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Quantitative analysis of hidden particles diffusing behind a scattering layer using speckle correlation

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Observing objects hidden behind scattering layers is a problem of general interest for biomedical applications, or in aerial, terrestrial and naval transport control and security. In optics, an opaque layer is such that each source point of the object of interest gives rise to a speckle figure, which spreads out over the entire field of view, making the source points non-localizable. We have shown, theoretically and experimentally, that it is possible to measure the diffusion coefficient of micrometric fluorescent beads hidden behind a totally scattering layer.

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Our new approach combines the spatio-temporal correlation methods typically used in fluorescence microscopy on living cells and the principle of the "memory effect": when a source point moves over the "memory effect "range the corresponding speckle figure shifts, while remaining otherwise unchanged. Experimentally, we used micrometric fluorescent beads diffusing in 2D at the surface of a water film. These beads are illuminated by an excitation speckle pattern, created when the incident laser beam passes through a TiO2 layer and each of them emits a detection speckle pattern, due to the fluorescence light passing through this same layer. Our method of analysis allows us to determine the diffusion coefficient of the beads and the number of beads simultaneously present. It is practically valid whatever the thickness of the diffusing layer, that is to say whatever the extent of the "memory effect".

References:
Quantitative analysis of hidden particles diffusing behind a scattering layer using speckle correlation
A. Sarkar, I. Wang, J. Enderlein, J. Derouard, A. Delon
LIPHY, Université Grenoble Alpes and CNRS, F-38000 Grenoble, France
Third Institute of Physics–Biophysics, Georg August University, 37077 Göttingen, Germany

More informations:
https://doi.org/10.1364/OE.401506