Non-Gaussian Correlations between Reflected and Transmitted Intensity Patterns Emerging from Opaque Disordered Media

Summary

When coherent light (like a laser) passes through or reflects off most materials, it scatters and creates what is known as a speckle pattern, a seemingly random intensity pattern. These speckles limit the precision of many types of imaging applications, including microscopic images of biological processes and ground-based telescopic views of space. Common sense suggests that reflected and transmitted light are completely uncorrelated—no information can be obtained regarding the transmitted light by measuring the reflected one and vice versa. However, some of our team recently predicted that interference effects should lead to such correlations. We experimentally investigate this prediction, and we find that this correlation not only exists but is much richer and more complicated than expected.We shine a helium-neon laser at a 45-degree angle onto a slab of glycerol with suspended particles of titanium dioxide and look for correlations between the speckle patterns of the transmitted and reflected light. By exploring a large range of sample thicknesses and scattering mean-free paths, we show that for large optical densities, the reflected and transmitted intensity profiles exhibit a long-range anticorrelation. For thinner systems, this is accompanied by a previously unforeseen long-range positive correlation. We develop a perturbative theory that describes both contributions and accurately represents the experimental results.The presence of correlations between the reflected and transmitted light proves that information of what is happening on the far side of an opaque medium can, in principle, be obtained by only measuring reflected light, thus opening the way to novel noninvasive imaging techniques.