Branched Flow of Light

Physicists Observe Branched Flow of Light

Physicists from the Technion-Israel Institute of Technology and the University of Central Florida have experimentally observed optical branched flow in liquid soap films.

Branched flow pattern shown on top of the interference color pattern generated by weak white light, making it possible to observe the potential landscape together with the branched flow. Image credit: Patsyk et al, doi: 10.1038/s41586-020-2376-8.

Waves propagating through a weak disordered potential with correlation length larger than the wavelength produce surprisingly long narrow filaments, or branches.

Instead of producing completely random speckle patterns, the slowly varying disordered potential gives rise to focused filaments that divide to form a pattern resembling the branches of a tree.

This phenomenon is called branched flow. It was first observed for electrons and for microwave cavities and it is generally expected for waves with vastly different wavelengths.

“We are familiar with the fact that waves spread when they propagate in a homogeneous medium. But for other kinds of mediums, waves can behave in very different ways,” said senior co-author Professor Miguel Bandres, a researcher in the College of Optics and Photonics at the University of Central Florida.

“When we have a disordered medium where the variations are smooth, like a landscape of mountains and valleys, the waves will propagate in a peculiar way.”

“They will form channels that keep dividing as the wave propagates, forming a beautiful pattern resembling the branches of a tree.”

In the study, Professor Bandres and colleagues coupled a laser beam to a soap membrane, which contains random variations in membrane thickness.

They discovered that when light propagates within the soap film, rather than being scattered, the light forms elongated branches, creating the branched flow phenomenon for light.

“In optics we usually work hard to make light stay focused and propagate as a collimated beam, but here the surprise is that the random structure of the soap film naturally caused the light to stay focused. It is another one of nature’s surprises,” said first author Anatoly Patsyk, a Ph.D. student in the Physics Department and Solid State Institute at the Technion-Israel Institute of Technology.

“There is nothing more exciting than discovering something new and this is the first demonstration of this phenomenon with light waves,” added co-author Professor Uri Sivan, from the Physics Department and Solid State Institute and the Russell Berrie Nanotechnology Institute at the Technion-Israel Institute of Technology.

“This goes to show that intriguing phenomena can also be observed in simple systems and one just has to be perceptive enough to uncover them. As such, bringing together and combining the views of researchers from different backgrounds and disciplines has led to some truly interesting insights.”

“The fact that we observe it with light waves opens remarkable new possibilities for research, starting with the fact that we can characterize the medium in which light propagates to very high precision and the fact that we can also follow those branches accurately and study their properties.”

“Now, with this observation we can think of a plethora of new ideas,” said senior co-author Professor Mordechai Segev, also from the Physics Department and Solid State Institute and the Russell Berrie Nanotechnology Institute at the Technion-Israel Institute of Technology.

“For example, using these light branches to control the fluidic flow in liquid, or to combine the soap with fluorescent material and cause the branches to become little lasers.”

“Or to use the soap membranes as a platform for exploring fundamentals of waves, such as the transitions from ordinary scattering which is always diffusive, to branched flow, and subsequently to Anderson localization.”

“There are many ways to continue this pioneering study. As we did many times in the past, we would like to boldly go where no one has gone before.”

The team’s paper was published in the July 1, 2020 issue of the journal Nature.

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A. Patsyk et al. 2020. Observation of branched flow of light. Nature 583, 60-65; doi: 10.1038/s41586-020-2376-8