Light-matter interactions simulated on supercomputer

Researchers at the University of Tsukuba discover an improved way to model interactions between matter and light at the atomic scale using the worlds fastest supercomputer.

Light-matter interactions are required to form the majority of crucial technologies, including lasers, light-emitting diodes (LEDs), and atomic clocks. However, usual computational approaches for modelling such interactions have limited capability.

As a result, researchers from the University of Tsukuba, Japan, have developed a technique that overcomes these limitations by simulating light–matter interactions at the atomic scale.

What makes light-matter interactions so difficult to simulate?

One reason for its complexity is that the phenomena associated with the interactions encompass many areas of physics, involving both the propagation of light waves and the dynamics of electrons and ions in matter. Another reason is that such phenomena can cover a wide range of length and time scales.

Given the multiphysics and multiscale nature of the problem, light–matter interactions are typically modelled using two separate computational methods. The first is electromagnetic analysis, whereby the electromagnetic fields of the light are studied; the second is a quantum-mechanical calculation of the optical properties of the matter. But these methods assume that the electromagnetic fields are weak and that there is a difference in the length scale.

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

Putting their simulation into effect

The researchers implemented their method in their in-house software SALMON (Scalable Ab initio Light–Matter simulator for Optics and Nanoscience), while optimising the simulation computer code to maximise its performance. They then tested the code by modelling light–matter interactions in a thin film of amorphous silicon dioxide, composed of more than 10,000 atoms. This simulation was carried out using almost 28,000 nodes of the fastest supercomputer in the world, Fugaku, at the RIKEN Centre for Computational Science in Kobe, Japan.

“We found that our code is extremely efficient, achieving the goal of one second per time step of the calculation that is needed for practical applications,” concluded Professor Yabana. “The performance is close to its maximum possible value, set by the bandwidth of the computer memory, and the code has the desirable property of excellent weak scalability.”

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

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