With the waves trapped, the researchers resolve a long-standing debate about the localization of light in three dimensions

In a dramatic increase in computing power, a team of researchers has solved a decades-old mystery about whether optical waves can be trapped in randomly packed three-dimensional micro- or nanoparticles. It’s a discovery that could open up new possibilities for lasers and photocatalysts among other applications.

Electrons within a material can move freely to conduct current or get trapped and act as insulators. This depends on the amount of randomly distributed defects the material has. When this concept, known as Anderson localization, was proposed in 1958 by Philip W. Anderson, it proved to be a game changer in contemporary condensed physics. The theory extended into both the quantum and classical realms, including electrons, acoustic waves, water and gravity.

However, exactly how this principle plays out in the trapping, or localization, of electromagnetic waves in three dimensions has not been clear despite 40 years of extensive study. Led by Prof. Hui Cao, the researchers have finally provided a definitive answer on the possibility of locating light in three dimensions. It is a discovery that could open up a wide range of avenues in both fundamental research and practical applications using 3D localized light. The results are published in Physics of nature.

Anderson’s 3D localization research of electromagnetic waves spanned several decades with numerous attempts and failures. There have been multiple experimental reports of 3D light localization, but they have all been questioned due to experimental artifacts or the observed phenomena have been attributed to physical effects other than localization.

These failures have led to intense debate as to whether the Anderson localization of electromagnetic waves even exists in 3D random systems. Since it is extremely difficult to eliminate all experimental artifacts to obtain conclusive results, Cao and his colleagues have resorted to the “humiliation of numerical simulation,” as Philip W. Anderson put it in his 1977 Nobel Prize lecture. However, running computer simulations of Anderson’s location in three dimensions has long proved a challenge.

“We haven’t been able to simulate large three-dimensional systems because we don’t have enough computing power and memory,” said Cao, an applied physics professor and John C. Malone professor of electrical engineering and physics. “And people have tried various numerical methods. But it hasn’t been possible to simulate such a large system to really show whether or not localization exists.”

But then Cao’s team recently partnered with Flexcompute, a company that has had a recent breakthrough in accelerating numerical solutions by orders of magnitude with their FDTD software Tidy3D.

“It’s amazing how fast the Flexcompute numerical solver works,” he said. “Some simulations that we predict would take months are done in just 30 minutes. This allows us to simulate many different random configurations, different system sizes, and different structural parameters to see if we can get three-dimensional light localization.”

Cao assembled an international team that included his longtime collaborator Prof. Alexey Yamilov at the Missouri University of Science and Technology and Dr. Sergey Skipetrov of the University of Grenoble Alpes in France. They worked closely with Prof. Zongfu Yu of the University of Wisconsin, Dr. Tyler Hughes and Dr. Momchil Minkov of Flexcompute.

Devoid of all the artifacts that have previously marred the experimental data, their study closes the long-running debate about the possibility of localizing light in three dimensions with accurate numerical results. First, they demonstrated that it is impossible to localize light in three-dimensional random clumps of particles made of dielectric materials such as glass or silicon, which explains the failures of the intense experimental efforts of recent decades. Second, they presented unequivocal evidence of Anderson localization of electromagnetic waves in random packs of metal spheres.

“When we saw Anderson’s location in the numerical simulation, we were thrilled,” Cao said. “It was incredible, considering there’s been such a long search by the scientific community.”

Metallic systems have long been ignored due to their light absorption. But even considering the loss of base metals such as aluminum, silver, and copper, Anderson’s location persists.

“Surprisingly, even though the leak wasn’t small, we can still see evidence of Anderson localization. That means it’s a very robust and strong effect.”

As well as solving some long-standing questions, the research opens up new possibilities for lasers and photocatalysts.

“The three-dimensional confinement of light in porous metals can enhance optical nonlinearities, light-matter interactions, and control random laser and targeted energy deposition,” Cao said. “So we anticipate that there could be many applications.”