The propagation of monochromatic light through a scattering medium produces speckle patterns in reflection and transmission, and the apparent randomness of these patterns prevents direct imaging through thick turbid media. Yet, since elastic multiple scattering is fundamentally a linear and deterministic process, information is not lost but distributed among many degrees of freedom that can be resolved and manipulated.Here we demonstrate experimentally that the reflected and transmitted speckle patterns are correlated, even for opaque media with thickness much larger than the transport mean free path, proving that information survives the multiple scattering process and can be recovered. The existence of mutual information between the two sides of a scattering medium opens up new possibilities for the control of transmitted light without any feedback from the target side, but using only information gathered from the reflected speckle.In multiply scattering materials, the random inhomogeneities in the refractive index scramble the incident wavefront, mixing colors and spatial degrees of freedom, resulting in a white and opaque appearance [1]. Under illumination with coherent light and for elastic scattering, interference produces large intensity fluctuations that is not averaged out by a single realization of the disorder, resulting in a seemingly random speckle pattern [2]. In principle the speckle pattern encodes all the information on the sample and the incident light [3]. A complete knowledge of the scattering matrix allows one to reverse the multiple scattering process and to recover the initial wavefront, thus permitting imaging through turbid materials [4,5]. Conversely, if the scattering matrix is not known, a multiply scattering material effectively behaves as an opaque screen.Speckle patterns are not as random as they appear at first sight. Interference between the possible scattering paths in the medium are known to produce spatial correlations between the intensity measured at different positions [6][7][8], and correlations of different ranges have been identified [9]. Short-range correlations determine the size of a speckle spot. Long-range correlations emerge as a consequence of constraints such as energy conservation or reciprocity [10][11][12]. Spatial correlations have not been used for imaging, a notable exception being the optical memory effect [13], a correlation of purely geometrical origin that has been exploited for non-invasive imaging through an opaque scattering layer [14,15].At first glance, as transmitted and reflected waves are expected to undergo very different multiple scattering sequences, correlations between transmitted and reflected wavefronts are expected to quickly average to zero. Very little attention has been given to cross-correlations between transmitted and reflected speckles, their existence being only mentionned in passing [16,17]. However, a recent theoretical study suggested that a long-range correlation should survive even for thick (opaque) scattering media [18]. Th...
Ghost imaging is an unconventional optical imaging technique that reconstructs the shape of an object combining the measurement of two signals: one that interacted with the object, but without any spatial information, the other containing spatial information, but that never interacted with the object [1,2]. Ghost imaging is a very flexible technique, that has been generalized to the singlephoton regime [3], to the time domain [4], to infrared and terahertz frequencies [5], and many more conditions [6]. Here we demonstrate that ghost imaging can be performed without ever knowing the patterns illuminating the object, but using patterns correlated with them, doesn't matter how weakly. As an experimental proof we exploit the recently discovered correlation between the reflected and transmitted light from a scattering layer [7,8], and reconstruct the image of an object hidden behind a scattering layer using only the reflected light, which never interacts with the object. This method opens new perspectives for non-invasive imaging behind or within turbid media.
Light scattering limits the penetration depth of non-invasive Raman spectroscopy in biological media. While safe levels of irradiation may be adequate to analyze superficial tissue, scattering of the pump beam reduces the Raman signal to undetectable levels deeper within the tissue.Here we demonstrate how wavefront shaping techniques can significantly increase the Raman signal at depth, while keeping the total irradiance constant, thus increasing the amount of Raman signal available for detection.
Spatial Light Modulators (SLMs) are widely used in several fields of optics such as adaptive optics. SLMs based on Liquid Crystal (LC) devices allow a dynamic and easy representation of two-dimensional phase maps. A drawback of these devices is their elevated cost, preventing a massive use of the technology. We present a more affordable approach based on the serial arrangement of vertical aligned LC devices, with characteristics of phase modulation similar to a widely used parallel aligned LC device. We discuss the peculiarities of the approach, the performance and some potential areas of applications.
Cataracts increase the amount of scattered light in the crystalline lens producing low-contrast retinal images and causing vision impairment. The Optical Memory Effect is a wave correlation of coherent fields, which can enable imaging through scattering media. In this work, we characterize the scattering properties of excised human crystalline lenses by measuring their optical memory effect and other objective scattering parameters, finding the relationship between them. This work has the potential to help fundus imaging techniques through cataracts as well as the non-invasive correction of vision through cataracts.
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