Disordered photonics

188 NATURE PHOTONICS | VOL 7 | MARCH 2013 | www.nature.com/naturephotonics The diffusion of light by random materials is an established phenomenon that is considered to be of little relevance in our daily lives. However, in the field of photonics, scattering, such as that created by dust or imperfections, is often particularly undesirable. So why should we bother performing research into disordered photonic materials? Although it is true that a better understanding of disorder can help overcome problems arising from scattering, researchers have recently discovered a wealth of interesting physics behind the behaviour of light rays in random materials. Light that is multiply scattered does not lose its wave character and can interfere both during and after the scattering process. Because the scattering is elastic, optical information is not lost. In addition, owing to reciprocity, multiple scattering is, in principle, fully reversible. This combination leads to a series of interesting physical effects and also creates enormous potential for new disorder-based optical applications. It is not possible to look through a white marble wall because the light rays inside are multiply scattered by wavelength-scale inhomogeneities. The rays from an object behind the wall will therefore be scattered several times before they reach your eye, thus completely blurring the image. Nevertheless, given that the scattering is elastic, the information on the object is still present; it is scrambled by the multiple scattering process and is thus contained in a ‘speckle-pattern’1 — a complex pattern arising from the interference between several randomly scattered waves (Fig. 1). If the ‘key’ to descrambling the information were known, it would in principle be possible to look through the wall, or at least to reconstruct the original image. The same concept could then be applied to look through fog, or form images of human organs through the skin. An important step in this direction was recently taken by Vellekoop and Mosk2, who developed an experimental technique for creating the descrambling key. Their approach is based on wavefront-shaping, which works as follows. First, a laser beam is directed onto a random sample, but passes beforehand through a phase matrix that makes it possible to control the phase of the beam along its cross-section, pixel by pixel. The light that has passed through the sample (undergoing multiple scattering events in the process) is then recorded and a smart optimization protocol is used to maximize the output at a single point. This slowly ‘teaches’ the phase matrix which random structure is needed at the input to compensate exactly for the random scattering of the sample. The descrambling ‘key’ is then embedded in the phase matrix and applied to the light before it enters the random medium. Not only does this concept make it possible to focus light2,3 through a Disordered photonics