Simulation Reveals Ablation Mechanisms

Gary Boas

To better understand laser ablation, researchers at the University of Montreal have developed a novel method to simulate thermodynamic trajectories of groups of atoms in a solid. The behavior of the atoms reveals the instabilities that result in the ejection of material and, the researchers suggest, the ablation mechanisms for atoms at different absorbed energy densities.

A model of the ablation of solids by femtosecond laser pulses suggests that the process involves three mechanisms dependent on the deposited energy. The topology of the target at early (left) and later (right) stages of ablation reveals different effects at different energy densities: unablated solid (I), homogeneous nucleation (II), fragmentation (III) and vaporization (IV, out of range). Courtesy of Danny Perez.

A number of theories have been proposed to explain the ablation of solids under femtosecond laser pulses. However, while working to resolve a practical issue restricting the use of ultrashort laser pulses in pulsed laser deposition applications, Danny Perez and Laurent J. Lewis found that the issue was only indirectly supported by theory and experiment. Thus, they turned their attention to the fundamental aspects of the problem to determine the mechanisms that give rise to ultrashort laser ablation of solids.

The simulations suggest that three mechanisms can contribute to ablation: homogeneous nucleation inside a metastable liquid, photomechanical fragmentation of the fast-expanding material and complete vaporization. The study is the first to demonstrate that photomechanical fragmentation can account for ablation under short laser pulses.

"Interestingly, these mechanisms can occur simultaneously at different depths under the surface of the target," Perez said. "The ablation process is thus more complex than what was previously thought."

The physics proved to be simpler than the pair expected, he said, because the isentrope on which the expansion occurs after the absorption of the pulse almost entirely determines the nature of the ablation mechanism. "Provided that the phase diagram is known, this -- in principle -- enables one to predict the ablation mechanism, some of the properties of the ejected material and the ablation depth."

The researchers have gone on to study in more detail the dynamics of the expansion and ablation processes. For example, they have investigated the influence of strong pressure waves emitted within the target and have considered a fourth ablation mechanism -- spallation --in which a tensile pressure wave causes a mechanically unstable state in the target and ejects solid pieces of the material. These and other findings will be published.