Wave Diffraction Research Articles

Scan-free time-of-flight-based three-dimensional imaging through a scattering layer.

Reconstructing an object's three-dimensional shape behind a scattering layer with a single exposure is of great significance in real-life applications. However, due to the little information captured by a single exposure while strongly perturbed by the scattering layer and encoded by free-space propagation, existing methods cannot achieve scan-free three-dimensional reconstruction through the scattering layer in macroscopic scenarios using a short acquisition time of seconds. In this paper, we proposed a scan-free time-of-flight-based three-dimensional reconstruction method based on explicitly modeling and inverting the time-of-flight-based scattering light propagation in a non-confocal imaging system. The non-confocal time-of-flight-based scattering imaging model is developed to map the three-dimensional object shape information to the time-resolved measurements, by encoding the three-dimensional object shape into the free-space propagation result and then convolving with the scattering blur kernel derived from the diffusion equation. To solve the inverse problem, a three-dimensional shape reconstruction algorithm consisting of the deconvolution and diffractive wave propagation is developed to invert the effects caused by the scattering diffusion and the free-space propagation, which reshapes the temporal and spatial distribution of scattered signal photons and recovers the object shape information. Experiments on a real scattering imaging system are conducted to demonstrate the effectiveness of the proposed method. The single exposure used in the experiment only takes 3.5s, which is more than 200 times faster than confocal scanning methods. Experimental results show that the proposed method outperforms existing methods in terms of three-dimensional reconstruction accuracy and imaging limit subjectively and objectively. Even though the signal photons captured by a single exposure are too highly scattered and attenuated to present any valid information in time gating, the proposed method can reconstruct three-dimensional objects located behind the scattering layer of 9.6 transport mean free paths (TMFPs), corresponding to the round-trip scattering length of 19.2 TMFPs.

Terahertz Scanless Hypertemporal Imaging

AbstractSince its first demonstration in 1995, terahertz time‐domain imaging has attracted an increasingly growing interest for its ability to reveal spectral fingerprints of materials and probe changes in refractive index and absorption, as well as detect the inner structure of complex objects via time‐of‐flight measurements. Practically, however, its widespread use has been hampered by the very long acquisition time typically required to spatially raster‐scan the object, and for each spatial point, record the field in time via a delay line. Here, this fundamental bottleneck is addressed by implementing a scanless single‐pixel imaging scheme, which sets the path for an unprecedented reduction of both system complexity and acquisition time. By properly exploiting natural wave diffraction, time‐to‐space encoding applied to terahertz point detection allows for an almost instantaneous capture of the terahertz waveforms, while multidimensional images are reconstructed via a computational approach. The scheme is a promising solution for the development of next‐generation fast and compact terahertz imagers perfectly suitable for high‐repetition‐rate laser sources.

Active sites of Te-hyperdoped silicon by hard x-ray photoelectron spectroscopy

Multiple dopant configurations of Te impurities in close vicinity in silicon are investigated using photoelectron spectroscopy, photoelectron diffraction, and Bloch wave calculations. The samples are prepared by ion implantation followed by pulsed laser annealing. The dopant concentration is variable and high above the solubility limit of Te in silicon. The configurations in question are distinguished from isolated Te impurities by a strong chemical core level shift. While Te clusters are found to form only in very small concentrations, multi-Te configurations of type dimer or up to four Te ions surrounding a vacancy are clearly identified. For these configurations, a substitutional site location of Te is found to match the data best in all cases. For isolated Te ions, this matches the expectations. For multi-Te configurations, the results contribute to understanding the exceptional activation of free charge carriers in hyperdoping of chalcogens in silicon.

Anapole-assisted ultra-narrow-band lattice resonance in slotted silicon nanodisk arrays

Anapole modes supported by well-designed dielectric nanostructures have attracted extensive attention in the field of nanophotonic applications owing to their unique strong near-field enhancement and non-radiative far-field scattering characteristics, yet it is still difficult to achieve high Q-factor resonance features with a narrow linewidth. In this work, a periodic slotted silicon nanodisk array is theoretically proposed to realize narrow linewidth and high Q-factor resonance in the near-infrared wavelength range. Through introducing the coupling between the anapole modes in the single dielectric nanostructure and the diffractive wave mode arising from the periodic array, the as-designed dielectric nanostructure synchronously manifests excellent spectral features with a bandwidth as narrow as about 2.0 nm, a large Q-factor of 599, an almost-perfect transmission amplitude of 96% and a relatively high electric field intensity (>2809 times) in the middle of the slotted silicon nanodisk. The as-designed nanostructure possessing these outstanding optical features can work as a high-efficiency refractive index sensor, whose sensitivity can reach 161.5 nm RIU−1 with its figure of merit attaining 80.8 RIU−1, efficiently distinguishing an index change of less than 0.01. The proposed slotted silicon nanodisk array exhibits tremendous potential for expanding applications such as label-free biochemical sensing, plasmonic refractive index sensing and surface enhancement spectroscopy.

Load effects on vessel-shaped fish cage steel structures, nets and connectors considering the effects of diffraction and radiation waves under irregular waves

Vessel-shaped fish cages are mainly composed of a large floating body, aquaculture nets and a number of steel frames. Due to the large scale of vessel-shaped fish cages, the velocity field induced by the diffraction waves and radiation waves cannot be ignored in the calculation of the loads on the net and steel frames. In turn, the loads can affect the wave-induced response of the floating body and hence the radiation waves as well as the sectional forces. In this paper, a novel method is proposed to consider the coupling effect. First, considering the irregular waves in short-term periods, the global response of the floating body is calculated in the time domain by the state-space method based on potential flow theory. Meanwhile, the velocity field considering the influence of the floating body is built according to the velocity transfer function. Then, the Morison formula is used to calculate the loads on the nets and the steel frames. Finally, the coupling achieved through numerical iterations is considered in the calculation of the twine tension, load effect of connector and the global cross-section under irregular waves. It is found that the diffraction and radiation waves make a significant difference in the twine tension and connector load effect.

Partial Areas Method in the Problem of Diffraction of an Electromagnetic Wave by a Longitudinal Partition in an Infinite Waveguide

Partial Areas Method in the Problem of Diffraction of an Electromagnetic Wave by a Longitudinal Partition in an Infinite Waveguide

Narrow resonance line-widths and high figure of merits via composite nanopillar dimer array sensors

The Ag/Si/SiO2 composite nanopillar dimer array sensor is put forward to form a surface lattice resonance (SLR) with strong intensity and narrow resonance line-width. In the Ag/Si/SiO2 composite nanopillar dimer array, Ag nanopillar dimers sit on Si nanopillar dimers, and Si nanopillar dimers sit on SiO2 nanopillar dimers, which sit on a quartz substrate. Simulated results demonstrate that the line-width can be as narrow as 2.1 nm. This is mainly attributed to the coupling between the combination of single Ag nanopillar dimers' local surface plasmon quadrupole (Q) resonance and Si nanopillar dimers' Mie resonance with the array's diffraction waves. However, for the Ag/Si/SiO2 composite nanopillar array (without dimers), the SLR intensity is too weak for the sensing applications. Compared to the Ag/Si/SiO2 composite nanopillar array, the superiority of the Ag/Si/SiO2 composite nanopillar dimer array is that the near-field coupling in the dimer can further improve Ag nanopillar dimers' Q resonance and Si nanopillar dimers' Mie resonance. Therefore, SLR intensity can be significantly enhanced with only a slight increase in line-width, thus the sensing detection range is wider. The array's period, the height and diameter of Ag/Si/SiO2 nanopillars have notable effects on the SLR. Benefiting from the narrow line-width, the figure of merit (FOM) can reach a value of 226. This work is important to improve the optical sensor based on SLRs.

Gaussian beam diffraction from radial structures: detailed study on the diffraction from sinusoidal amplitude radial gratings.

In this work, we report a comprehensive theoretical investigation on the diffraction of a Gaussian beam from structured radial apertures. In particular, the study of near- and far-field diffraction of a Gaussian beam from an amplitude radial grating having a sinusoidal profile provides new theoretical insights and possible applications. We observe a high self-healing feature at far-field for the Gaussian beam in the diffraction from amplitude radial structures. It is also shown that by increasing the spokes number of the grating, the strength of the self-healing decreases, and reforming of the diffracted pattern into a Gaussian beam occurs at longer propagation distances. The energy flow towards the central lobe of the diffraction pattern and its dependence on the propagation distance are also investigated. In the near-field regime, the diffraction pattern is very similar to the intensity distribution in the central area of the radial carpet beams generated in the diffraction of a plane wave from the same grating. It is shown that by optimally choosing the waist radius of the Gaussian beam, in the near-field regime, it is possible to have a petal-like diffraction pattern, which has been experimentally used in multiple-particle trapping. Compared to radial carpet beams, since in this case there is no energy in the geometric shadow of the radial spokes of the grating, the main part of the power of the incident Gaussian beam is transferred to the main intensity spots of the petal-like pattern, which significantly increases the multi-particle trapping efficiency. We also show that regardless of the grating spokes number, at the far field, the diffraction pattern becomes a Gaussian beam, and its power share reaches 2/3 of the total power passed through the grating.

Blast-resistant design approach for RC bridge piers

Piers are the critical components to support the superstructure of bridge, but susceptible to the intentional and accidental explosions due to its easy accessibility. The blast-induced damage to the pier may cause the entire bridge to collapse, resulting in huge casualties and economy loss, and thus it is necessary to perform the blast-resistant design for the bridge pier. At present, the analytical model and blast-resistant design approach for blast-loaded reinforced concrete (RC) pier considering both the direct shear and flexural responses were studied. Firstly, six shots of field ground explosion test on 1/5-scale circular and square RC columns were carried out, in which the TNT charges with the scaled distances of 0.86–1.22 m/kg1/3 were adopted. Then, the corresponding numerical simulations incorporating the newly developed Structured Arbitrary Lagrangian Eulerian (SALE) solver were conducted, and the validity of finite element (FE) analysis approach was verified by comparing the experimental incident and reflected overpressure-time histories acting on the periphery of pier with deviations for most measuring points less than 20%. Afterwards, total 176 explosion scenarios were numerically simulated, and a blast loading distribution model on the pier for ground explosions was proposed, which considers the pier cross-section shapes (circular and square) and pier diameter/edge length (0.2–1.6 m), the scaled distances of explosive charge (0.3–2.3 m/kg1/3), as well as the blast wave reflection and diffraction by the pier. Furthermore, a Timoshenko beam-based multi-degree of freedom (MDOF) analytical model was established, and validated by both the existing explosion tests on RC columns and refined numerical simulations of blast-loaded bridge piers with corresponding deviations no more than 5%. Finally, a blast-resistant design procedure incorporating the above-proposed blast loading distribution and MDOF models was given, providing a reliable and efficient blast-resistant evaluation and design tool for bridge piers.

Experimental study of the motion of a shock wave in the plasma of a pulsed volume discharge in air

The motion of quasi-plane shock waves with Mach numbers = 2.20–3.50 in the plasma of a nanosecond combined volume discharge in air at an initial pressure of 10–100 Torr has been experimentally studied on the basis of high-speed shadow registration of the flow field. The dynamics of shock–wave configurations after the discharge at various stages of an unsteady supersonic flow, which is formed after the diffraction of a plane shock wave by a rectangular obstacle, is studied. An increase in the velocity of the shock wave front over a time interval of up to 15 𝜇s in a plasma region of 9–40 mm long and its dependence on the plasma parameters is found. An analysis of relaxation processes in plasma showed that the acceleration of the shock wave front can be caused by air heating due to the quenching of electronically excited nitrogen molecules, in which the internal energy is converted into thermal energy at times up to 30 𝜇s.

Wave Diffraction in an Elastic Medium with a Spherical Cavity Supported by a Thin Shell

Wave Diffraction in an Elastic Medium with a Spherical Cavity Supported by a Thin Shell

Effects of a thin vertical porous barrier on the water wave scattering by a porous breakwater

Porous breakwaters are introduced to reduce the impact of wave load on various marine structures and dissipate unwanted wave energy by creating a tranquility zone. This type of marine structure can have practical implications for the development of artificial breeding or nursing grounds for fish and other sea animals. We theoretically investigated the water wave scattering by a surface-piercing porous breakwater in the presence of a thin vertical porous barrier in the framework of linear potential flow theory. Flow through the vertical porous barrier is considered using the model of Yu [“Diffraction of water waves by porous breakwaters,” J. Waterw., Port, Coastal, Ocean Eng. 121, 275–282 (1995)] under the assumption of fine pores. We use the eigenfunction expansion method to obtain solutions of the associated boundary value problems analytically. We develop semi-analytical solutions of the proposed mathematical problem using matching boundary conditions along boundaries of individual successive regions of the system. For wave scattering by a barrier-breakwater system with focus on wave energy dissipation, the effect of various wave and structural parameters on the reflection and transmission coefficients, dissipation coefficient, and wave force on the system are studied. The practical interest in this problem stems from the result that for the barrier-breakwater system, there exists a critical value of wave inclination angle below which wave reflection is minimum and wave energy dissipation is maximum. The results have implications for the proposed problem to harness energy from ocean waves and other problems related to the design and enhancement of the hydrodynamic performance of the breakwater in the marine environment.

Theoretical and Experimental Investigation of Plane Wave Diffraction from an Amplitude Modified Fresnel Zone Plate

Theoretical and Experimental Investigation of Plane Wave Diffraction from an Amplitude Modified Fresnel Zone Plate

Experimental investigation on the effect of a barrier wall on unconfined hydrogen explosion

27 m3 unconfined hydrogen explosion experiments are conducted to investigate the effect of barrier wall on explosion characteristics. The results show that the flame propagation velocity is highly related with the development of the flame instability, not only dependent on the laminar burning velocity. In front of the wall, due to the wave reflection, the overpressure peak Pmax and positive overpressure impulse IP increase while the positive overpressure duration time tdur decreases. In the rear of the wall, due to the wave diffraction, Pmax and IP decrease while tdur increases. The time reaching overpressure peak tpeak increases on both sides of the wall. Pmax is about twice the incident overpressure on the wall surface because of the pressure reflection. The enhancement or mitigation effect of the barrier wall only works in a certain range. The mitigation effect is highly related with the relative height between gas cloud and barrier wall.

High-Q transmission characteristics in terahertz guided-mode magnetic resonance system

Introduction: Guided mode resonance is generated by coupling wave diffractions with the waveguided mode. The guided mode resonances provide narrow-linewidth and resonance intensity for high quality factor (Q-factor) optical resonators.Methods: we demonstrate the high-Q guided mode resonances propagating on a low-loss, terahertz guided-mode magnetic resonance system, which are periodic square lattices of U-shaped split ring resonators (SRRs) on quartz substrates.Results: By choosing a judicious array period, two distinct frequency guided mode resonances and a magnetic dipole resonance with high Q-factor are observed. The interaction of the two resonances at similar frequencies produces a total transmission peak.Discussion: The dependences of the magnetic dipole resonance on the lattice period and structural parameters are investigated and discussed. The frequency difference between these two guided mode resonances widens with increasing Lattice period. The sharp spectral feature of each resonance results in the abrupt degradation of the spectral edge transmission. The proposed scheme is promising for efficient THz sensing, THz switching, and slow-light devices.

Hydroelasticity analysis of a vessel-shaped fish cage under incident, diffraction and radiation wave fields

Vessel-shaped fish cages are promising large aquaculture structures developed in recent years, with an overall length of nearly 400 m. In this paper, a coupled hydroelasticity model of a vessel-shaped fish cage is used to calculate the motion and structural response in the time domain. First, the floating body of the cage is discretized into a multimodule system to calculate the frequency-domain hydrodynamic loads. Then, the multimodule system is connected by equivalent elastic beams to consider the hydroelastic behavior in the time domain. The hydrodynamic loads of the multimodule system are transformed from the frequency-domain loads. Moreover, based on the velocity field transfer functions and the motion of the multimodule system, coupling wave fields considering incident, diffraction and radiation waves are built and used to calculate the loads on the net and steel frame. By iterating the motion response of the multimodule system and the hydrodynamic loads on the net and steel frame in the time domain, the balanced hydroelasticity response of the whole cage is finally obtained. The results show that the hydroelasticity effects have a significant influence on the vertical displacement and cross-sectional load effects of the vessel-shaped fish cage.

Plane-wave least-squares diffraction imaging using short-time singular spectrum analysis

Abstract Diffractions are seismic waves generated by small-scale heterogeneities in the subsurface. These are often superimposed by strong reflections so that they are not visible on the image, leading to misinterpretation and incorrect localization of the scatterers. Therefore, the separation of diffracted and reflected waves is a crucial step in identifying these small-scale diffractors. To realize the separation of diffraction and imaging, a least-squares reverse time migration method of plane waves (PLSRTM) optimized with short-time singular spectrum analysis (STSSA) was developed in this work. The proposed STSSA algorithm exploits the properties of singular spectral analysis (SSA) to separate linear signals. By establishing the Hanning window and the energy compensation function, it also compensates for the shortcomings of SSA in local dip processing and convergence of linear signals. As there is no clear boundary between reflected and diffracted waves, the energy loss during separation leads to a slow convergence rate of the diffraction wave imaging technique. We use STSSA as a constraint for PLSRTM, which greatly improves the imaging quality for diffraction waves. The tests with the Sigsbee2A model and noisy seismic data have shown that our method can effectively improve the resolution of diffraction wave imaging and that the constraint of STSSA increases the robustness to noisy data.

Application of Classified Elastic Waves for AE Source Localization Based on Self-Organizing Map

Acoustic emission (AE) source localization has been used to visualize progress failures generated in a wide variety of materials. In the conventional approaches, AE source localization algorithms assume that the AE signal is propagated as a straight line. However, it is supposed that progress failures form heterogeneity of elastic wave velocity distributions. Hence, with the conventional source localization, it is expected that the localization accuracy is reduced in heterogeneous materials since diffraction and refraction waves are generated. Thus, if the straight propagation waves are classified, conventional source localizations are performed in the heterogeneous materials. The self-organizing map (SOM) is one of the unsupervised learning methods, and the SOM has potential to classify straight propagation waves for the source localizations. However, the application of classified AE signals in source localization is not popular. If classified AE signals are applied to the time difference of arrival (TDOA) method, which is the popular localization method, it is expected that number of visualized sources are decreased because the algorithm does not consider the selection of the propagated wave. Although ray tracing has potential to localize a larger number of sources than the TDOA method, it is expected that the localized sources are less accurate in comparison with results of the TDOA method. In this study, classified waves were applied to two of the source localizations, and model tests based on pencil-lead breaks (PLBs) generating artificial AE sources were conducted to validate the performance of the source localizations with classified waves. The results of the validation confirmed that the maximum error in the TDOA method is larger in comparison with ray tracing conducted with 20 mm intervals of source candidates. Moreover, ray tracing localizes the same number of sources as the number of PLB tests. Therefore, ray tracing is expected to more practically localize AE sources than the TDOA method if classified waves are applied.

Wave diffraction and radiation from a semi-submersible floating foundation for wind turbines: A semi-analytical study

Many marine structures are composed of vertical axisymmetric floats. In this paper, a semi-analytical model based on the linear potential flow theory and an eigenfunction expansion method is developed to study wave diffraction and radiation by an array of cylindrical structures. Each structure can be formed by three coaxial cylinders of different dimensions. Based on the semi-analytical model, a constrained matrix equation of motion is presented and solved to evaluate the performance of multiple interconnected cylindrical structures. In order to verify the accuracy of the semi-analytical model, a typical OC4-DeepCwind floating offshore wind turbine (FOWT) is selected for validation. The validated model is, then, applied to study the effect of base column submergence depth, radius, and thickness on the motion response of a semi-submersible platform. Although the results of this study are of significance for the selection of the optimum semi-submersible FOWT for specific locations (with specific prevailing wave directions), the focus of this work was placed on the semi-analytical model itself, which is efficient in modeling the interaction of the wave field and can be used in future FOWT projects.

Numerical Study of Mode Conversion between Fundamental Lamb and Rayleigh Waves

This article reports the findings of a numerical study of mode conversion between fundamental Lamb and Rayleigh waves and vice versa in quarter and half spaces. Fundamental Lamb wave (A0 and S0) propagation in a plate attached to a quarter space and the subsequent mode conversion to a Rayleigh wave was studied using finite element analysis. B-Scans show that a beat-like phenomenon can be observed for the R → L conversion and a generation length can be observed for the L → R conversion. The quarter-space model was also used to study the Rayleigh to Lamb mode conversion. Two hypotheses were developed based on the observed mode conversion efficiencies: (a) the main driving factor of mode conversion between Lamb and Rayleigh waves is the grazing incidence of bulk modes and (b) due to scattering and diffraction, the modes will require a generation length to stabilize in amplitude. Both hypotheses were tested and confirmed using numerical models, including a half-space model to study the diffraction of bulk waves from an incident Lamb wave. The results are of significance for nondestructive evaluation of complex structures where such structural discontinuities exist and it becomes important to understand the fundamental mode conversion phenomenon.