Branched Flow of Light

Observation of branched flow of light. (Source: Technion)

A team of researchers from Technion – Israel Institute of Tech­nology has observed branched flow of light for the very first time. The study was carried out by Anatoly (Tolik) Patsyk in colla­boration with Miguel A. Bandres, who was a postdoctoral fellow at Technion when the project started. The research was led by Technion President Uri Sivan and Mordechai (Moti) Segev of Technion’s Physics and Electrical Engi­neering Faculties, the Solid State Institute, and the Russell Berrie Nano­technology Institute.

When waves travel through landscapes that contain distur­bances, they naturally scatter, often in all directions. Scattering of light is a natural phenomenon, found in many places in nature. For example, the scattering of light is the reason for the blue color of the sky. As it turns out, when the length over which distur­bances vary is much larger than the wavelength, the wave scatters in an unusual fashion: it forms channels (branches) of enhanced inten­sity that continue to divide or branch out, as the wave propa­gates. This pheno­menon is known as branched flow. It was first observed in 2001 in electrons and had been suggested to be ubi­quitous and occur also for all waves in nature, for example – sound waves and even ocean waves. Now, Technion researchers are bringing branched flow to the domain of light: they have made an experi­mental obser­vation of the branched flow of light.

“We always had the intention of finding something new, and we were eager to find it. It was not what we started looking for, but we kept looking and we found something far better,” says Miguel Bandres. “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. When we have a disordered medium where the varia­tions are not random but 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 their research, the team 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,” says Tolik Patsyk. The ability to create branched flow in the field of optics offers new and exciting opportunities for inves­tigating and under­standing this universal wave pheno­menon.

“There is nothing more exciting than disco­vering something new and this is the first demons­tration of this phenomenon with light waves,” says Uri Sivan. “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.”

Moti Segev looks to the future. “I always educate my team to think beyond the horizon,” he says, “to think about something new, and at the same time – look at the experimental facts as they are, rather than try to adapt the experiments to meet some expected behavior. Here, Tolik was trying to measure something completely different and was surprised to see these light branches which he could not initially explain. He asked Miguel to join in the experiments, and together they upgraded the experiments consi­derably – to the level they could isolate the physics involved. That is when we started to under­stand what we see. It took more than a year until we under­stood that what we have is the strange phenomenon of “branched flow”, which at the time was never considered in the context of light waves.”

“Now, with this observation – we can think of a plethora of new ideas. 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”, Segev adds. (Source: Technion)

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

Link: Solid State Institute (M. Segev), Technion-Israel Institute of Technology, Haifa, Israel

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