When direct vision is obstructed, detecting an object usually involves either using mirrors or actively controlling some of the properties of light used for illumination. In our paradigm, we show that a highly scattering wall can transfer certain statistical properties of light, which, in turn, can assist in detecting objects even in non-line-of-sight conditions. We experimentally demonstrate that the transformation of spatial coherence during the reflection of light from a diffusing wall can be used to retrieve geometric information about objects hidden around a corner and assess their location. This sensing approach is completely passive, assumes no control over the source of light, and relies solely on natural broadband illumination.
In classical diffraction theory, Babinet's principle relates the electromagnetic fields produced by complementary sources. This theorem was always formulated for single-point quantities, both intensities or field amplitudes, in conditions where the full spatial coherence is implicitly assumed. However, electromagnetic fields are, in general, partially coherent, and their spatial properties are described in terms of two-point field-field correlation functions. In this case, a generalized Babinet's principle can be derived that applies to the spatial coherence functions. We present both the derivation and the experimental demonstration of this generalized Babinet theorem.
Spatial coherence function (SCF) is a complex function of two spatial coordinates that, in general, carries more information than the bare intensity distribution. A fast and quantitatively accurate measurement of the SCF is extremely important for a range of applications in optical sensing and imaging. Here, we demonstrate an efficient two-step procedure for measuring the full-field complex coherence function. The measurement relies on an optimized design of a wavefront shearing interferometer capable of characterizing spatially inhomogeneous fields over an extended angular domain. The measurement precision is confirmed by the excellent agreement with a numerical estimation based on Fresnel calculations. We demonstrate that the sensitivity and the measurement range afforded by our instrument permits us to reliably describe the differences in the complex coherence functions that are due to subtle modifications in the shape, position, and orientation of radiation sources.
Optical vortex beams are under considerable scrutiny due to their demonstrated potential for applications ranging from quantum optics to optical communications and from material processing to particle trapping. However, upon interaction with inhomogeneous material systems, their deterministic properties are altered. The way these structured beams are affected by different levels of disturbances is critical for their uses. Here, for the first time, we quantify the degradation of perfect optical vortex beams after their interaction with localized random media. We developed an analytical model that (1) describes how the spatial correlation and the phase variance of disturbance affect the phase distribution across the vortex beams and (2) establishes the regimes of randomness for which the beams maintain the memory of their initial vorticity. Systematic numerical simulations and controlled experiments demonstrate the extent of this memory effect for beams with different vorticity indices.
Coagulation monitoring relies on in vitro tests where the clot formation is induced using external stimuli. We report an optical method capable of revealing the propensity of coagulation based solely on the natural dynamics of erythrocytes in whole blood. In contrast to traditional techniques, our approach provides means to assess the blood coagulability without the need to chemically trigger the coagulation. Results of correlations with standard clinical methods suggest that this optical assay could be used for continuous management of blood coagulation during clinical procedures.
High-frequency viscoelasticity of blood can be measured passively by optical means and described by simple microrheological models in the regime where the erythrocytes test their immediate surroundings.
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