Unforeseen correlated relation between transmitted and reflected light in opaque media

What does happen when coherent light passes through or reflects off a material? It scatters and creates a seemingly random pattern, called a speckle pattern. Theses 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, recent work predicted that interference effects should lead to such correlations. An international team from University of Exeter and from Langevin Institute (ESPCI Paris, PSL University, CNRS) experimentally investigated this prediction, and found that this correlation not only exists but is also much richer and more complicated than expected. This work has been published in Physical Review X

Scientists used an helium-neon laser and sent the beam onto a partially transparent slab of glycerol with suspended particles of titanium dioxide. They captured the speckle patterns both in transmission and in reflexion. By exploring a large range of sample thicknesses and opacity, they managed to show that for large optical densities, the reflected and transmitted intensity profiles exhibit a long-range anticorrelation, i.e. when on value increases the other decreases. For thinner systems, the system exhibits a previously unforeseen long-range positive correlation. The team developed 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.

Associated publication:

Starshynov, Panaguia-Diaz, Fayard, Goetschy, Pierrat, Carminati and Bertolotti., Non-Gaussian Correlations between Reflected and Transmitted Intensity Patterns Emerging from Opaque Disordered Media, Physical Review X, 2018

DOI:https://doi.org/10.1103/PhysRevX.8.021041

Contact :

Exeter University : j.bertolotti@exeter.ac.uk
ESPCI Paris : remi.carminati@espci.fr