Early plume expansion in atmospheric pressure midinfrared laser ablation of water-rich targets.

We have developed a one-dimensional fluid dynamics model for the ablation of water-rich targets by nanosecond infrared laser pulses at atmospheric pressure. To describe the laser-target interaction and the plume expansion dynamics, in light of recent experimental results the model incorporates phase explosion due to superheating and the nonlinear light absorption properties of water. In the model, the phase explosion is treated as a prolonged process that lasts for a finite time. Once a thin layer beneath the target surface exceeds the phase explosion temperature, this layer is transformed from target material into a mixture of water vapor and droplets and become part of the plume. This process is sustained for some time until the laser energy cannot maintain it. The simulation results show that as a result of two different phase transition mechanisms, i.e., surface evaporation and phase explosion, a first, slower plume expansion phase is followed by a more vigorous accelerated expansion phase. The calculated time evolution of the shock front at various fluence levels agrees well with the experimental observations of Apitz and Vogel [I. Apitz and A. Vogel, Appl. Phys. A. 81, 329 (2005)]. This model sheds light on the effect of phase explosion in laser ablation dynamics and its results are relevant for material synthesis, surface analysis, and medical (surgery) applications.