Efficient method to simulate light-matter interactions at the atomic scale

The light-matter interaction is an essential subject for the disciplines of physical and chemical sciences and optical and electrical engineering. The invention of the laser in the early 1960s led to several innovations in these fields. Since then, laser technologies have developed in various directions.

In the field of optical science, it is becoming increasingly important to observe and manipulate matter on an atomic scale using ultrashort pulsed light.

Light-matter interactions are difficult to simulate because the phenomena related to light-matter interaction are multiphysical in nature, involving the propagation of light waves and the dynamics of electrons and ions in matter. Three physical laws are involved: electromagnetism for light fields, quantum mechanics for electrons, and Newtonian mechanics for ionic motion.

Now, in a study published in The International Journal of High performance computing applications, a research team led by the University of Tsukuba describes a very efficient method for simulating light-matter interactions at the atomic scale.

Due to the multiphysics and multiscale nature of the problem, two distinct computational approaches have been developed. The first is electromagnetic analysis, in which matter is treated as a continuous medium, and the second is the ab initio quantum calculation of the optical properties of materials. These two approaches suppose a weakness of the light field (theory of disturbances in quantum mechanics) and a difference in length scale (macroscopic electromagnetism). However, the utility and capacity of these traditional computational approaches are limited in current research.

“Our approach provides a unified and improved way to simulate light-matter interactions” says lead author of the study, Professor Kazuhiro Yabana. “We accomplish this feat by simultaneously solving three key physical equations: Maxwell’s equation for electromagnetic fields, the time-dependent Kohn-Sham equation for electrons, and Newton’s equation for ions.”

The researchers implemented the method in their internal software SALMON (Scalable Ab initio Light-Matter simulator for Optics and Nanoscience). They thoroughly optimized the simulation computer code to maximize its performance. They then tested the code by modeling light-matter interactions in a thin film of amorphous silicon dioxide made up of more than 10,000 atoms. This simulation was performed using nearly 28,000 nodes of the world’s fastest supercomputer, Fugaku, at the RIKEN Center for Computational Science in Kobe, Japan.

“We have found that our code is extremely efficient, reaching the goal of one second per time step of the computation which is necessary for practical applications” said Professor Yabana. “The performance is near its maximum possible value, defined by the computer memory bandwidth, and the code has the desirable property of excellent low scalability.”

Although the team simulated light-matter interactions in a thin film in this work, their approach could be used to explore many phenomena in optics and photonics at the nanoscale.

Journal reference

  1. Yuta Hirokawa, Atsushi Yamada, Shunsuke Yamada, Masashi Noda, Mitsuharu Uemoto, Taisuke Boku, Kazuhiro Yabana; Large-scale ab initio simulation of light-matter interaction at the atomic scale at Fugaku, High performance computing applications. DOI: 10.1177 / 10943420211065723
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