Abstract
The evolution of coherent waves in two-dimensional systems with a random potential can give rise to an interesting phenomenon known as Branched Flow. In this process, a wave scatters from a weak random potential with correlation length longer than the wavelength, and forms focused channels that keep dividing as the wave propagates, creating a pattern resembling the branches of a tree. This phenomenon was observed for electrons [1–3], for a specific example of microwaves [4] and for ocean waves [5], but thus far never for light at optical frequencies. Furthermore, the statistical features of branched flow were predicted theoretically but were never observed in any experimental system. Here, we present the first observation of branched flow in optics, prove that the experiments represent branched flow, and study the statistical features. In our experiments, we couple an optical beam to a thin liquid soap film and observe its evolution within this thin membrane. The light experiences scattering from thickness variations in the soap film, which acts as a two-dimensional medium with a random potential. The beam propagates and scatters from the random thickness variations, forming focused branches that keep dividing, ending up in a pattern that resembles the branches of a tree, as shown in Fig.1a. To view the thickness variations directly, we construct a white light microscope illuminating the thin soap film from above, and observe the colorful map shown in Fig. 1b. The colors in Fig. 1b are true colors, and they emerge due to the reflection of white light from the thin soap film, indicating the local thickness (Fig.1c).
© 2019 IEEE
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