Nanotechnology

With the wave trapped, the researchers settled a long-standing debate

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June 17, 2023

(Nanowerk News) With a dramatic increase in computing power, the research team has solved the decades-long mystery of whether optical waves can be trapped in randomly packed three-dimensional microparticles or nanoparticles. This is a discovery that could open up new possibilities for lasers and photocatalysts among other applications.

Electrons in a material can move freely to flow current or get trapped and act as an insulator. It depends on the number of randomly distributed defects that 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 condensate physics. This theory extends to the quantum and classical realms, including electrons, acoustic waves, water, and gravity.

However, exactly how this principle plays into the trapping, or localization, of electromagnetic waves in three dimensions remains unclear – despite 40 years of extensive research. Led by Prof. Hui Cao, the researchers finally gave a definite answer whether light can be localized in three dimensions. This is a discovery that could open many avenues in both fundamental research and practical applications using 3D local light.

The results are published in Natural Physics (“Localization of Anderson’s electromagnetic waves in three dimensions”).

The search for Anderson’s 3D localization of electromagnetic waves has spanned decades with many attempts and failures. There are several experimental reports of 3D light localization, but all of them are questionable because experimental artifacts, or observed phenomena are associated with physical effects other than localization. This failure led to heated debate about whether Anderson localization of electromagnetic waves even exists in 3D random systems.

Because it is extremely difficult to eliminate all experimental artifacts to get conclusive results, Cao and his co-workers resorted to “an embarrassing numerical simulation,” as Philip W. Anderson put it in his 1977 Nobel Prize lecture. However, running computer simulations of Anderson localization in three dimensions has long proved challenging.

“We can’t simulate large three-dimensional systems because we don’t have enough computing power and memory,” said Cao, the John C. Malone Professor of Applied Physics and Professor of Electrical Engineering and Physics. “And people have tried various numerical methods. But it’s impossible to simulate such a large system to really show whether localization is present or not.”

But then Cao’s team recently teamed up with Flexcompute, a company that recently had a breakthrough in accelerating numerical solutions by an order of magnitude with their Tidy3D FDTD Software.

“It’s amazing how fast the Flexcompute numeric solver works,” he says. “Some of the simulations that we estimated would take days, turned out to be 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 obtain a three-dimensional light localization.”

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

Free of all the artifacts that previously distorted experimental data, their study closes a long debate about the possibility of light localization in three dimensions with accurate numerical results. First, they demonstrated that it is impossible to localize light in a random three-dimensional collection of particles made of dielectric materials such as glass or silicon, which explains the failure of intensive experimental efforts in recent decades. Second, they presented unambiguous evidence of the localization of Anderson’s electromagnetic waves in random packing of metal spheres.

“When we saw Anderson’s localization in the numerical simulation, we were very excited,” said Cao. “It’s amazing, considering there has been such a long pursuit by the scientific community.”

Metal systems have long been neglected because of their absorption of light. But even given the loss of common metals like aluminum, silver, and copper, Anderson’s localization persists.

“Surprisingly, even though the losses were not small, we can still see evidence of Anderson’s localization. That means it is a very strong and powerful effect.”

As well as resolving some old questions, this research opens up new possibilities for lasers and photocatalysts.

“Three-dimensional confinement of light in porous metals can enhance optical nonlinearity, light-matter interactions, and control random gain and targeted energy deposition.” said Cao. “So we hope there will be many applications.”



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