Multiple Reflections Research Articles

Multiband Josephson effect in an atomic scale Pb tunnel junction

Multiband superconductivity plays an important role in many emergent novel superconductors and has attracted great interest over the years. Various related experimental aspects have been intensely researched, but a quantitative understanding on the Cooper-pair transport remains still elusive, despite its fundamental and technological importance. We study a Josephson junction with a scanning tunneling microscope (STM), where both tip and sample are Pb, a prototypical type I two-band superconductor. We map the properties of the junction across a wide range of normal state conductances revealing in-gap features originating from multiple Andreev reflections (MARs) and the Josephson effect. We present the theoretical framework to extract the transmission through the transport channels and describe the Cooper-pair tunneling with quantitative precision through two superconducting bands. This paves the way for the understanding of increasingly complicated superconductors. Published by the American Physical Society 2024

Maximizing light-to-heat conversion of Ti3C2Tx MXene metamaterials with wrinkled surfaces for artificial actuators

MXene, a promising photothermal nanomaterial, faces challenges due to densely stacked nanosheets with high refractive index (RI). To maximize photothermal performance, MXene metamaterials (m-MXenes) are developed with a superlattice with alternating MXene and organic layers, reducing RI and inducing multiple light reflections. This approach increases light absorption, inducing 90% photothermal conversion efficiency. The m-MXene is coated onto liquid crystal elastomer (LCE) fibers, as actuating platforms via a dip-coating (m-MXene/aLCE fiber), exhibiting excellent light-driven actuating owing to the synergetic effect of the patterned m-MXene laysers by structural deformation. The m-MXene/aLCE fibers lift ~6,900 times their weight and exhibit a work density 6 times higher than that of human skeletal muscle. It is applied to artificial muscles, grippers, and a bistable structure (a shooting device, and switchable gripper). Our study offers an effective strategy to enhance light absorption in 2D nanomaterials and contributes to advancements in photothermal technologies in various fields.

Lightweight, thermally insulating SiBCN/Al2O3 ceramic aerogel with enhanced high-temperature resistance and electromagnetic wave absorption performance

Owing to tunable dielectric properties, light weight and high porosity, polymer-derived SiBCN ceramic aerogels possess significant application prospects in electromagnetic wave (EMW) absorption and thermal insulation. However, due to inadequate oxidation resistance, the structural collapse and performance deterioration of SiBCN aerogels will easily occur in high-temperature aerobic environments, limiting their application. Herein, to address this issue, a novel and straightforward strategy based on typical polymer-derived-ceramic (PDC) aerogel method and impregnation with boehmite sol was proposed for synthesizing SiBCN/Al2O3 composite ceramic aerogels. The microstructure, phase composition, thermal insulation, oxidation resistance and EMW absorption properties of SiBCN/Al2O3 ceramic aerogels were investigated. The resulting SiBCN/Al2O3 composite aerogel demonstrates superior high-temperature structural stability, exhibiting an ultra-low linear shrinkage of only 6.5 % following heat treatment at 1200 °C for 2 h in air. Additionally, the composite aerogel shows a low thermal conductivity of 0.039 W/mK and a low density of 0.112 g/cm3. The SiBCN/Al2O3 composite aerogel, composed of dielectric SiBCN, conductive free carbon, and insulating alumina, demonstrates outstanding EMW absorption properties with a minimum reflection loss of −48.6 dB and an effective bandwidth of 5.8 GHz. The enhanced microwave absorption performance is mainly attributed to the improved impedance matching, multiple reflection, and enhanced interfacial polarization resulting from the introduction of Al2O3. Given prominent oxidation resistance, thermal insulation and EMW absorption properties, the SiBCN/Al2O3 composite aerogel paves the way for developing microwave absorption and thermal insulation integrated material in high-speed vehicles.

Efficient Hydrogen Peroxide Production: Enhancing Electron Transfer on PANI/ CdS Photocatalysts in Aqueous Solution

The conventional anthraquinone process for hydrogen peroxide production is energy-intensive and complex. We present a sustainable alternative: a photocatalytic technology driven by solar energy for the green and efficient generation of H2O2. In this study, we synthesized polyaniline (PANI), a conductive polymer, and combined it with CdS nanoparticles (CdS-NP) using a hydrothermal method to create PANI/CdS-NP composites. The unique hollow raspberry-like structure of the composite enhances light utilization through multiple reflections and refractions. PANI significantly improved the photoresponse and photocatalytic activity of CdS-NP. Under visible light and an air flow of 20mL·min-1, the 10wt%-PANI/CdS-NP composite exhibited a remarkable photocatalytic hydrogen peroxide production rate of 371.33 μmol·h-1·g-1, surpassing the pristine CdS-NP by 1.74 times, and all without any sacrificial agents. Work function calculations revealed that the electron transport between CdS and PANI follows the type II heterojunction photocatalytic mechanism, with the one-step two-electron ORR being the dominant pathway. This work not only contributes to the theoretical understanding of photocatalytic H2O2 synthesis but also offers a promising route toward the green and sustainable production of H2O2.

Curvature Regularization for Non-Line-of-Sight Imaging From Under-Sampled Data.

Non-line-of-sight (NLOS) imaging aims to reconstruct the three-dimensional hidden scenes by using time-of-flight photon information after multiple diffuse reflections. The under-sampled scanning data can facilitate fast imaging. However, the resulting reconstruction problem becomes a serious ill-posed inverse problem, the solution of which is highly likely to be degraded due to noises and distortions. In this paper, we propose novel NLOS reconstruction models based on curvature regularization, i.e., the object-domain curvature regularization model and the dual (signal and object)-domain curvature regularization model. In what follows, we develop efficient optimization algorithms relying on the alternating direction method of multipliers (ADMM) with the backtracking stepsize rule, for which all solvers can be implemented on GPUs. We evaluate the proposed algorithms on both synthetic and real datasets, which achieve state-of-the-art performance, especially in the compressed sensing setting. Based on GPU computing, our algorithm is the most effective among iterative methods, balancing reconstruction quality and computational time.

Trace gas detection system based on multi-reflection differential Helmholtz cell and VMD-airPLS algorithm

In this paper, a photoacoustic spectroscopy sensor based on multi-reflection differential Helmholtz cell and Variational Mode Decomposition-adaptive iteratively reweighted penalized least squares (VMD-airPLS) algorithm is proposed to improve the signal-to-noise ratio (SNR) and reduce the response time of the system. The Helmholtz photoacoustic cell, fabricated from brass, allows multiple reflections of the laser light within its resonant cavity, effectively increasing the gas absorption path by a factor of ∼ 6. The VMD-airPLS algorithm suppresses incoherent noise and slowly varying baseline noise over time. To validate the performance of the system, methane is measured using a laser with a wavelength of 1653.7 nm. Experimental results demonstrate a significant enhancement in the sensitivity of system, up to approximately 2.2 times, and an improvement in the SNR by up to approximately 4.2 times compared to single-pass system. The minimum detection limit for methane is ∼ 96.79 ppb.

Construction of 2D/2D heterostructure with porous few layer g-C3N4 and NiCo2O4 nanosheets towards electromagnetic wave absorption

Developing high-performance materials with ultra-high reflection loss (RL ≤ -65 dB) for electromagnetic wave (EMW) absorption is pivotal to address electromagnetic pollution, but it is still challenging. Herein, a two-dimensional (2D)/2D PFL g-C3N4/NiCo2O4 heterostructure with porous few-layer g-C3N4 (PFL g-C3N4) and ultrathin NiCo2O4 nanosheets has been prepared by self-assembly, hydrothermal reaction, and calcination strategies. The 2D/2D heterostructure exhibits extraordinary EMW absorption property, achieving a reflection loss (RL) value of -68.59 dB and an effective absorption bandwidth (EAB) of 1.92 GHz at a matching thickness of 2.5 mm. As expected, the porous few layer g-C3N4 nanosheets provide rich interface which increases multiple reflection paths and enhances the conduction loss. The cooperative effects of magnetic loss, dielectric loss, and impedance matching contribute to the exceptional EMW absorption property of PFL g-C3N4/NiCo2O4 (2D/2D) heterostructure. The potential of PFL g-C3N4/NiCo2O4 in actual radar stealth is verified through radar scattering cross-section (RCS) simulation. This work paves the way for utilizing 2D/2D heterostructure as an ultra-efficient EMW absorbers.

Effect of powder and absorptivity assumptions in the high-fidelity modeling of Laser Powder Bed Fusion processes

Laser Powder Bed Fusion (LPBF) technology represents the most promising metal additive manufacturing process to be scaled up to an industrial level for producing full batches of parts within the automotive aerospace and health sectors, among others. Despite this higher technological readiness, imperfections such as warping, porosities or undesired non-equilibrium microstructures can arise unexpectedly due to the complex thermal histories inherent to LPBF. Being able to anticipate to such imperfections is thus of high importance. With this purpose, numerical modeling strategies can be employed to significantly reduce experimental work on the melt pool scale. This allows to quantify the odds and magnitude of these process unwanted effects. More precisely, Computational Fluid Dynamics (CFD) enables high-fidelity models on the laser-matter interaction during LPBF processing. In this work, a modeling approach is presented in which the driving forces for melt formation are included in a CFD workspace, where multiple reflections of the laser on the melt pool Liquid-Gas Interface (LGI) are considered by means of an absorptivity function of the LGI depth. This simplification and the use of adaptive mesh strategies allows the model to be computed within a reasonable time. Results and applicability of the model on the determination of the melting mode (conduction or keyhole) are displayed and compared with experimental measurements of the melt pool width and depth for 316L and Ti64 single tracks produced on a LPBF machine. Likewise, further aspects such as total absorbed power are assessed.

Multifunctional Anisotropic Aerogels for Intelligent Electromagnetic Wave Absorption

AbstractThe rapidly developing modern society has higher requirements for intelligent electromagnetic wave absorbing (EMA) materials than ever before. Herein, lightweight, multifunctional metal‐free carbon‐based aerogels (RCPs) with a longitudinal honeycomb porous framework and a transverse neatly layered structure are obtained by heat‐treating graphene oxide/oxidized carbon nanotubes/hexachlorocyclotriphosphazene complex to simultaneously achieve the tunable EMA, flame retardancy, and thermal insulation. The anisotropic structure ensures the tunable properties of the carbon aerogels along with tilt angles according to environmental needs. The honeycomb porous structure effectively promotes multiple reflections and scatterings of electromagnetic waves compared with the layered structure, maximizing the penetration inside the material and thus a high EMA performance. The optimized R10C2P‐4 exhibits a minimum reflection loss in the longitudinal direction of−61.5 dB, while the value is as low as −22.4 dB in the transverse direction. Furthermore, the porous structure adequately inhibits the spread of heat, accompanied by the phosphorus species induced by hexachlorocyclotriphosphazene pyrolysis, endowing a good flame retardancy and thermal insulation with a low thermal conductivity of 20.6 mW m−1 K−1 in the longitudinal direction, rather than the transverse direction. This work provides an ingenious insight into the design of multifunctional EMA materials, adapting flexibly to various application requirements.

Experimental Study on Tree Shape Identification and Interaction Among Trees

Forests play a crucial role in mitigating global warming and maintaining climate balance, necessitating a comprehensive understanding of forest dynamics. However, many optical characteristics related to forests remain poorly understood, particularly models for determining forest parameters such as tree species, shape, and inter-tree distances. Although several models exist for estimating leaf area index and photosynthetically active radiation, fewer models address structural forest parameters. This research aims to develop a model using visible and near-infrared radiometer data from Earth observation satellites to estimate forest parameters, including tree species, shape, height, and spacing. Results show that as the distance between trees increases, the impact of multiple reflections decreases significantly. Elliptical tree shapes exhibit approximately three times higher multiple reflection effects compared to conical shapes, indicating potential for distinguishing tree shapes through radiometric data. For canopy shapes, shorter, thicker trees experience more significant reflection effects than taller, thinner trees, suggesting the feasibility of estimating tree height. Overall, the impact of multiple reflections ranges from a few to 10% of TOA radiance, necessitating its consideration when calculating forest reflectance to ensure accurate forest parameter retrieval from satellite observations.

Preparation of high performance microwave absorbing materials based on waste peanut shells

Abstract The emergence of advanced wave-absorbing materials has led to a growing interest in biomass-derived porous carbon due to its abundant availability, low density, and eco-friendly nature. This study utilized peanut shell biomass waste to produce porous carbon material (KPS) through a two-step carbonation-activation process. We investigated the influence of varying ratios of carbonized peanut shells (CPS) to potassium hydroxide (KOH) activator on the pore structure and morphology of KPS. The comprehensive analysis of the electromagnetic parameters revealed that KPS sample achieved a remarkable reflection loss of −42.38 dB and an effective bandwidth of 2.58 GHz with the thickness of 2.5 mm when the quality ratio of CPS to activator KOH was 1:3 and the carbonation temperature was 600 °C. Notably, the KPS material demonstrated substantial specific surface area of 1037.589 m2 g−1 and complex pore architecture, facilitating the development of conductive network and promoting multiple reflections of electromagnetic waves. This research highlighted the potential of efficiently utilizing recyclable resources to create microwave-absorbing materials that are not only simple to produce but also environmentally sustainable.

Height Measurement for Meter-Wave MIMO Radar Based on Sparse Array Under Multipath Interference

For meter-wave multiple-input multiple-output (MIMO) radar, the multipath of target echoes may cause severe errors in height measurement, especially in the case of complex terrain where terrain fluctuation, ground inclination, and multiple reflection points exist. Inspired by a sparse array with greater degrees of freedom and low mutual coupling, a height measurement method based on a sparse array is proposed. First, a practical signal model of MIMO radar based on a sparse array is established. Then, the modified multiple signal classification (MUSIC) and maximum likelihood (ML) estimation algorithms based on two classical sparse arrays (coprime array and nested array) are proposed. To reduce the complexity of the algorithm, a real-valued processing algorithm for generalized MUSIC (GMUSIC) and maximum likelihood is proposed, and a reduced dimension matrix is introduced into the real-valued processing algorithm to further reduce computation complexity. Finally, sufficient simulation results are provided to illustrate the effectiveness and superiority of the proposed technique. The simulation results show that the height measurement accuracy can be efficiently improved by using our proposed technique for both simple and complex terrain.

Nickel-Induced Dual Carbon Networks Encapsulating Phase Change Materials for Photothermal Conversion and Storage.

Bifunctional phase change materials (PCMs) with efficient energy storage and photothermal conversion capabilities have tremendous potential to be applied in advanced thermal management. However, classical organic PCMs with high latent heat are challenged by poor light harvesting, low thermal conductivity, and leakage risks. Here, we design a unique dual-carbon network with Ni nanoparticles (NPs), confined carbon nanotubes (CNTs) shuttling in carbon honeycombs (CH), namely, CH@Ni-CNTs, to encapsulate paraffin wax (PW) that can facilitate the light capture and photothermal conversion dynamics. Benefiting from the physical adsorption of the hierarchical porous structure, the obtained PW/CH@Ni-CNTs composite PCMs show a high phase change enthalpy of 131.0 J g-1 and long-lasting thermal stability of up to 300 heating-cooling cycles. Moreover, an outstanding photothermal energy conversion efficiency of 96.9% is achieved due to the synergistic effect of the dual carbon network and confined Ni NPs. The CNTs shuttled CH network affords multiple reflection chambers and a thermal conductive pathway, while the localized surface plasmon resonance (LSPR) effects of Ni NPs concentrate the incident light energy to generate and accelerate the transport of active "hot electron", thus collectively contributing to the excellent photothermal properties of the composite PCMs. This study presents a bifunctional Ni-induced dual-carbon network system for the controllable preparation of composite PCMs, and it sheds light on the photothermal conversion mechanisms.

Boosted photoinduced charge separation in FeNi2S4@Cd0.9Zn0.1S Step-scheme heterojunction for photothermal assisted photocatalytic water splitting into hydrogen

Rational design of photocatalysts with photothermal effect to maximize light utilization is pivotal in achieving superior photocatalytic efficiency. In this work, by coating Cd0.9Zn0.1S (CZS) nanorods on hollow FeNi2S4 (FNS) microspheres with photothermal effect, a novel FeNi2S4@Cd0.9Zn0.1S (FNS@CZS) Step-scheme (S-scheme) heterojunction photocatalyst was constructed for efficient photothermal-assisted hydrogen (H2) evolution via simultaneously employing light and thermal energy. The hollow structure of FNS microsphere not only provides abundant reactive sites but also serves as a supportive substrate for CZS nanorods, effectively inhibiting their agglomeration. And the unique hollow structure of FNS allows for multiple reflections and refractions of visible light, enhancing the local temperature of the photocatalyst. This effectively minimizes thermal losses within the composite system, thereby enhancing the efficiency of light energy utilization. Furthermore, the formation of the S-scheme heterojunction facilitates the efficient separation of photogenerated charge carriers and enhances the activity of redox reactions, thereby boosting the overall photocatalytic activity. Experimental results reveal that the H2 production rate of the FNS@CZS heterojunction reaches 12.9 mmol·g−1·h−1, which is 25.1 times higher than that of pristine CZS. By controlling the experimental temperature, the impact of the photothermal effect on the photocatalytic H2 production rate has been clarified. According to relevant evidence from infrared thermography and DFT calculations, comprehensive analyses and discussions were conducted on the photothermal effect and S-scheme charge transfer mechanisms. This study offers guidance on designing photothermal-enhanced photocatalysts to achieve satisfactory H2 production activity.

Study of the optical rotatory of potassium titanyl phosphate using the advanced dual-wavelength polarimetric method

A dual-wavelength high-accuracy universal polarimeter was applied to the circular birefringence and optical activity measurement in potassium titanyl phosphate (KTP) nonlinear crystal. The experimental setup used two single-mode He-Ne lasers with close wavelengths of 594 and 633 nm as light sources. Measurement has been carried out for two crystal settings in directions of a 45-degree relative angle to the [100] and [010] crystallographic axes. Multiple light reflections inside the crystal sample were considered when processing the results of the polarimetric measurements. The results have been analysed using the optical transmission function for the polariser-sample-analyser system, and 2D intensity contour maps made it possible to determine the phase parameters, systematic errors, and eigenwaves ellipticity. It was found that the gyration tensor component of the KTP crystal is equal to g12 = 1.4 ⋅ 10−5 which in terms of optical rotatory power corresponds to the very small magnitude of the rotation value of 2.3 deg/mm.

Fabry-Perot Effect Suppression in Gas Cells Used in THz Absorption Spectrometers. Experimental Verification.

A standard measuring gas cell used in absorption spectrometers is a cylinder enclosed by two transparent windows. The Fabry-Perot effects caused by multiple reflections of terahertz waves between these windows produce significant variations in the transmitted radiation intensity. Therefore, the Fabry-Perot effects should be taken into account to correctly measure absorption spectra in Bouguer law-based absorption spectroscopy. One approach to reducing the Fabry-Perot effects is based on inserting an additional external movable window with the standard measuring gas cell. This was proposed and numerically analyzed in our previous work. This paper is aimed at the experimental validation of this method when using amplitude modulation (AM) spectroscopy. Also, a comparison of the efficiency of reducing the Fabry-Perot effects using this method is experimentally compared to frequency modulation spectroscopy. The latter was shown to effectively reduce the Fabry-Perot effects compared to AM spectroscopy with the standard measuring gas cell, and the use of the external movable window was shown to further improve the elimination of Fabry-Perot effects.

Numerical Modeling on Ocean-Bottom Seismograph P-Wave Receiver Function to Analyze Influences of Seawater and Sedimentary Layers

It is challenging to apply the receiver function method to teleseisms recorded by ocean-bottom seismographs (OBSs) due to a specific working environment that differs from land stations. Teleseismic incident waveforms reaching the area beneath stations are affected by multiple reflections generated by seawater and sediments and noise resulting from currents. Furthermore, inadequate coupling between OBSs and the seabed basement and the poor fidelity of OBSs reduce the signal-to-noise ratio (SNR) of seismograms, leading to the poor quality of extracted receiver functions or even the wrong deconvolution results. For instance, the poor results cause strong ambiguities regarding the Moho depth. This study uses numerical modeling to analyze the influences of multiple reflections generated by seawater and sediments on H-kappa stacking and the neighborhood algorithm. Numerical modeling shows that seawater multiple reflections are mixed with the coda waves of the direct P-wave and slightly impact the extracted receiver functions and can thus be ignored in subsequent inversion processing. However, synthetic seismograms have strong responses to the sediments. Compared to the waveforms of horizontal and vertical components, the sedimentary responses are too strong to identify the converted waves clearly. The extracted receiver functions correspond to the above influences, resulting in divergent results of H-kappa stacking (i.e., the Moho depth and crustal average VP/VS ratio are unstable and have great uncertainties). Fortunately, waveform inversion approaches (e.g., the neighborhood algorithm) are available and valid for obtaining the S-wave velocity structure of the crust–upper mantle beneath the station, with sediments varying in thickness and velocity.

Analysis of Scattering Mechanisms in SAR Image Simulations of Japanese Wooden Buildings Damaged by Earthquake

The difficulty in identifying collapsed houses and damaged structures in synthetic aperture radar (SAR) images after natural disasters represents a significant challenge in the monitoring of urban structural deformation using SAR. SAR image simulation was conducted on a three-dimensional model of a typical wooden building in Japan to analyze the scattering mechanism of the structure in collapsed and uncollapsed states. Based on the physical properties of the buildings, a correlation was established between the simulated SAR image feature signals and the geometric structures of the buildings. The findings indicate that SAR scattering is more uniform for uncollapsed structures, which is predominantly influenced by their geometry. At low incidence angles, single reflections were the predominant phenomenon, whereas at high incidence angles, multiple reflections became more prevalent. The uncollapsed building’s facade formed a dihedral angle, exhibiting bright lines in the SAR image. Multiple reflections occurred at the edges of the building and floor junctions. These findings follow the theoretical predictions. In the case of the collapsed buildings, multiple reflections occurred with greater frequency, and irregular scattering was observed. Notwithstanding the augmented scattering pathways, some walls nevertheless manifested single reflections. The collapsed structures demonstrated a reduced sensitivity to alterations in the angle of incidence.

Reliable and Resilient Wireless Communications in IoT-Based Smart Agriculture: A Case Study of Radio Wave Propagation in a Corn Field

In the past few years, one of the largest industries in the world, the agriculture sector, has faced many challenges, such as climate change and the depletion of limited natural resources. Smart Agriculture, based on IoT, is considered a transformative force that will play a crucial role in the further advancement of the agri-food sector. Furthermore, in IoT-based Smart Agriculture systems, radio wave propagation faces unique challenges (such as attenuation in vegetation and soil and multiple reflections) because of sensor nodes deployed in agriculture fields at or slightly above the ground level. In our study, we present, for the first time, several models (Multi-slope, Weissberger, and COST-235) suitable for planning radio coverage in a cornfield for Smart Agriculture applications. We received signal level measurements as a function of distance in a corn field (R3 corn stage) at 0.9 GHz and 2.4 GHz using two transmitting and two receiving antenna heights, with both horizontal and vertical polarization. The results indicate that radio wave propagation in a corn field is influenced not only by the surrounding environment (i.e., corn), but also by the antenna polarization and the positions of the transmitting and receiving antennas relative to the ground.

Dielectric synergistic gradient metamaterials enable exceptional ultra–wideband microwave absorption and antibacterial properties

Interface structure design and multi–component strategies for regulating electromagnetic parameters to achieve efficient microwave absorption remain a challenge. In this study, a composite film with a heterogeneous structure, PAN@PPy@Ti3C2Tx@GO (PPTG) was fabricated through electrospinning, in situ oxidative polymerization, and dip–coating processes, achieving excellent electromagnetic wave absorption and antibacterial properties. When the filler content is only 20 wt%, the PPTG film exhibits an ultra–wide bandwidth, with a minimum reflection loss value (RLmin) of −49.98 dB (2.2 mm, 16.72 GHz), and the maximum effective absorption bandwidth (EAB) reaches 8.0 GHz (2.5 mm). The outstanding absorption performance is attributed to quarter–wavelength theory, polarization, multiple reflections and scattering, impedance matching, metamaterial properties, quasi–antenna effect, conductive loss, and magnetic loss. Excitingly, the radar cross–section (RCS) of the PPTG film can be reduced by up to 25.72 dB•m2, indicating excellent radar stealth properties. Moreover, the PPTG film exhibits excellent antibacterial properties. This research, based on the MXene absorber, provides new insights into the development of lightweight, efficient, and antibacterial microwave–absorbing materials.