Backscattering Enhancement Research Articles

Enhancement of time-dependent coherent backscattering: Leveraging phase information

Enhancement of time-dependent coherent backscattering: Leveraging phase information

Development of a Polymer Ultrasound Contrast Agent Incorporating Nested Carbon Nanodots

Polymer microbubbles have garnered broad interest as potential theranostic agents. However, the capabilities of polymer MBs can be greatly enhanced, particularly regarding the imaging performance and functional versatility of the platform. This study investigates integrating fluorescent carbon nanodots within polylactic acid (PLA) microbubbles. First, the formulations are characterized by their size, microbubble counts, zeta potential, and resonance frequency. Then, the fluorescence capabilities, nanoparticle loading, and acoustic capabilities are examined. Unmodified (U-), carboxylated (C-), and aminated graphene quantum dots (A-GQDs) were separately suspended and synthesized at a 2% w/w ratio with PLA in the organic phase of the water/oil/water double emulsion process. The new microbubbles were characterized using an AccuSizer, Zetasizer, scanning electron microscopy, fluorescence microscopy and fluorimetry, a custom-built acoustic setup, and clinical ultrasound. The GQD microbubbles were sized between 1.4 and 1.9 µm (U = 1.90, C = 1.44, A = 1.72, Unloaded = 2.02 µm). The U-GQD microbubble exhibited a higher bubble concentration/mg PLA ( p < .05) and the A-GQD microbubbles exhibited the greatest shift in zeta potential. Electron microscopy revealed smooth surfaces and a spherical shape, showing that the nanoparticle addition was not deleterious. The A-GQD microbubbles were specifically detectable using DAPI-filtering with fluorescence microscopy and had the highest TRITC-filtered fluorescence. The C-GQD microbubbles had the highest loading efficiency at 59.4% ( p < .05), and the lowest max acoustic enhancement at 5 MHz (U = 19.8, C = 17.6, A = 18.9, Unloaded = 18.5 dB; p < .05). Additionally, all microbubbles were visible and susceptible to inertial cavitation utilizing clinical ultrasound. The A-GQDs showed promise toward improving the theranostic capabilities of the microbubble platform. They have imbued the most advantageous fluorescence capability and slightly improved backscatter enhancement while retaining all the necessary capabilities of an ultrasound contrast agent. Future studies will investigate the coloading potential of A-GQDs and drug within microbubbles.

Ultra-low loss Rayleigh scattering enhancement via light recycling in fiber cladding

Rayleigh backscattering enhancement (RSE) of optical fibers is an effective means to improve the performance of distributed optical fiber sensing. Femtosecond laser direct-writing techniques have been used to modulate the fiber core for RSE. However, in-core modulation loses more transmission light, thus limiting the sensing distance. In this work, a cladding-type RSE (cl-RSE) structure is proposed, where the femtosecond laser is focused in the fiber cladding and an array of scatterers is written parallel to the core. The refractive-index modulation structure redistributes the light in the cladding, and the backward scattered light is recovered, which enhances the Rayleigh backscattered signal with almost no effect on the core light. Experimentally, it was demonstrated that in an effectual cl-RSE structure, the insertion loss was reduced to 0.00001 dB per scatterer, corresponding to the lowest value for a point scatterer to date. The cl-RSE structure accomplished measurements up to 800°C. In particular, the temperature measurement fluctuation of the cl-RSE fiber portion is only 0.00273°C after annealing. These results show that the cl-RSE structure has effective scattering enhancement, ultra-low loss, and excellent high-temperature characteristics, and has great potential for application in Rayleigh scattering-enhanced distributed fiber sensing.

Scattering properties of protoplanetary dust analogs with microwave analogy: Rough compact grains

Context. Scattering simulations of perfect spheres are not sufficient to explain the observations of scattered light from protoplanetary and debris disks, especially when the dust sizes are on the same order of magnitude as the wavelength used to perform the observations. Moreover, examples of grains collected from the Solar System have proved that the morphology of interstellar dust is irregular. These pieces of evidence lead us to consider that the morphologies of the dust that participates in these circumstellar disks are more complex than those of spheres. Aims. We aim to measure and simulate the scattering properties of six rough compact grains to identify how their morphology affects their scattering properties. These grains are intended to be dust analogs of protoplanetary and debris disks. Their convexity ranges from 75% to 99%. Methods. Grains were 3D printed using stereolithography, and their shape and refractive index were controlled. These analogs were measured with our microwave-scattering experiment (microwave analogy) at wavelengths ranging from 16.7 mm to 100 mm, leading to size parameters from X = 1.07 to X = 7.73. In parallel, their scattering properties were simulated with our finite-element method (FEM), which contained the same geometric file as the 3D printed grains. Results. We retrieved five scattering properties of these grains, that is, the phase function, the degree of linear polarization (DLP), and three other Mueller matrix elements 〈Sij〉. Two types of studies were performed. First, a study of the scattering properties averaged over several orientations of grains at different wavelengths. Second, a study of the same scattering properties, for which a power-law size distribution effect was applied. Conclusions. The very good correspondence between the measured and simulated Mueller matrix elements demonstrated the accuracy of our measurement setup and the efficiency of our FEM simulations. For the first study, DLP proved to be a good indicator of the grain morphology in terms of convexity and shape anisotropy. For the second study, backscattering enhancements of the phase function were related to the grain convexity. The maximum DLP and its negative polarization branches as well as the 〈S22〉/〈S11〉 levels were related to the shape anisotropy of our grains.

Coherent and low-coherent enhanced backscattering in tissue models

We perform simulations of coherent backscattering enhancement, explicitly accounting for the finiteness of spatial coherence of radiation. Three plausible models are considered, which produce the broadening of the coherent backscattering angular profile with the loss of spatial coherence. These models show fair agreement with data obtained earlier for biological media. Additionally, the exact solution for the double scattering term is obtained; its numerical magnitude is shown to be non-dominant for evaluations in the biomedical area due to high scattering anisotropy.

Double Telescope Atmospheric Turbulence Lidar System Based on Backscattering Enhancement Effect

Double Telescope Atmospheric Turbulence Lidar System Based on Backscattering Enhancement Effect

Quantifying the coherent backscatter enhancement of non-spherical particles with discrete dipole approximation.

The prevailing backscattering peak associated with the scattering phase function of large non-absorptive particles can be interpreted with the coherent backscatter enhancement (CBE) theory, but has not been explicitly quantified with numerical simulations based on solving Maxwell's equations. In this paper, representative numerical simulations performed with the discrete-dipole-approximation (DDA) model are used to quantify the effect of CBE on the single-scattering phase function. For each scattering case, the particle volume was divided into multiple thin slices parallel to the incident beam. The dipole polarizations in the j'th slice in response to the incident field of the i'th slice were computed, and then the corresponding contribution to the scattering phase function was calculated. Interference between conjugate terms representing reversible wave paths is constructive at the backscattering direction, which corresponds to the CBE. Subsequently, the contribution of CBE to the scattering phase function was quantified by comparing the electric fields calculated with and without the interference between conjugate terms. Results from these numerical simulations are consistent with conclusions obtained from the CBE theory. The simulations also quantitatively explain why it is difficult to identify a CBE-induced backscattering peak for the phase function of small particles and strong-absorptive particles.

Backscattering enhancements by partially exposed cylindrical shells due to reflected flexural waves at air–water interfaces

For smooth curved objects in contact with a flat interface, the propagation paths of supported surface elastic waves can be altered by the interface, thereby enhancing the characteristics of the backscattered echo returns. In this study, backscattering experiments are conducted on an empty stainless steel cylindrical shell that breaks through an air–water interface and is insonified from below at grazing incidence. The shell has a thickness-to-radius ratio of 5%. The frequencies of interest fall within the vicinity of and below the coincidence frequency, which corresponds to a ka range of 13.8–20.1. The experimental results demonstrate that, for the partially exposed cylindrical shell, the flexural waves can traverse the interface and circumnavigate the shell multiple times. The flexural waves experience mode transitions upon passing through the interface, existing as a0− waves below the interface and a0 waves above the interface. Simultaneously, the flexural waves undergo partial reflection each time they pass through the interface, guiding them to follow a reverse path and generating a periodic series of echo returns, which leads to the enhancement of backscattering. The shift in resonance frequency in relation to the changing exposure is analyzed and predicted using the phase matching theory.

Main Mechanisms of Celestial Bodies Negative Polarization Formation: A Review

The scattered light by the vast majority of celestial bodies without atmospheres has a characteristic feature: A negative branch of linear polarization degree at little phase angles. Researchers have proposed many theoretical mechanisms to explain this feature. This review describes the main mechanisms that form a negative branch of linear polarization degree. The results of ground-based observations of the negative branch of the degree of linear polarization of various objects of the solar system are described. Scattering by single particles, shadow effect, coherent backscattering enhancement, and effects of the near field are considered. The review will be useful for all researchers studying the scattering of light by celestial bodies.

Backscattering enhancements by partially exposed spheres due to reflected subsonic Rayleigh waves at air-water interfaces.

Air-water interfaces can enable distinct target scattering mechanisms different from the mechanism under free field conditions. In this study, backscattering experiments are performed by lowering an acrylic or polymethylmethacrylate sphere through the air-water interface into the water and insonifying the sphere from below at grazing incidence. Pronounced backscattering enhancements associated with the subsonic Rayleigh wave propagation mechanism are observed before the specular reflection point of the sphere reaches the water. The results indicate that, for a partially exposed sphere, subsonic Rayleigh waves can pass through the air-water interface and circumnavigate the sphere multiple times. The phase velocities of Rayleigh waves are different when propagating above and below the air-water interface. Moreover, subsonic Rayleigh waves are partially reflected when passing through the air-water interface, generating wavefronts that propagate in the reverse direction.

Enhanced Backscatter Fibers for Sensing in Telecom Networks

We discuss the application of enhanced backscattering fiber in telecom networks. Such fibers can greatly increase the potential of telecom networks to be used as sensors of network health and its surrounding environment. We describe the effects of increased attenuation and multipath interference (MPI) observed in such fibers as the fiber length and backscattering enhancement increases. For MPI we apply both a well known, approximate three bounce treatment as well as a full solution using coupled mode equations, and we verify the validity of the approximate solution. Our analysis allows us to obtain a sensor reach as a function of the backscattering enhancement level and apply this to interrogation schemes for which the minimum required signal power level and the maximum allowable MPI noise level are known. To show that the backscattering enhancement in our fiber does not interfere with telecom signals, we measure the penalty for signals propagating within and near the enhancement bandwidth in a 10 km length of continuously enhanced backscattering fiber. We find that signals only a few nm away from the enhancement bandwidth can propagate without OSNR penalty.

Polarization properties of the coherent backscattering enhancement for large nonspherical particles with random orientation

Polarization properties of the coherent backscattering enhancement for large nonspherical particles with random orientation

Low-profile and compact retroreflector enabled by a wide-angle and high-efficiency metalens

In this paper, a low-profile metasurface retroreflector based on an ultra-thin wide-angle and high-efficiency metalens is proposed and investigated. Owing to its stable high transmission magnitude and the insensitive phase shift of the proposed meta-atoms within a continuous wide-angle range, the designed metalens can efficiently focus both normal and various oblique incident angle waves to a pre-designed curved trajectory continuously. By placing a concave metal mirror which is exactly coincide with the determined focused spots, the cascaded structure thus can realize backscattering enhancement working at a continuous wide-angle range. A prototype is also fabricated, measured and characterized and the experimental results verify that the proposed retroreflector can perform an excellent backscattering enhancement within a stable 3 dB monostatic RCS for almost ±30° angle range. Moreover, the relationship between the focusing performances of wide-angle metalens (e.g., focused spot power intensity, focusing efficiency and positions) and the backscattering enhancement of cascaded wide-angle retroreflector is established and highlighted. Besides, its excellent backscattering enhancement (the absolute RCS level and angular range) can be compared to some traditional widely-used radar retroreflectors like trihedral corner reflector or Luneburg lens reflector for the similar physical size. The proposed metalens retroreflector also posses of a noticeable working bandwidth of about 0.2 GHz within X band. The presented retroreflective device is promising to develop into a low-profile, light-weight and cost-effective wide-angle retroreflector thus enabling various applications for modern microwave engineering.

Improving OFDR Distributed Fiber Sensing by Fibers With Enhanced Rayleigh Backscattering and Image Processing

This article investigates using optical fibers with enhanced backscattering profiles to improve distributed fiber sensor performance and reduce instrumentation costs. Using a femtosecond (fs) laser direct writing technique, the Rayleigh backscattering profile of a standard telecom fiber was enhanced by more than 40 dB to improve the signal-to-noise ratio (SNR) for optical frequency domain reflectometry (OFDR). The enhanced backscattering signals enable effectively distributed strain measurements using a low-cost tunable laser (TL). Median filtering is applied to denoise cross correlation results to further improve measurement outcomes. Results presented in this article show that a TL with a 1-nm tuning range, which is far less than the tuning range used in commercial OFDR interrogators, can perform effectively distributed strain measurements using sensing fibers with enhanced backscattering profiles. The sensing fiber with over 40-dB backscattering enhancement achieved 4.8-cm spatial resolution in strain measurements with a root mean square accuracy of less than 2.70 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> when 10–50 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> were exerted to the sensing fiber. Results presented in this article reveal both the potential and limitations of sensing fibers with enhanced backscattering for OFDR-based distributed fiber sensors.

Coherent backscatter enhancement in bistatic Ku- and X-band radar observations of dry snow

Abstract. The coherent backscatter opposition effect (CBOE) enhances the backscatter intensity of electromagnetic waves by up to a factor of 2 in a very narrow cone around the direct return direction when multiple scattering occurs in a weakly absorbing, disordered medium. So far, this effect has not been investigated in terrestrial snow in the microwave spectrum. It has also received little attention in scattering models. We present the first characterization of the CBOE in dry snow using ground-based and spaceborne bistatic radar systems. For a seasonal snowpack in the Ku-band (17.2 GHz), we found backscatter enhancement of 50 %–60 % (+1.8–2.0 dB) at a zero bistatic angle and a peak half-width at half-maximum (HWHM) of 0.25∘. In the X-band (9.65 GHz), we found backscatter enhancement of at least 35 % (+1.3 dB) and an estimated HWHM of 0.12∘ in the accumulation areas of glaciers in the Jungfrau–Aletsch region, Switzerland. Sampling of the peak shape at different bistatic angles allows estimating the scattering and absorption mean free paths, ΛT and ΛA. In the VV polarization, we obtained ΛT=0.4±0.1 m and ΛA=19±12 m at the Ku-band and ΛT=2.1±0.4 m and ΛA=21.8±2.7 m at the X-band, assuming an optically thick medium. The HH polarization yielded similar results. The observed backscatter enhancement is thus significant enough to require consideration in backscatter models describing monostatic and bistatic radar experiments. Enhanced backscattering beyond the Earth, on the surface of solar system bodies, has been interpreted as being caused by the presence of water ice. In agreement with this interpretation, our results confirm the presence of the CBOE at X- and Ku-band frequencies in terrestrial snow.

Quasi-omnidirectional retroreflective metagrating for TE-polarized waves based on wave-vector reversions.

Structuring elements of gratings brings more freedom in manipulating diffraction waves, e.g., retroreflection using diffraction orders other than the 0th order. Most retroreflective metagratings (RMs) can achieve retroreflection only under one particular direction, limiting their applications. In this paper, we propose a quasi-omnidirectional RM based on wave-vector reversion for TE-polarized waves. The metagrating element is composed of four rotationally-symmetric sub-elements, which is composed of one probe and two directors on its two sides. The substrate-air-metal layer can reverse kz while directors can reverse kx. Therefore, the wave-vector k of reflected waves can be completely reversed by the sub-element, providing necessary momentum for retroreflection. The -2nd diffraction order of the metagrating is tailored to channel out waves with reversed k, leading to retroreflection. Due to the element's four-fold rotational symmetry, retroreflection can be achieved along four directions, covering all of the four quarters of azimuth angle. We demonstrate prototypes in Ku band, and the average backscattering enhancement compared with a metal plane with the same area (SAMP) along the four directions reaches up to 31.3 dB with incident angle 50.0° at 15.0 GHz. Both simulated and measured results verify our design. This work provides another perspective on retroreflection and may find applications in retroreflective functional devices.

Passive and Active Multiple Scattering of Forests Using Radiative Transfer Theory With an Iterative Approach and Cyclical Corrections

In this article, a unified framework of vegetation scattering using radiative transfer (RT) theory for passive and active remote sensing of vegetated land surfaces, especially those associated with moderate-to-large vegetation water contents (VWCs), e.g., forest field, is presented. The framework allows for modeling passive and active microwave signatures of the vegetated field with the same physical parameters describing the vegetation structure. RT equations are solved by a numerical iterative approach for both passive and active configurations. This approach allows including higher order scattering, which represents multiple scattering. In fields such as forests with large VWCs, associated with large scattering albedo and optical thickness, multiple scattering effects are critical. In the active iterative approach, cyclical terms are identified and backscattering enhancement is included by doubling contributions from cyclical terms. The method is applied to aspen trees in forest fields to compute the brightness temperatures and backscattering coefficients for passive and active remote sensing configurations, respectively. In the passive configuration, for forest field with VWC of 15 kg/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{2}$ </tex-math></inline-formula> , the deviation between the zeroth-order brightness temperature, i.e., the tau–omega model results, and multiple scattering results around 40° observation angle, can be as large as 50 K for vertical polarization and 35 K for horizontal polarization. In the active configuration, the deviation between first-order results, which is identical to the distorted Born approximation, and the multiple scattering results around 40° incidence angle, is about 1.6 dB for VV and 0.7 dB for HH polarization. Multiple scattering is shown to be crucial for accurate forward modeling, especially over forested areas. The proposed approach is thus suitable for vegetation scattering with large VWCs. Furthermore, the proposed model is validated with the passive and active L-band sensor (PALS) acquired in SMAPVEX12 measurements in 2012, which demonstrates the applicability of this model.

Backscattering by broadside and tilted intermediate thickness empty cylindrical metal shells in water: Experiments and theory

In prior research the backscattering properties of empty metal shells in water were investigated over a wide frequency range [Morse et al., J. Acoust. Soc. Am. 103, 785–794 (1998); Morse and Marston, J. Acoust. Soc. Am. 111, 1289–1294 (2002)]. The detailed high-frequency features of the observed and modeled enhancements depended on the shell thickness. In the present research backscattering is measured, and modified models are introduced for a steel shell intermediate in thickness between those considered in the aforementioned prior investigations. Some of the outcomes of this investigation may be summarized as follows: (a) for broadside scattering the contribution from a coincidence-frequency related guided wave is easily observed and is prominent within a ray-theory based model; (b) supersonic antisymmetric guided waves are evident at high frequencies; (c) such antisymmetric guided waves also especially result in easily noticed meridional wave backscattering enhancements when the shell is tilted as was previously demonstrated for other tilted shells. [Work supported by ONR.]

A global study of hygroscopicity-driven light-scattering enhancement in the context of other in situ aerosol optical properties

Abstract. The scattering and backscattering enhancement factors (f(RH) and fb(RH)) describe how aerosol particle light scattering and backscattering, respectively, change with relative humidity (RH). They are important parameters in estimating direct aerosol radiative forcing (DARF). In this study we use the dataset presented in Burgos et al. (2019) that compiles f(RH) and fb(RH) measurements at three wavelengths (i.e., 450, 550 and 700 nm) performed with tandem nephelometer systems at multiple sites around the world. We present an overview of f(RH) and fb(RH) based on both long-term and campaign observations from 23 sites representing a range of aerosol types. The scattering enhancement shows a strong variability from site to site, with no clear pattern with respect to the total scattering coefficient. In general, higher f(RH) is observed at Arctic and marine sites, while lower values are found at urban and desert sites, although a consistent pattern as a function of site type is not observed. The backscattering enhancement fb(RH) is consistently lower than f(RH) at all sites, with the difference between f(RH) and fb(RH) increasing for aerosol with higher f(RH). This is consistent with Mie theory, which predicts higher enhancement of the light scattering in the forward than in the backward direction as the particle takes up water. Our results show that the scattering enhancement is higher for PM1 than PM10 at most sites, which is also supported by theory due to the change in scattering efficiency with the size parameter that relates particle size and the wavelength of incident light. At marine-influenced sites this difference is enhanced when coarse particles (likely sea salt) predominate. For most sites, f(RH) is observed to increase with increasing wavelength, except at sites with a known dust influence where the spectral dependence of f(RH) is found to be low or even exhibit the opposite pattern. The impact of RH on aerosol properties used to calculate radiative forcing (e.g., single-scattering albedo, ω0, and backscattered fraction, b) is evaluated. The single-scattering albedo generally increases with RH, while b decreases. The net effect of aerosol hygroscopicity on radiative forcing efficiency (RFE) is an increase in the absolute forcing effect (negative sign) by a factor of up to 4 at RH = 90 % compared to dry conditions (RH &lt; 40 %). Because of the scarcity of scattering enhancement measurements, an attempt was made to use other more commonly available aerosol parameters (i.e., ω0 and scattering Ångström exponent, αsp) to parameterize f(RH). The majority of sites (75 %) showed a consistent trend with ω0 (higher f(RH = 85 %) for higher ω0), while no clear pattern was observed between f(RH = 85 %) and αsp. This suggests that aerosol ω0 is more promising than αsp as a surrogate for the scattering enhancement factor, although neither parameter is ideal. Nonetheless, the qualitative relationship observed between ω0 and f(RH) could serve as a constraint on global model simulations.

Vibrational energy distribution in plate excited with random white noise

In Statistical Energy Analysis (SEA) and more generally in all statistical theories of sound and vibration, the establishment of diffuse field in subsystems is one of the most important assumption. Diffuse field is a special state of vibration for which the vibrational energy is homogeneously and isotropically distributed. For subsystems excited with a random white noise, the vibration tends to become diffuse when the number of modes is large and the damping sufficiently light. However even under these conditions, the so-called coherent backscattering enhancement (CBE) observed for certain symmetric subsystems may impede diffusivity. In this study, CBE is observed numerically and experimentally for various geometries of subsystem. Also, it is shown that asymmetric boundary conditions leads to reduce or even vanish the CBE. Theoretical and numerical simulations with the ray tracing method are provided to support the discussion.