Enhanced Secrecy in Optical Communication using Speckle from Multiple Scattering Layers

We study the secrecy of an optical communication system with two scattering layers, to hide both the sender and receiver, by measuring the correlation of the intermediate speckle generated between the two layers. The binary message is modulated as spatially shaped wavefronts, and the high number of transmission modes of the scattering layers allows for many uncorrelated incident wavefronts to send the same message, making it difficult for an attacker to intercept or decode the message and thus increasing secrecy. We collect 50,000 intermediate speckle patterns and analyze their correlation distribution using Kolmogorov-Smirnov (K-S) test. We search for further correlations using the K-Means and Hierarchical unsupervised classification algorithms. We find no correlation between the intermediate speckle and the message, suggesting a person-in-the-middle attack is not possible. This method is compatible with any digital encryption method and is applicable for codifications in optical wireless communication (OWC)

Probing the position-dependent optical energy fluence rate in 3D scattering samples

The accurate determination of the position-dependent energy fluence rate of light is crucial for the understanding of transport in anisotropically scattering and absorbing samples, such as biological tissue, seawater, atmospheric turbulent layers, and light-emitting diodes. While Monte Carlo simulations are precise, their long computation time is not desirable. Common analytical approximations to the radiative transfer equation (RTE) fail to predict light transport, and could even give unphysical results. Here, we experimentally probe the position-dependent energy fluence rate of light inside scattering samples where the widely used $P_1$ and $P_3$ approximations to the RTE fail. We study samples that contain anisotropically scattering and absorbing spherical scatterers, namely microspheres ($r = 0.5$ $\rm{\mu}$m) with and without absorbing dye. To probe the position-dependent energy fluence rate, we detect the emission of quantum dots that are excited by the incident light and that are contained in a thin capillary. By scanning the sample using the capillary, we access the position dependence. We present a comprehensive analysis of experimental limitations and (systematic) errors. Our measured observations are compared to the results of analytical approximations of the solution of the radiative transfer equation and to Monte Carlo simulations. Our observations are found to agree well with the Monte Carlo simulations. The $P_3$ approximation with a correction for forward scattering also agrees with our observations, whereas the $P_1$ and the $P_3$ approximations deviate increasingly from our observations, ultimately even predicting unphysical negative energies

Focusing light through a free-form scattering medium

Imaging and transport light through scattering opaque media is a hot topic pursued in multiple fields, ranging from nanotechnology to life sciences. A promising technique to do this is wavefront shaping (WFS), where the light propagation through a scattering medium is controlled by interference [1][2]. Recently, the potential of WFS was even extended to, for instance, time-varying samples [3][4]. In most cases to date, WFS has been done on the quintessential scattering sample geometry, namely in slabs. Real-world applications, however, require samples to have any shape – “free-form scattering optics” – that defies current theories. Here, we present the study of an opaque sample of TiO2 particles suspended in silicone. Exploiting the flexibility of silicone, we are able to modify the geometry of the sample and measure the enhancement of the intensity η in a point of the speckle pattern. Using this opportunity, we compare the performance of a flat and a free form sample. These experimental measurements will be compared with a newly formulated theory of light transport in free form scattering media

Enhanced transparency in strongly scattering media

The well-known optical theorem describes that extinction energy lost from a light beam is equal to scattering in all directions, and possible absorption [1]. Recently, our group has generalized the theorem to multiple incident beams [2], with which we discovered it is possible to either enhanced transparency or enhanced extinction in a scattering system with multiple beams, controlling the relative phase and angle between them. This effect is called mutual transparency or extinction. Here, we present an experimental study of this effect. We use a strongly scattering sample made from a strip of silicon (see figure 1.b ). A liquid crystal phase retarder is used to control the phase difference between two incident beams and using an unbalanced March-Zehnder interferometer, we can control the relative angle with a movable mirror

Unravelling the conductance path through single-porphyrin junctions

Porphyrin derivatives are key components in natural machinery enabling us to store sunlight as chemical energy. In spite of their prominent role in cascades separating electrical charges and their potential as sensitizers in molecular devices

Single-molecule transport of fullerene-based curcuminoids

We present experimental and theoretical studies of single-molecule conductance through nonplanar fullerocurcuminoid molecular dyads in ambient conditions using the mechanically controllable break junction technique. We show that molecular dyads with bare fullerenes form configurations with conductance features related to different transport channels within the molecules, as identified with filtering and clustering methods. The primary channel corresponds to charge transport through the methylthio-terminated backbone. Additional low-conductance channels involve one backbone side and the fullerene. In fullerenes with four additional equatorial diethyl malonate groups attached to them, the latter transport pathway is blocked. Density functional theory calculations corroborate the experimental observations. In combination with nonequilibrium green functions, the conductance values of the fullerocurcuminoid backbones are found to be similar to those of a planar curcuminoid molecule without a fullerene attached. In the nonplanar fullerocurcuminoid systems, the highest-conductance peak occurs partly through space, compensating for the charge delocalization loss present in the curcuminoid system