Physicists first time succeeded to observe the branching luminous flux — i.e. the division of a single beam of light on a wide variety of focused component. Scientists have found that this effect occurs during the propagation of the laser beam in a soap film. The results of a study published in the journal Nature.
Branched flows can occur when a wave propagates in a weak potential field, the value of which varies randomly in space, and the correlation length (i.e. the characteristic distance within which the scatter values are about the same) — greater than the wavelength. Under these conditions, the set of small variations of the external field is able to dissipate the initial flow smoothly, but rather to turn it into several narrower streams expressed. Those, in turn, under the influence of the same mechanism can be split into more narrow beams — a picture resembling the branches of a tree.
It is believed that this effect should occur in waves of any type. Scientists have observed for electrons and microwave radiation, and also attracted to the description of ocean waves and acoustic phenomena. Despite this, until recently, the branching of the thread has not been observed in optics for electromagnetic waves in the vicinity of the visible range.
Anatoly Patsyuk (Anatoly Patsyk) from the Israeli Institute of technology and Andres Miguel (Miguel Bandres) from the University of Central Florida, along with colleagues was first experimentally discovered and investigated the branching of the stream to optical radiation. With the help of fiber optic physicists have sent a beam of laser with a wavelength of 532 nm (corresponding to green light) on soap film — a thin layer of liquid which is enclosed between two layers of surfactant molecules.
The membrane thickness was varied randomly in space (in the course of the experience — in the range of 50-550 nm, which is comparable to the wavelength) is led to a change in the effective refractive index of the medium, and with it the nature of the light propagation. To measure variations of the film thickness, the scientists lit it on three wavelengths (red, green and blue region of the spectrum), and received the image through a microscope, and then based on the resulting image recreated roughness of the membrane. With the help of the microscope, the authors were tracking the spread of the laser beam and, below, not giving the beam a significant impact, at the same time to observe the heterogeneity of the film — used a weak white light. Because the experience occurred in the laboratory, the variation of the film thickness has varied with time under the influence of weak air flow is led to the shake of the secondary rays and instability of the picture.
In the result, the physicists were able to observe and many times reproduce the branching of the light beam under the influence of varying the optical properties of the film. In addition, the results of experience, scientists qualitatively confirmed previously obtained by other researchers, the ratio between the distance at which the splitting occurs for the first time, and characteristics of the environment — the statistical variation of the potential value of the length of correlation, and noted some properties of the emergent secondary rays — in particular, they are compared with the original beam in a homogeneous medium is much less susceptible to diffraction broadening (i.e., a spontaneous increase of the transverse dimensions of the beam with distance) — the course of the experiment the width of the secondary beams almost remained at the scale of ten diffraction lengths.
The authors emphasize that done open and experimental technique opens up a whole direction for further research by giving the optical medium of various properties (for example, changing the curvature of the surface of the soap film, or by using nonlinear effects), it is possible to obtain a wide set of experimental data that will clarify and develop the theoretical model (including elements related to the General theory of relativity).
The soap film has attracted the attention of researchers not only from the point of view of optics, in March, we wrote about how scientists have measured acoustic sound pressure bursting soap bubbles, and in 2017 — told, with a solution of soap films coped mathematics.