Opaque Medium Research Articles

Image reconstruction through a nonlinear scattering medium via deep learning

Image reconstruction through the opaque medium has great significance in fields of biophotonics, optical imaging, mesoscopic physics, and optical communications. Previous researches are limited in the simple linear scattering process. Here, we develop a nonlinear speckle decoder network, which can reconstruct the phase information of the fundamental frequency wave via the nonlinear scattering signal. Further, we validate the ability of our model to recover simple and complex structures by using MNIST and CIFAR data sets, respectively. We then show that the model is able to restore the image information through different sets of nonlinear diffusers and reconstruct the image of a kind of completely unseen object category. The proposed method paves the way to nonlinear scattering imaging and information encryption.

GPR Mapping of Cavities in Complex Scenarios with a Combined Time-Depth Conversion.

The paper deals with a combined time-depth conversion strategy able to improve the reconstruction of voids embedded in an opaque medium, such as cavities, caves, empty hypogeal rooms, and similar targets. The combined time-depth conversion accounts for the propagation velocity of the electromagnetic waves both in free space and in the embedding medium, and it allows better imaging and interpretation of the underground scenario. To assess the strategy's effectiveness, ground penetrating radar (GPR) data referred to as an experimental test in controlled conditions are accounted for and processed by two different approaches to achieve focused images of the scenario under test. The first approach is based on a classical migration algorithm, while the second one faces the imaging as a linear inverse scattering approach. The results corroborate that the combined time-depth conversion improves the imaging in both cases.

Three-phase VOF modeling of water jet – molten metal interaction

Numerical study of water jet impact on a pool of high-temperature metallic melt is carried out. Experimental studies in this field encounter difficulties in visualizing the flow and performing measurements in hot and opaque medium, while theoretical studies by multifluid models have severe limitations in describing the flow structure and interphase processes. VOF method allows one to alleviate many limitations, as well as reproduce the flow details that are hardly amenable to direct experimental measurements. The work is focused on the validation of VOF predictions against small-scale experimental data on water jet interaction with molten lead–bismuth eutectic alloy; also, features of the vapor-water cavity formation and evolution in the pool are analyzed. Simulations are performed by a solver developed in OpenFOAM software. Results are presented for three-dimensional simulations of subcooled water jet interaction with LBE melt in a vessel, which has a 10-mm thick slice configuration with semi-circular bottom. Validation is performed against two tests by Sibamoto et al, for a water jet impacting the melt at the velocity of 4.7 and 6.2 m/s. Cross-verification with two multi-fluid codes is performed. Reasonable agreement with respect to cavity geometry and evolution stages is demonstrated, further research directions are discussed. Results obtained are relevant to nuclear safety problems for advanced lead-cooled fast reactors.

Terahertz large-area unidirectional surface magnetoplasmon and its applications

Unidirectional surface waves in nonreciprocal plasmonic platforms with nonlocal effects have been a topic of significant interest and some controversy. In this study, we present a scheme to achieve unidirectional surface magnetoplasmons (USMPs) with large modal areas at terahertz frequencies. Such large-area USMPs (LUSMPs) exist in a metal-UENZ (uniaxial-ε\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\varepsilon $$\\end{document}-near-zero)-Si-InSb structure under external magnetic field, where the effect of nonlocality is included. The field of the LUSMP extends almost uniformly in the UENZ layer with a thickness of wavelength scale, thus its modal size can be represented by the UENZ-layer thickness. Due to the modal energy primarily distributed in the thick UENZ layer, the nonlocality-induced leakage of the LUSMP is significantly reduced by an order of magnitude, compared to previous USMP existing at interface between InSb and opaque isotropic medium. Due to their large modal sizes, such LUSMPs can be efficiently excited by terahertz radiations directly from free space. In addition, LUSMPs offer high degree of freedom for manipulating terahertz waves, such as energy squeezing and trapping. Based on LUSMPs, a terahertz free-space isolator is also developed. Our findings have important implications to the development of innovative plasmonic devices in terahertz regime.

Neural Networks Push the Limits of Luminescence Lifetime Nanosensing.

Luminescence lifetime-based sensing is ideally suited to monitor biological systems due to its minimal invasiveness and remote working principle. Yet, its applicability is limited in conditions of low signal-to-noise ratio (SNR) induced by, e.g., short exposure times and presence of opaque tissues. Herein this limitation is overcome by applying a U-shaped convolutional neural network (U-NET) to improve luminescence lifetime estimation under conditions of extremely low SNR. Specifically, the prowess of the U-NET is showcased in the context of luminescence lifetime thermometry, achieving more precise thermal readouts using Ag2 S nanothermometers. Compared to traditional analysis methods of decay curve fitting and integration, the U-NET can extract average lifetimes more precisely and consistently regardless of the SNR value. The improvement achieved in the sensing performance using the U-NET is demonstrated with two experiments characterized by extreme measurement conditions: thermal monitoring of free-falling droplets, and monitoring of thermal transients in suspended droplets through an opaque medium. These results broaden the applicability of luminescence lifetime-based sensing in fields including in vivo experimentation and microfluidics, while, hopefully, spurring further research on the implementation of machine learning (ML) in luminescence sensing.

An Optoelectronics-Based Compressive Force Sensor with Scalable Sensitivity.

There is an increasing need to accurately measure compressive force for biomedical and industrial applications. However, this need has not been fully addressed, as many sensors are bulky, have high power requirements, and/or are susceptible to electromagnetic interference. This paper presents an optoelectronics-based force sensor that can overcome the limitations of many sensors in the market. The sensor uses a light emitting diode (LED) to transmit visible broad-spectrum light into a photoresistor through an optically clear spacer on top of an elastomeric medium. In the absence of an external force, the light path is mostly blocked by the opaque elastomeric medium. Under a compressive force, the clear spacer compresses the elastomer, moving itself into the light path, and thus increasing the overall light transmission. The amount of light received by the photoresistor is used to quantify compressive force based on elastomer displacement/compression and a priori knowledge of elastomer stiffness. This sensing scheme was tested under eight different configurations: two different sized sensors with four types of elastomers per size (20A neoprene, 30A neoprene, 50A neoprene, and 75A styrene-butadiene rubber (SBR)). All configurations measured force with R2 > 0.97, RMSE < 1.9 N, and sensitivity values ranging from 17 to 485 N/V. This sensing scheme provides a low-cost, low-power method for accurate force sensing with a wide force range.

Collimated Whole Volume Light Scattering in Homogeneous Finite Media.

Crepuscular rays form when light encounters an optically thick or opaque medium which masks out portions of the visible scene. Real-time applications commonly estimate this phenomena by connecting paths between light sources and the camera after a single scattering event. We provide a set of algorithms for solving integration and sampling of single-scattered collimated light in a box-shaped medium and show how they extend to multiple scattering and convex media. First, a method for exactly integrating the unoccluded single scattering in rectilinear box-shaped medium is proposed and paired with a ratio estimator and moment-based approximation. Compared to previous methods, it requires only a single sample in unoccluded areas to compute the whole integral solution and provides greater convergence in the rest of the scene. Second, we derive an importance sampling scheme accounting for the entire geometry of the medium. This sampling strategy is then incorporated in an optimized Monte Carlo integration. The resulting integration scheme yields visible noise reduction and it is directly applicable to indoor scene rendering in room-scale interactive experiences. Furthermore, it extends to multiple light sources and achieves superior converge compared to independent sampling with existing algorithms. We validate our techniques against previous methods based on ray marching and distance sampling to prove their superior noise reduction capability.

LiquidO: Detection and Imaging in Opaque Medium

LiquidO: Detection and Imaging in Opaque Medium

Sensing the position of a single scatterer in an opaque medium by mutual scattering.

We investigate the potential of mutual scattering, i.e., light scattering with multiple properly phased incident beams, as a method to extract structural information from inside an opaque object. In particular, we study how sensitively the displacement of a single scatterer is detected in an optically dense sample of many (up to N = 1000) similar scatterers. By performing exact calculations on ensembles of many point scatterers, we compare the mutual scattering (from two beams) and the well-known differential cross-section (from one beam) in response to the change of location of a single dipole inside a configuration of randomly distributed similar dipoles. Our numerical examples show that mutual scattering provides speckle patterns with an angular sensitivity at least 10 times higher than the traditional one-beam techniques. By studying the "sensitivity" of mutual scattering, we demonstrate the possibility to determine the original depth relative to the incident surface of the displaced dipole in an opaque sample. Furthermore, we show that mutual scattering offers a new approach to determine the complex scattering amplitude.

Retinalike Foveated Imaging through an Opaque Scattering Medium

Imaging through an opaque scattering medium, such as biological tissue, is of broad interest for many applications from physics to life sciences. Measuring the transmission matrix (TM) of the scattering medium is a versatile tool for this purpose. Previously, researchers are used to improving the image resolution by measuring a gigantic TM with a long sampling time. However, the low resolution and low SNR of the reconstructed images are still two obstacles to the adoption of such a technique. The foveal centralis of the human eye produces sharper vision than that from the retina's peripheral region. Inspired by this feature, we report the implementation of foveated scattering imaging with a multiresolution TM whose space-variant resolution feature can be flexibly adjusted. The enhancement of linear resolution and SNR of the foveated scattering imaging system by factors of 4 and 8.3, respectively, is demonstrated experimentally. Our bioinspired imaging method heuristically introduces foveal vision into scattering imaging, and thus paves the way for both high resolution and high SNR imaging through scattering medium concurrently monitoring the entire field of view.

Rayleigh–Bénard Convection in a Radiating Fluid

Abstract Linear and weakly nonlinear stability analyses of Rayleigh–Bénard convection (RBC) in a radiating Newtonian fluid are studied in the paper. The optical properties of the Newtonian fluid are considered to be independent of the wavelength of radiation. A gray medium thus assumed allows us to consider two asymptotic cases: (a) optically thin fluid medium (transparent) and (b) optically thick fluid medium (opaque). Using the solution in terms of a truncated Fourier series representation, we arrive at the analytical expression for the Rayleigh number and examine the thermal radiation properties. A modified Lorenz model, which has in it the influence of the radiation parameters, is derived. The analytically intractable three-dimensional Lorenz model is then projected into the one-dimensional Stuart–Landau equation. The analytical solution of the Stuart–Landau equation is used to quantify the heat transport. It is shown that the radiation inhibits the primary instability of convection in both transparent and opaque media. However, the delay of convection is more in the opaque medium compared to that in the transparent medium. Inclusion of a transparent medium creates a “heat-sink-like situation,” whereas the opaque medium leads to an “enhanced-thermal-diffusivity situation.” Both these situations result in diminished heat transport in the RBC system. The analytical expression of the Hopf–Rayleigh number is obtained by linearizing the modified Lorenz model around one of its postonset critical points. This number provides information about the onset of chaos in the dynamical system. The impact of the radiation effect is to delay the appearance of chaos.

Imaging Complex Targets through a Scattering Medium Based on Adaptive Encoding

The scattering of light after passing through a complex medium poses challenges in many fields. Any point in the collected speckle will contain information from the entire target plane because of the randomness of scattering. The detailed information of complex targets is submerged in the aliased signal caused by random scattering, and the aliased signal causes the quality of the recovered target to be degraded. In this paper, a new neural network named Adaptive Encoding Scattering Imaging ConvNet (AESINet) is constructed by analyzing the physical prior of speckle image redundancy to recover complex targets hidden behind the opaque medium. AESINet reduces the redundancy of speckle through adaptive encoding which effectively improves the separability of data; the encoded speckle makes it easier for the network to extract features, and helps restore the detailed information of the target. The necessity for adaptive encoding is analyzed, and the ability of this method to reconstruct complex targets is tested. The peak signal-to-noise ratio (PSNR) of the reconstructed target after adaptive encoding can be improved by 1.8 dB. This paper provides an effective reference for neural networks combined with other physical priors in scattering processes.

The Backscattering Polarization for a Sphere with a Two-Scale Relief of Rough Surface

The paper presents the results of studies of the polarization of backscattered optical radiation for a model of a sphere with a wavy surface covered with arbitrarily oriented ellipsoidal pores. The calculations were performed by the Monte Carlo method (ROKS-RG program, geometrical optics approximation). Surface roughness of objects is taken into account using local geometries. It is shown that for structures whose pores contain a vitreous medium, the presence of vacuum voids between a transparent and opaque medium determines the nature of the phase dependence of the degree of polarization. In the absence of vacuum, the polarization is always positive and, for small phase angles, depends significantly on the optical characteristics of an opaque medium. The presence of vacuum is a catalyst for the growth of the p-component of the reflected radiation. This leads to a significant decrease in the amplitude of the positive polarization branch. Differences in the optical characteristics of opaque materials are also leveled out, and in some cases negative values of the degree of polarization are observed in the region of small phase angles. Keywords: polarization, geometrical optics approximation, Monte Carlo method, rough surface, Two-Scale Relief.

A model of a reflective surface under the Shklovsky-de Genet topology

A rough surface is studied. Its reflective features are related to the geometry of a random allocation of a solid opaque medium, the surface of which separates it from a translucent medium. A model of a real rough surface is suggested. It is based on the Shklovsky-de Genet topology, which uses the concept of a connecting cluster, which is divided into a skeleton (core) and dead ends (drop points). A step parameter is chosen as the roughness value in the model, as it is reliably detected using a profilometer. It is demonstrated that data concerning the step parameter allows to analytically evaluate the spectral reflection coefficient of surfaces with a known reflection indicatrix in a wide range of wavelengths.

Neutrino physics with an opaque detector

In 1956 Reines & Cowan discovered the neutrino using a liquid scintillator detector. The neutrinos interacted with the scintillator, producing light that propagated across transparent volumes to surrounding photo-sensors. This approach has remained one of the most widespread and successful neutrino detection technologies used since. This article introduces a concept that breaks with the conventional paradigm of transparency by confining and collecting light near its creation point with an opaque scintillator and a dense array of optical fibres. This technique, called LiquidO, can provide high-resolution imaging to enable efficient identification of individual particles event-by-event. A natural affinity for adding dopants at high concentrations is provided by the use of an opaque medium. With these and other capabilities, the potential of our detector concept to unlock opportunities in neutrino physics is presented here, alongside the results of the first experimental validation.

Detection and Drug Susceptibility Testing of Neisseria gonorrhoeae Using Isothermal Microcalorimetry.

Background: Gonorrhea is a frequently encountered sexually transmitted disease that results in urethritis and can further lead to pelvic inflammatory disease, infertility, and possibly disseminated gonococcal infections. Thus, it must be diagnosed promptly and accurately. In addition, drug susceptibility testing should be performed rapidly as well. Unfortunately, Neisseria gonorrhoea is a fastidious microorganism that is difficult to grow and requires culturing in an opaque medium. Methods: Here, we used isothermal microcalorimetry (IMC) to monitor the growth and the antimicrobial susceptibility of N. gonorrhoea. Results: Using IMC, concentrations of N. gonorrhoea between 2000 and 1 CFU·mL−1 were detected within 12 to 33 h. In addition, drug susceptibility could be monitored easily. Conclusions: The use of isothermal microcalorimetry provides an interesting and useful tool to detect and characterize fastidious microbes such as N. gonorrhoea that require media incompatible with optical detection conventionally used in many commercial systems.

Simulation of Fluid Dynamics Monitoring Using Ultrasonic Measurements

The simulation of the propagation of ultrasonic waves in a moving fluid will improve the efficiency of the ultrasonic flow monitoring and that of the in-service monitoring for various reactors in several industries. The most recent simulations are mostly limited to 3D representations of the insonified volume but without really considering the temporal aspect of the flow. The advent of high-performance computing (HPC) now makes it possible to propose the first 4D simulations, with the representation of the inspected medium evolving over time. This work is based on a highly accurate double simulation. A first computational fluid dynamics (CFD) simulation, performed in previous work, described the fluid medium resulting from the mixing of hot jets in a cold opaque fluid. There have been many sensor developments over the years in this domain, as ultrasounds are the only method able to give information in an opaque medium. The correct design of these sensors, as well as the precise and confident analysis of their measurements, will progress with the development of the modeling of wave propagation in such a medium. An important parameter to consider is the flow temperature description, as a temperature gradient in the medium deflects the wave path and may sometimes cause its division. We develop a 4D wave propagation simulation in a very realistic, temporally fluctuating medium. A high-performance simulation is proposed in this work to include an ultrasonic source within the medium and to calculate the wave propagation between a transmitter and a receiver. The analysis of the wave variations shows that this through-transmission setup can track the jet mixing time variations. The steps needed to achieve these results are described using the spectral-element-based numerical tool SPECFEM3D. It is shown that the low-frequency fluctuation of the liquid metal flow can be observed using ultrasonic measurements.

Acoustically induced transparency for synchrotron hard x-ray photons

The induced transparency of opaque medium for resonant electromagnetic radiation is a powerful tool for manipulating the field-matter interaction. Various techniques to make different physical systems transparent for radiation from microwaves to x-rays were implemented. Most of them are based on the modification of the quantum-optical properties of the medium under the action of an external coherent electromagnetic field. Recently, an observation of acoustically induced transparency (AIT) of the 57Fe absorber for resonant 14.4-keV photons from the radioactive 57Co source was reported. About 150-fold suppression of the resonant absorption of photons due to collective acoustic oscillations of the nuclei was demonstrated. In this paper, we extend the AIT phenomenon to a novel phase-locked regime, when the transmitted photons are synchronized with the absorber vibration. We show that the advantages of synchrotron Mössbauer sources such as the deterministic periodic emission of radiation and controlled spectral-temporal characteristics of the emitted photons along with high-intensity photon flux in a tightly focused beam, make it possible to efficiently implement this regime, paving the way for the development of the acoustically controlled interface between hard x-ray photons and nuclear ensembles.

Three-Dimensional Imaging of Dissolution-Driven Convection in Simple Porous Media Using X-ray CT Scanning

Motivated by geological carbon dioxide (CO2) storage and the associated process of CO2 convective mixing in brine, we present an experimental method to visualize dissolution-driven convection in a three-dimensional porous medium. Owing to the difficulty of imaging the time-dependent convective process within an opaque medium, experiments so far have largely focused on two-dimensional systems. However, this imposes a strong restriction on the lateral spreading of the plumes. To explore the flow pattern within a three-dimensional medium, we present an experimental procedure employing X-ray computed tomography that enables the visualization of the spatial and temporal evolution of the mixing process non-invasively. To mimic the dissolution of CO2 in brine, we use potassium iodide (KI) in water. Here, we show the results with high spatial and temporal resolution for an experiment with a homogeneous packing of glass beads (Ra=5300). We discuss the evolution of the average solute mass fraction, compare it to numerical results of a purely diffusive process and report a strong enhancement of the dissolution process when convection dominates over diffusive mass transfer effects.

Diffuse photon remission from thick opaque media of the high absorption/scattering ratio beyond what is accountable by the Kubelka-Munk function.

The Kubelka-Munk (KM) theory of diffuse photon remission from opaque media is widely applied to quality-control processes. Recent works based on radiative transfer revealed that the KM function as the backbone parameter of the method may saturate at strong absorption to cause the KM approach to be unfit to predict the change of diffuse reflectance from the medium at strong absorption. We demonstrate by empirical means based on Monte Carlo results that diffuse photon remission from a strong-absorbing medium depends simply upon the absorption/scattering ratio when evaluated over a large area centered at the point of illumination differing in geometry from those convenient for the KM approach. Our empirical prediction gives ∼11% mean errors of the diffuse photon remission from thick opaque medium having an absorption coefficient ranging 0.001 to up to 1000 times stronger than the reduced-scattering coefficient. A slight modification to the KM function in terms of the absorption weighting and absorption-scattering coupling for use within the KM approach also noticeably improves the prediction of diffuse photon remission from thick opaque medium of strong absorption. Our empirical model and the KM approach using the modified KM function were compared against measurements from a thick opaque medium, of which the absorption coefficient was changed over four orders of magnitude.