Effects of Thin Films on the Dynamics of Nanosecond Laser Ablation

Laser ablation plays an important role in machining, manufacturing, materials deposition and characterization. Prior studies predominantly focus on examining laser interaction with bulk, i.e., optically thick, structures. At the same time, nanostructures can drastically change optical and thermal dynamics, as such new regimes of laser ablation may be expected in nanostructured systems. However, detailed study of laser ablation in nanoscale systems is missing. Here, we report a discovery of nanostructure induced control over explosive boiling phase transition under intense laser illumination. We study laser ablation of nanometer thick films by high fluence nanosecond lasers and demonstrate that nanostructures drastically modify the onset of explosive boiling onset.

We study laser ablation of three different types of targets by a ~1J nanosecond (4.5 – 20 ns) laser. The first two targets are bulk Silicon and Titanium wafers. The third is a 100 nm thick titanium film deposited on Silicon. We then measure crater depth as a function of the laser fluence (up-to 75 kJ/cm2). For all three targets we observe phase transition as the laser energy is increased. Specifically, above certain fluence threshold, crater depth drastically increases, which indicates an onset of explosive boiling leading to rapid material removal. We find that the thin (<100 nm) metal films significantly modify the depth of laser produced craters and shift the explosive boiling threshold. Counterintuitively, while ablating the Titanium wafer produces deeper craters than those for the Silicon wafer, adding a Titanium film on top of the Silicon reduces the ablation depth.

We expect that such unprecedented dynamics is due to a coupled interplay between thermal conduction, laser absorption and plasma screening. To examine this hypothesis, we perform time resolved imaging and spectroscopy of the plasma plumes produced during ablation. For this purpose, a gated CCD camera coupled to a high resolution spectrometer is used. By measuring plume dynamics and we extract electron temperature at given points in time. We show that the plasma plume expands at speeds on the order of 10 km/s and has electron temperatures over 10,000K.

With the help of the experimental data, we develop a numerical model that takes into account laser absorption, thermal diffusion, evaporation, boiling and laser-plasma interplay. This model allows predicting complex dynamics of laser ablation and can be used to explain phase transitions observed experimentally. The results show that at the initial stages of ablation (first 10s of nanoseconds) the ejected material is produced through evaporation that is limited to the target surface. This evaporated material forms the plasma that shields the surface from the laser pulse. At later stages heat penetrates deeper into the target, and leads to boiling that causes most of the volume of the crater. We believe the separation of these two stages is what allows the thin film to have such an effect on laser ablation. Our general approach can potentially be extended to study ablation of targets with even more complex structure such as metasurfaces.

To summarize, we have discovered that nanoscale films can modify the threshold of explosive boiling, as compared to bulk targets. The experimental results correlate well with a semi-analytical model we developed. The observed effects are significant even at high fluences and need to be taken into account to accurately predict the ablation process.