Branched Flow of Light

Credit: Credit: American Technion Society

Light with its eternal presence all around us and having unique characteristics has not only helped the sustainability of the living system on the Earth but also have enthralled scientists from centuries and has been most dominating subject of scientific study. The phenomena exhibited by light due to its interaction with matter like reflection, refraction, diffraction, interference and polarization which man encounter in everyday life have always fascinated scientists to explore for their explanation.  Similarly, the topics such as speed of light in vacuum/medium as well as its dual nature (particle or wave) are still being debated in the scientific community towards a final answer.

Propagation of light

Geometrical optics (also known as Newtonian optics) due to its limitation to explain many of the optical phenomena observed when light interacted with matter was followed by Huygens’s wave optics.  Wave optics could explain successfully almost all the optical phenomena.  Geometrical optics as a limited case of wave optics, has always endorsed the rectilinear flow of light i.e., “light always flow in a straight line”. Light is an electromagnetic wave (having both electric and magnetic field components). When waves travel through landscapes that contain disturbances, they naturally scatter, often in all directions. Scattering of light is also a natural phenomenon, found in many places in nature. For example, scattering of light is the reason for the blue color of the sky. Scattering of light is also said to follow the straight line nature light. All different phenomena exhibited by light depend critically on the different between the wavelength of light and the size of the object with which light is interacting.

Now branched flow

When waves propagate through a weak disordered potential with correlation length larger than the wavelength, they form channels (branches) of enhanced intensity that keep dividing as the waves propagate. Branched flow was first observed in 2001 with electrons, and had been suggested to be ubiquitous and occur also for all waves in nature, for example sound waves and even ocean waves.  A team of researchers from the Technion – Israel Institute of Technology discovered that 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. Now, researchers are bringing branched flow to the domain of light: they have made an experimental observation of branched flow of light. It is a known fact that that waves spread when they propagate in a homogeneous medium but for other kinds of media, waves can behave in very different ways. With a disordered medium where the variations are smooth, like a landscape of mountains and valleys, the waves will propagate in a peculiar way as they will form channels that keep dividing as the wave propagates, forming a beautiful pattern resembling the branches of a tree (Figure).  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.

Scientists say 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. According to scientists, there is nothing more exciting than discovering something new and this is the first demonstration of this phenomenon with light waves. The ability to create branched flow in the field of optics offers new and exciting opportunities for investigating and understanding this universal wave phenomenon. According to scientists, the observed branched flow of light for the very first time is a beautiful phenomenon which will allow for new and exciting research opportunities in the fields of Optics and Optofluidics.  There are so many ways to continue this pioneering study and now, with this observation –scientists are planning to work on 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.

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