The transition from normal vaporization to phase explosion during laser ablation of aluminum was investigated using a nanosecond Nd:YAG laser. The threshold nature of phase explosion was observed by a discontinuous jump in the ablation depth at approximately 5.2J∕cm2. Ablation was imaged using a shadowgraph technique that was capable of probing ablation with nanosecond exposure time and nanosecond time delay resolution with respect to laser heating. Images above the threshold captured a mixture of vapor and droplets generated by phase explosion, which began near the end of the laser pulse without a significant time lag.

Visualization of Nd : YAG laser ablation of aluminium targets was performed by a shadowgraph apparatus capable of imaging the dynamics of ablation with nanosecond time resolution. Direct observations of vaporization, explosive phase change and shock waves were obtained. The influence of vaporization and phase explosion on shock wave velocity was directly measured. A significant increase in the shock wave velocity was observed at the onset of phase explosion. However, the shock wave behaviour followed the form of a Taylor–Sedov spherical shock below and above the explosive phase change threshold. The jump in the shock wave velocity above phase explosion threshold is attributed to the release of stored enthalpy in the superheated liquid surface. The energy released during phase explosion was estimated by fitting the transient shock wave position to the Taylor scaling rules. Results of temperature calculations indicate that the vapour temperature at the phase explosion threshold is slightly higher than the critical temperature at the early stages of the shock wave formation. The shock wave pressure nearly doubled when transitioning from normal vaporization to phase explosion.

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Phase explosion is an explosive liquid-vapor phase change that occurs during short pulse laser ablation. Phase explosion results from homogenous vapor nucleation in a superheated liquid phase as the surface temperature approaches the thermodynamic critical temperature, Tc. For a metastable liquid, the upper limit of superheating is approximately 0.9Tc, above which the rate of homogeneous nucleation rises dramatically. Prior to reaching the superheat limit however, a “dielectric transition” is expected to occur at approximately 0.8Tc. The dielectric transition is the transition of an electrically conductive material to a non-conducting state due to large fluctuations in material properties. One consequence of the dielectric transition is that the material will become semi-transparent. Until now, little work has been performed to understand the role of the dielectric transition in laser ablation, and many questions remain about how the surface will rise above 0.8Tc if the surface is semitransparent and only weakly absorbing. This work investigates the role of the dielectric transition with a one-dimensional numerical model for heat transfer and phase change and includes the effect of the metal to dielectric transition. The model is used to simulate heating of aluminum by a Nd:YAG laser with a 7 nanosecond pulse width (FWHM) at the fundamental wavelength of 1064 nm. Calculations of the transient temperature field, melt depth, and depth of the dielectric layer are obtained. Estimates of the absorption coefficient of a metal surface above the metal-dielectric transition are made from correlations found in the research literature. The value of the absorption coefficient is shown to be a critical parameter for determining the energy density required to reach 0.9Tc.