Computational Simulations Could Enable Intelligent Design of New Laser Applications
In this day and age, the term laser is easily recognizable and probably brings to mind one of the many items in our daily lives that use the intense light or heat, or both, offered by various laser technologies. Several of those technologies employ only the highly precise light source of lasers. Among the most well known are supermarket checkout scanners, CD players, gun sights, and presentation pointers. But many more applications take advantage of the heat aspect of a laser rather than the light, including optical and cosmetic surgeries, high-precision glass and metal cutting, and the heat and shock treatment of various materials to allow for precise surface hardening.
As is the case with most modern science, much of the research being done on laser technologies is being aided by the use of computational simulations.
Leonid Zhigilei and co-researchers Chengping Wu, Eaman Tahir Abdul Karim, and Maxim Shugaev at the University of Virginia used Kraken (now decommissioned) at the National Institute of Computational Sciences (NICS) to investigate the distinct characteristics of short pulse laser interactions with a metal target (silver, Ag) covered by a transparent overlayer. As part of their investigation, they developed a highly scalable message-passing interface (MPI) parallel code, which they optimized for use on Kraken. With this code, they simulated the interactions between a short pulse laser and a silver target covered by a transparent overlayer.
Details of their project are provided in the 2014 paper titled "Atomistic simulation study of short pulse laser interactions with a metal target under conditions of spatial confinement by a transparent overlayer" in Journal of Applied Physics.
They ran the simulations for a broad range of laser intensities (or fluences), which, without the transparent overlayer, would normally cover the regimes of melting and resolidification, photomechanical spallation, and phase explosion of the surface regions of the metal target.
“Photomechanical spallation,” explains Zhigilei, “is the removal or ejection of particles driven by the relaxation of laser-induced stresses.” He goes on to explain that phase explosion simply signifies “the explosive release of vapor in a part of the target metal which is quickly heated above the limit of thermodynamic stability of the superheated liquid.”
Zhigilei notes that “the fundamental understanding of the mechanisms of complex structural and phase transformations occurring in the irradiated targets is often lacking.” Thus, without atomic-level molecular dynamics simulations “it is difficult to design a reliable continuum-level description of the laser–target materials interactions.”
“Large-scale molecular dynamics simulations,” Zhigilei says, “can provide unique insights into the collective atomic dynamics responsible for the laser-induced transformations of the target.” And he adds, “Using molecular dynamics can lead to the discovery of new processes and mechanisms that were not anticipated before running the simulations.”
“This improved understanding of the fundamental mechanisms of the laser–materials interactions is important from the practical point of view,” says Zhigilei, “because it enables an intelligent design of new laser applications.”
The results of the simulations revealed that the spatial confinement by the transparent overlayer suppresses the generation of an unloading tensile wave and leads to the formation of a thin, nanocrystalline layer at the interface between the transparent overlayer and the silver target. Also, at higher laser fluences, a detachment of the metal target from the confining overlayer occurred.
Among the primary challenges to this research, the foremost issue arises from the limiting factor of time and length scales that are accessible for the simulations. Essentially, because the molecular dynamics technique is based on the integration of the equations of motion for all of the atoms within the system, the simulation is limited to a smaller number of atoms, and a shorter period of time, than would be preferred for an ideal investigation of the laser–target interactions.
Understandably then, one area for future research lies in the design of new, multiscale computational approaches that can overcome the time and length scale limitations. The advancement of the computational model, Zhigilei explains, may enable investigation of new practically important and scientifically intriguing phenomena.
“The next steps that are both challenging and exciting may involve the extension of the domain of these atomistic molecular dynamics simulations to include laser interactions with complex, multicomponent systems such as nanocomposite materials or biological tissue,” Zhigilei says.
- Karim, E. T., Shugaev, M., Wu, C., Zhibin, L., Hainsey R. F., Zhigilei, L. V. (2014). Atomistic simulation study of short pulse laser interactions with a metal target under conditions of spatial confinement by a transparent overlayer. Journal of Applied Physics, 115. doi: 10.1063/1.4872245
- Wu, C., Zhigilei, L. V. (2013). Microscopic mechanisms of laser spallation and ablation of metal targets from large-scale molecular dynamics simulations. Applied Physics A, 114, 11-32. doi: 10.1007/s00339-013-8086-4
Jacob Pieper, science writer, NICS, JICS
Article posting date: 5 August 2014
About JICS and NICS: The Joint Institute for Computational Sciences (JICS) was established by the University of Tennessee and Oak Ridge National Laboratory (ORNL) to advance scientific discovery and state-of-the-art engineering, and to further knowledge of computational modeling and simulation. JICS realizes its vision by taking full advantage of petascale-and-beyond computers housed at ORNL and by educating a new generation of scientists and engineers well versed in the application of computational modeling and simulation for solving the most challenging scientific and engineering problems. JICS runs the National Institute for Computational Sciences (NICS), which had the distinction of deploying and managing the Kraken supercomputer. NICS is a leading academic supercomputing center and a major partner in the National Science Foundation's eXtreme Science and Engineering Discovery Environment, known as XSEDE. In November 2012, JICS sited the Beacon system, which set a record for power efficiency and captured the number one position on the Green500 list of the most energy-efficient computers.