. The bubble growth dynamics are also pulse-duration dependent for pulse lengths shorter than the thermal conduction time. For ultrashort pulses (shorter than the acoustic transit time across the absorber), the bubble dynamics approach a limiting growth trajectory. The process is a cooperative effect of both thermal and pressure interactions. We also find that variation of the absorber's mechanical parameters, such as the thermal expansion coefficient, can significantly alter not only the shock waves but also the bubble growth dynamics. (C) 2000 American Institute of Physics. [S0021-8979(00)05618-8].