Material phase change under extreme domain confinement in laser material interaction
Laser has been widely applied in the science and industry fields because of the good spatial and temporal coherence and narrow spectrum. And the spatial confinement is common in the laser-assisted manufacturing field and it results in the change of the manufacturing process. However, due to the short time duration and high energy, the underlying physics is hard to be probed by experiment. Molecular dynamic (MD) simulation is employed to investigate the phenomenon related to the spatial confinement which includes the shock wave, wall confinement in cold sintering and laser induced breakdown spectroscopy (LIBS) enhancement and the tip confinement in surface nanostructuring.
Existence of the shock wave affects the phase change and stress wave development and propagation significantly. It suppressed the bubble growth and shortened their lifetime. No effect from shock wave on the stress wave in solid was observed. The absorption depth and laser fluence played important roles in the stress wave formation and evolution. Secondary stress wave in the target occurred because of the ablated cluster re-deposition.
The final cold-sintered structure was found to be nanocrystalline. Smaller nanoparticles were easy to reconstruct, but the final structure was more destructed, and structural defects were observed. For larger particles, the final cold-sintered structure was partially nanocrystalline. The orientation-radial distribution function (ODF) was developed to investigate the degree of orientation twisting. It was proved to be more comprehensive than radial distribution function (RDF) in structure analysis for the additional angle information it provides.
Spatial confinement was found effective in improving the sensitivity of laser-induced breakdown spectroscopy (LIBS). The temperature, pressure and number density of the shock wave were observed to increase dramatically immediately after the reflection from the wall. The reflected shock wave and the forward-moving shock wave had a strong collision, and such an atomic collision/friction made the velocity of the shock wave decreases to almost zero after reflection. A temperature rise as high as 218 K was observed for the shock wave front after the wall reflection. More importantly, the temperature of the plume is enhanced dramatically from 89 K to 132 K. Also this high temperature was maintained for quite a long time. This explains the sensitivity enhancement in the spatial confinement of LIBS.
Nanoscale-tip based laser surface nanostructuring is a promising technique for ultrahigh density data storage and nanoelectronics industries. The phase change (e.g. melting, phase explosion, and solidification/re-crystallization) within the tip-substrate region is extremely confined at the scale of a few nm. Such extreme domain constraint could significantly affect the structuring process and the tip apex profile. On the other hand, little is known about this extremely confined phase change by both computer modeling and experiment. In this work, systematic atomistic modeling is employed to explore the tip-confinement effect on the surface nanostructuring. Material ablation is trapped by the tip and the number of atoms flying out from the substrate decreases due to the tip-confinement. Although large atom-clusters are observed in tip-free scenario, no such clusters are observed in tip-based surface nanostructuring. This is favorable for making nanoscale surface structures. Tip apex oscillation occurs because of its interaction with the substrate. The effect of tip-substrate distance and laser fluence on the surface nanostructuring is investigated in detail. The profile of the cone-shape crater in the substrate is not affected much by the tip-substrate distance. Instead, the laser fluence plays a dominant role in the final crater shape. The protrusion around the crater is affected by both the tip-substrate distance and the laser fluence. For the case of tip-substrate distance d= 7 nm, the protrusion is flatter but wider than d= 1 nm and 2 nm. The tip-confinement also affects the recrystallization process after laser heating. The recrystallization time is longer for the case with tip confinement due to the interaction between the tip and the substrate. The tip apex is distorted during laser ablation. Both the tip-substrate distance and the laser fluence play import roles in the distortion. For the case of laser fluence E= 10 J/m2, the tip apex is reshaped to be blunt. The nanotip-laser interaction (near-field focusing) could change negatively due to the tip-apex reshaping, and this change will induce undesirable surface nanostructure change. Although the tip-substrate confinement significantly prolongs the solidification/recrystallization process in the substrate, it has little negative effect on the defects formed in the nanostructuring region.