Thermodynamic evolution of phase explosion during high-power nanosecond laser ablation

Metallic surface temperature at the very beginning of the laser-induced breakdown obtained from black-body radiation

We present experimental results of cooper surface temperature measured nanosecond by nanosecond at the very beginning of the laser-induced breakdown experiment. The experiment is conducted by Nd:YAG laser for three irradiances: 1.4×10^{9}, 3.2×10^{11}, and 3.7×10^{12} W/cm^{2}. The temperatures of the metal surface, before plasma is created, are in the interval 7 400-11 200 K. These values are close to most estimates for the critical temperature of copper. The moment when plasma was created is recognized applying a bifurcated optical cable. After creation, temperature of the plasma increases attaining values over 50 000 K. All temperatures are derived relying on continuous spectra emitted from the bright spot illuminated by the laser.

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.

Time-resolved shadowgraphs of material ejection in intense femtosecond laser ablation of aluminum

The dynamic process of intense 50 fs laser ablation of aluminum is investigated by ultrafast time-resolved microscopy. A stripe pattern preceding phase explosion is clearly seen in the shadowgraph of 1 ns time delay. Intermittent material ejections are observed within the ejected plume after 2.5 and 7 ns time delay, respectively, which may be attributed to the material response to the generation of an extremely strong thermoelastic wave. Similar processes are also recorded in the ablation of silicon and glass samples, except for the glass samples, the intermittent material ejections are not found.