Light waves can be 'frozen' or 'trapped' in 3D materials, shows new study

Previous attempts to achieve three-dimensional (3D) Anderson localization of electromagnetic waves have faced challenges and resulted in debates and questionable results. However, a new study puts these debates to rest. 

A team of researchers led by Prof. Hui Cao from Yale University has conclusively demonstrated the three-dimensional localization of vector electromagnetic waves in random aggregates of overlapping metallic spheres.

This study settles the long-running debate over whether optical waves may be trapped or frozen in three-dimensional, randomly packed nano- or micro-particles.

Teaming up with Flexcompute

For four decades, various experimental and numerical attempts have been made to find 3D Anderson localization of electromagnetic waves. Despite various efforts, previous reports have been called into question, as they can be attributed to observational artifacts or other physical processes rather than localization.

Numerical simulations have also proven challenging to run due to the computational complexity of the problem. Cao and her team decided to team up with Flexcompute, a company that just made a breakthrough in order-of-magnitude numerical solution acceleration with their FDTD Software Tidy3D.

In a press release, Cao said, "It's amazing how fast the Flexcompute numerical solver runs. Some simulations that we expect would take days to do, it can do in just 30 minutes. This allows us to simulate many different random configurations, different system sizes, and different structural parameters to see whether we can get three-dimensional localization of light."

Cao and her international team of researchers included Dr. Tyler Hughes and Dr. Momchil Minkov at Flexcompute, Prof. Zongfu Yu at the University of Wisconsin, Prof. Alexey Yamilov at the Missouri University of Science and Technology, and Dr. Sergey Skipetrov from the University of Grenoble Alpes in France. 

Trapping light waves

Cao and her team used Flexcompute's FDTD Software Tidy3D to demonstrate the 3D localization of vector electromagnetic waves. Their research had two main findings. 

They first showed that the localization of 3D random aggregates of particles made of dielectric materials, such as glass or silicon, is impossible. This explained why previous experiments failed to demonstrate 3D Anderson localization.

Next, they focused on random aggregates of overlapping metallic spheres. Their simulations demonstrated the presence of localization in the metallic spheres, which was in stark contrast to the absence of localization seen in dielectric spheres.

Despite the absorption of light in metallic systems, which has historically led to their relative neglect, the study demonstrates the persistence of Anderson localization in these systems. The researchers observed evidence of Anderson localization even when considering the inherent light absorption of common metals like aluminum, silver, and copper.

This suggests that Anderson localization in metallic systems is a robust phenomenon, capable of overcoming the challenges posed by light absorption.

"When we saw Anderson localization in the numerical simulation, we were thrilled. It was incredible, considering that there has been such a long pursuit by the scientific community," said Cao. 

This opens up new avenues for research into lasers, photocatalysts, and Anderson localization. 

Their findings are published in the journal Nature Physics. 

Study abstract:

Anderson localization is a halt of diffusive wave propagation in disordered systems. Despite extensive studies over the past 40 years, Anderson's localization of light in three dimensions has remained elusive, leading to the question of its very existence. Recent advances have enabled finite-difference time-domain calculations to be sped up by orders of magnitude, allowing us to conduct brute-force numerical simulations of light transport in fully disordered three-dimensional systems with unprecedented dimension and refractive index difference. We show numerically three-dimensional localization of vector electromagnetic waves in random aggregates of overlapping metallic spheres, in sharp contrast to the absence of localization for dielectric spheres with a refractive index up to 10 in air. Our work opens a wide range of avenues in both fundamental research related to Anderson localization and potential applications using three-dimensional localized light.