Values Of Group Velocity Research Articles

Detecting Cracks on Plates Using Reflections of Fundamental Symmetric and Anti-symmetric Lamb Waves from Defect

Introduction: Ultrasonic Lamb waves can achieve long-range inspection on plate-like structures and can even detect unreachable cracks and defects. There are multiple modes under a certain generation frequency with most of them dispersive. Method: The dispersion curves of Lamb wave in a steel plate were given by numerical calculation. A 2-D finite element (FE) model of a steel plate was promoted. The fundamental symmetric and antisymmetric Lamb waves were generated by uploading symmetrical and anti-symmetrical displacements on one side of the plane. The wave reflects when it comes across the defect and the edge of the plate. Mode conversion will occur after the asymmetrical discontinuities of the structure. Result: The theoretical and numerical results of the values of group velocities of S0 and A0 in a steel plate show well agreement. The maximum amplitude of the converted A0 wave from the interaction of the incident S0 wave with the cracks was observed. The amplitude will first rise then fall under any depth of the defects. The relationship between the defect reflection and the length as well as the depth of defects were simulated based on the FE model. Phase reversion and phase delay were discovered by controlling the defect size changing in one dimension. The reflection coefficient increases linearly with the defect length while the depth is constant, as well as the defect depth while the length is constant. method: Using finite element (FE) method to simulate Lamb wave motion in a plate, as well as experiments on steel plates with artificial defects. Conclusion: Experiments on steel plates with artificial defects including thorough holes and tapered holes were carried out using ISONICA STAR. The results also show that when the defect size changes in one dimension such as the thorough holes, the reflection coefficient increases linearly correspondingly. However, the testing results of the taper holes were inconsistent with the regulation as the size increases in both the length and the depth. result: The values of group velocities were calculated by time-of-flight method to identify specific modes. The results were calibrated and showed well agreement with theoretical data. Mode conversion will appear when the wave comes across asymmetric discontinuities of the structure.

Dispersion-dependent superluminal propagation and photon drag in GaAs/AlGaAs quantum dot molecule

Controlling the basic properties of light such as polarization, angular momentum, and speed through various optical media leads to interesting phenomena in various fields such as quantum optics, photonics, plasmonics, and quantum information technology. Few of the phenomenon include, for instance, light storage, ultra-slow and ultra-fast propagation, optical transparency, and quantum entanglement via quantum coherence or superposition. The understanding of these phenomenon of milestone implications paved the way for the invention of novel classical and quantum technology. In this article, we present detailed theoretical and numerical demonstration on the dispersion-dependent phenomenon of slow and fast light, optically induced transparency, and photon drag. For this, we employ a three-level scheme of an ensemble of GaAs\\AlGaAs double quantum dot molecule in cascade configuration. In particular, by tuning the medium dispersion we predict interesting interplay between ultra-slow and ultra-fast light propagation with vanishing, infinite, ultra-positive and ultra-negative group velocities, and optical-assisted quantum transparency. The quantum coherence of the dressed states modifies considerably the optical properties of the system in view of dispersion, absorption, electron energy loss, transmission, and photon drag. An interesting interplay between ultra-slow to ultra-fast propagation is found by tuning various optical parameters. The calculated superluminal value of group velocity is m s−1. We also predict light propagation at vanishing and infinite group velocity. The calculated light drag is rad/m.

Analysis of the interaction between torsion wave and phononic crystal in pipes

This article presents an analysis of torsional wave propagation in phononic crystal-based metamaterials. Such structures hold great promise as tools for wave manipulation. The unit cell of the presented structure was meticulously examined to obtain dispersion curves, revealing the presence of bandgaps and negative group velocities. The calculations of effective material parameters, based on eigenfrequencies, are presented to substantiate the existence of the bandgap and the negative value of group velocity. Time-domain simulations were conducted at various frequencies to analyze the wave behavior. In the case of negative group velocity, the wave inside the metamaterial propagates in the same direction as the incident wave. To verify the existence of negative velocity, a 2D Fast Fourier Transform (FFT) was performed, and the results from the 2D FFT data analysis align perfectly with the eigenfrequency analysis.

Bolt Axial Stress Measurement Based on the Dual-Mode Electromagnetic Acoustic Transducer

To further simplify the bolt stress measurement process, the feasibility of using a dual-mode EMAT that excites both longitudinal and shear waves is investigated to achieve rapid and accurate measurement. The propagation characteristics of cylindrical guided waves in a rod are studied by a Semi-analytical Finite Element model. Through the combination of the theoretical formula and the finite element simulation, the non-uniform distribution of axial stress is directly used to calculate the time of flight of cylindrical guided waves without building any approximate model. The effect of stress distribution on guided wave modes with different energy concentration regions is analyzed. The propagation characteristics of guided waves and the theoretical values of group velocity of these two modes are studied by cylindrical guided wave dispersion curve and mode shape analysis. Then, by combining the static stress distribution inside the bolt with the theoretical formula, the influence of the different distributions of guided wave energy in the cross section of the two modes on the velocity ratio was analyzed. Finally, the feasibility of the theory is verified by the experiment. The result of the experiment in the aluminum bolt verified the effectiveness of the dual-mode EMAT on stress measurement and the novel ultrasonic testing models proposed in this paper.

Hydrostatic pressure effect on melting temperature and lattice thermal conductivity of bulk and nanowires of indium arsenide

The Morelli–Callaway model was used to calculate the lattice thermal conductivity (LTC) of indium arsenide in both zinc blende and wurtzite phases of bulk and nanowire (NW) forms under applied hydrostatic pressures. Calculations were performed for NWs with diameters of 50, 63, 66, 100, and 148 nm in the temperature range of (0–400) K. The melting temperature and hydrostatic pressure phase diagram of the bulk and NW forms were predicted using the Clapeyron equation. A new method was developed to examine various related parameters, such as bulk modulus and mass density. The influence of pressure on melting temperature, melting enthalpy, melting entropy, surface energy, and stress. Results indicate that the calculated values of group velocity increased with the increase in NW size. The melting temperature dropped sharply with the rise in pressure. The pressure and temperature dependencies of the LTC were obtained, and they decreased with applied hydrostatic pressure.

Experimental Method for Simultaneous Determination of the Lamb Wave A0 Modes Group and Phase Velocities.

Determining Lamb wave dispersion curves when measuring phase and group velocity values at a fixed frequency is now a common and relevant task. In most cases, in order to solve such a problem, it is necessary to know the exact properties of the material, particularly its thickness. In experimental methods, Lamb wave parameters are evaluated directly from the test materials. This paper proposes a new and simple experimental algorithm for A0 mode group and phase velocity determination based on signal filtering and zero-crossing estimating. The main idea is to capture the zero-crossing instances of the signals closest to the signal envelope peaks and use these time instances to determine the phase and group velocities. The reliability of the proposed method was evaluated using simulated and experimental signals propagating in an aluminum plate. Theoretical modeling has shown that the proposed method enables the calculation of the A0 mode group and phase velocities with a mean relative error of less than 0.7%. An accuracy of 0.8% was observed during the experimental measurements.

Speed of sound in the IEC tissue-mimicking material and its maintenance solution as a function of temperature

Tissue-Mimicking Material (TMM) is defined on IEC International Standards and applied in assessing ultrasonic diagnostic and therapeutic equipment's basic safety and essential performance. One of the TMM that fits IEC standards specification has its recipe described at IEC 60601-2-37, and it is fabricated using glycerol (11.21 %), deionized water (82.95%), benzalkonium chloride (0.47 %), silicon carbide (0.53 %), aluminum oxide 0.3 μm (0.88%), aluminum oxide 3.0 μm (0.94 %), and agar (3.08 %). Glycerol is the component responsible for adjusting the TMM's speed of sound. Moreover, it is recommended to store TMM in a closed container immersed in a mixture of water (88.1 %)/glycerol (11.9 %) to prevent it from drying out and avoiding air contact. The literature points out TMM measurements underwater can alter the speed of sound property as TMM tends to lose glycerol. Herein, the authors proposed to assess the viability of measuring the TMM speed of sound in the water/glycerol maintenance solution. First, the authors characterized the maintenance solution's speed of sound for a temperature range of 20 °C to 45 °C. Then, the group velocity of a set of TMM was measured underwater and in the maintenance solution for the same temperature range. The respective group velocity expanded uncertainty was calculated. The results indicate it is feasible to measure TMM in the maintenance solution, achieving group velocity values with no statistical difference from the ones measured underwater in the temperature range of 20 °C to 40 °C.

Angular dependency of the polarization rotation in a coherent atomic medium

The dependency of polarization rotation with electromagnetically induced transparency (PREIT) in 87Rb vapor on the angular mismatch between the probe and the pump beams has been studied experimentally and theoretically for a V-type configuration. We have observed a nonlinear variation of the PREIT angle with the angular mismatch. The angular mismatch enhanced the residual Doppler broadening effect, as well as diminishes the two photon coherence contribution, in the medium. We have also systematically studied the effect of optical depth (OD) and the pump beam spot size variations to understand how the number of interacting atoms affect the two photon coherence contribution of the polarization rotation. The value of group velocity of the probe beam has been calculated from the experimentally observed spectrum. We have modeled a three-level V-type system to explain the experimentally observed phenomena by considering the effect of OD and spot size with angular mismatch in the medium. We have considered two-dimensional Maxwell–Boltzmann distribution to calculate the susceptibilities for all atoms of all velocity components. The characteristic variation of the angle of PREIT with the angular mismatch matches well with the experimentally observed result.

Evaluation of materials used for vascular anastomoses using shear wave elastography

Cardiovascular disease is the leading cause of mortality in the United States. Peripheral artery disease (PAD) affects a substantial portion of the elderly population (>70 years). PAD can be symptomatic or asymptomatic, and treatment is directed towards improving functional vessel patency and limb salvage. Revascularization surgical treatments are needed in some cases, and it is not always possible to use autologous veins for vascular grafting. In this case, synthetic materials may be used. Synthetic materials such as expanded polytetrafluoroethylene (ePTFE) and poly (ethylene terephthalate) (Dacron) have been used. These materials are much stiffer compared to normal arterial vessels so developing alternative materials is an active area of research. One such potential material is a p olyvinyl alcohol cryogel reinforced arterial model (PRAM). In this study we used shear wave elastography (SWE) techniques to compare the mechanical behavior of excised aortas and anastomoses formed between the aortas and the Dacron, ePTFE, and PRAM materials. Different initial longitudinal strains (0%–40%) and transmural pressures (20–180 mmHg) were used to test a wide range of parameters that are within physiological ranges. We found that the PRAM materials had similar mechanical behavior in terms of group velocity measured in the vessel wall with respect to the excised aortas compared to the Dacron and ePTFE that had higher values of group velocity. The ratios of the group velocity for the PRAM material with respect to the aortas ranged from 0.83 to 1.13 where the Dacron and ePTFE had ranges of 1.12–2.22 and 1.91–3.10 for the same stretch and pressure ranges. The PRAM materials provide an alternative vascular material with better mechanical matching with excised aortas.

Ice detection by ultrasonic guided waves

This paper describes a method for inflight ice detection using ultrasonic guided waves that is little studied until now. Unlike existing systems, the proposed method can immediately detect icing at the place of formation. The detection area can also include whole structural segments and the proposed sensor can be integrated into a wing structure without any effect on the flow around the aerofoil or increase in drag. The proposed detection method is based on excitation with a burst signal and a sine sweep. The system was tested on a plate made from carbon fibre-reinforced plastic (CFRP) and two leading edges of a NACA 0012 profile. Ice was applied to the CFRP-plate in a cooling chamber using regular water and built up onto the leading edges in an ice wind tunnel using super cooled water droplets. Piezo ceramic patches and discs were affixed to the testing model structures for use as transceivers and receivers, respectively, to initiate the guided waves and convert them back into electric signals. The influence of variation in ice layer thickness and temperature on the sensor output signal was then investigated using the average and maximum absolute sensor output and runtime to calculate the group velocity, which was then compared to calculated group velocity values. Based on the test results, the proposed method was determined to be sensitive to the build-up of ice and, thus, effective in principle.

Elastic, dipole–dipole interaction and viscosity impact on vibrational properties of anisotropic hexagonal microtubule lattice

The paper investigates microtubules lattice properties taking into consideration elastic, dipole–dipole interaction of tubulins and viscosity. A microtubule is modeled as a system of bound tubulins, forming a skewed hexagonal two-dimensional lattice. Wave frequencies and group velocities have been calculated. Calculations have been performed for various directions of wave front propagation: helix, along the protofilament, and anti-helix. Three different wave polarization directions have been considered. It has been shown that the direction of the wave polarization influences the frequency and wave group velocity values in the lattice considerably. The impact of dipole–dipole interaction greatly depends on the direction of the wave polarization; thus, it is only moderate for the longitudinally (LA) polarized waves while it is sufficient for the transversely (TA), and out-of-plane (ZA) polarized waves. Moreover dipole–dipole interaction may result in the waves which are able to cause the rupture of microtubules. With viscosity considered, lattice oscillations become harmonically damped only over a certain wavelength range when longitudinal polarization occurs. Out of this range as well as for the other polarization directions, lattice deviations from equilibrium are dampened exponentially. Taking viscosity into consideration also results in a noticeable decrease in frequency and increase in the group wave velocity when the waves are longitudinally polarized. Reverse wave domains which may be associated with a possible phenomenon of negative refraction have been determined for hexagonal microtubule lattice.

Deterministic Phonon Transport Predictions of Thermal Conductivity in Uranium Dioxide With Xenon Impurities

We present a method for solving the Boltzmann transport equation (BTE) for phonons by modifying the neutron transport code Rattlesnake which provides a numerically efficient method for solving the BTE in its self-adjoint angular flux (SAAF) form. Using this approach, we have computed the reduction in thermal conductivity of uranium dioxide (UO2) due to the presence of a nanoscale xenon bubble across a range of temperatures. For these simulations, the values of group velocity and phonon mean free path in the UO2 were determined from a combination of experimental heat conduction data and first principles calculations. The same properties for the Xe under the high pressure conditions in the nanoscale bubble were computed using classical molecular dynamics (MD). We compare our approach to the other modern phonon transport calculations, and discuss the benefits of this multiscale approach for thermal conductivity in nuclear fuels under irradiation.

Seismoacoustic effects of the Hovsgol earthquake (Mw = 4.9) of December 5, 2014

The results of study of the Hovsgol earthquake with M w = 4.9, which occurred on December 5, 2014, in the northern part of Hovsgol Lake in Mongolia, are presented. An infrasonic signal of ~140 s long was recorded by the Tory infrasonic station for the first time for the Baikal Rift System. On the basis of the source parameters and focal mechanism of this earthquake determined, displacements in the epicentral zone of the Hovsgol earthquake are modeled. It is shown that they could not have produced an infrasonic sound. The use of acceptable values of group velocities of infrasonic waves (0.28–0.35 km/s) demonstrates that the signal source was located approximately midway between the Tory station and the epicenter of the Hovsgol earthquake, indicating it was a secondary source. Based on the data on the azimuth and arrival time of the acoustic wave at the Tory station, the location of this secondary source is determined to have been on the northern slopes of the Khamar Daban Range. The infrasonic signal formed most likely by interaction between seismic waves from the earthquake and the mountain relief.

Enhanced Phase Sensitivity in Plasmonic Refractive Index Sensor Based on Slow Light

Fano spectrum exhibits an intriguing feature of narrow asymmetrical resonance profile, which can be used in designing an ultra sensitive refractive index sensor. Here, in this letter, an ultra compact surface plasmon sensor is investigated. The sensor is based on Metal-Dielectric-Metal (MDM) waveguide geometry and connected to a pair of stub resonators. The distance between resonators is adjusted in such a manner that their increased coupling leads to an asymmetrical profile resonance, which is known as Fano resonance. The Fano resonance is very sensitive to any change in refractive index of the material. To measure the sensitivity of the device, stubs are filled with liquid/gaseous material under test. The structure is numerically simulated by the Finite difference time-domain method and the value of sensitivity is obtained as high as S = 510 nm/refractive index unit with a narrow line width of 19 nm and large quality factor Q ≈ 80. Fano resonance is generally accompanied with sharp dispersion, which leads to the generation of slow light. To account an effect of slow light in the waveguide phase, sensitivity is also analyzed as a function of a group delay. The large value of phase sensitivity is reported, which is equal to S O = 41.9 rad for per unit change in refractive index unit. The phase sensitivity rises linearly with the increasing value of group delay or decreasing value of group velocity. Thus, the device is well suited for designing on-chip optical sensors, optical buffers, switches, modulators, and so on.

Detection of fundamental shear horizontal mode in plates using fibre Bragg gratings

Shear horizontal guided waves are attractive for structural health monitoring applications in view of the non-dispersive behaviour of the fundamental mode, possibly higher frequencies of operation and a less complex multi-modal structure. One of the key issues with the deployment of shear horizontal guided waves of modes for structural health monitoring applications is the general non-availability of techniques to sense this family of modes effectively. This article demonstrates the detection of fundamental shear horizontal waves in an aluminium plate by placing a fibre Bragg grating along a direction perpendicular to that of propagating guided elastic waves. In order to uniquely identify the three fundamental plate-guided modes, we map the experimentally measured group velocities as detected by the fibre Bragg grating to theoretically obtained group velocity dispersion curves. We find that the experimentally measured group velocity values using time-of-flight measurements from a perpendicularly placed fibre Bragg grating are in agreement with the theoretical curve for the [Formula: see text] mode. Possible extension of these results to feature-guided modes is also discussed.

Acoustic scattering by a two-layer cylindrical tube immersed in a fluid medium: Existence of a pseudo wave

The present paper studies the acoustic signal backscattered by an air-filled copper–solid polymer two-layer cylindrical tube immersed in water. The work is done from the calculation of the backscattered pressure, an inverse Fourier Transform, which allows us to obtain an impulse signal. Smoothed pseudo Wigner–Ville and Concentrated spectrogram representations have been chosen to analyze the scattering phenomenon. For reduced frequencies ranging from 0.1 to 200, the resonance trajectories and time–frequency images have shown the presence of the guided waves. The bifurcation of the A0 wave into the A0− and the A0+ waves has also been observed. The authors provide the phase and the group velocities of guided waves and investigate the differences between curves. The findings are then compared with those obtained for the copper and the solid polymer one-layer cylindrical tubes. Group velocity values have also been extracted from smoothed pseudo Wigner–Ville and Concentrated spectrogram time–frequency images. A good agreement with the theory has, therefore, been observed. The study of acoustic backscattering by a copper–solid polymer two-layer tube has revealed the interaction and the coupling of guided waves, specially the presence of a pseudo A1 wave; which is a very interesting, remarkable phenomenon.

Wideband Slow Surface Plasmons in Double Resonator Plasmonic Grating Waveguide

We propose a theoretical and numerical investigation of surface plasmon polaritons supported by a metal–insulator–metal (MIM) waveguide, periodically coupled to a series of adjacent resonators. The slow light characteristics of the MIM-coupled resonator is analyzed by controlling separation or displacement between resonators. By tailoring this displacement factor, group velocity of slow wave structures is reduced to a great extent with a large value of the normalized delay bandwidth product (NDBP = 0.667) that measures the capacity of the device. A low value of group velocity ( ${v}_{g} = c/7$ ) is reported over an ultralarge bandwidth ( $\Delta f = 21$ THz). An unusual U-shaped group index—frequency curves with a constant group index ${n}_{g}$ is obtained, which leads to a nearly-zero value of group velocity dispersion followed by positive and negative peaks. The structure is also analyzed for changing the depth of the resonators. The structure can be further modified to suit different applications in optical switching device, dispersion compensating units, and nonlinear optics devices.

Ray tracing of multiple transmitted/reflected/converted waves in 2-D/3-D layered anisotropic TTI media and application to crosswell traveltime tomography

To overcome the deficiency of some current grid-/cell-based ray tracing algorithms, which are only able to handle first arrivals or primary reflections (or conversions) in anisotropic media, we have extended the functionality of the multistage irregular shortest-path method to 2-D/3-D tilted transversely isotropic (TTI) media. The new approach is able to track multiple transmitted/reflected/converted arrivals composed of any kind of combinations of transmissions, reflections and mode conversions. The basic principle is that the seven parameters (five elastic parameters plus two polar angles defining the tilt of the symmetry axis) of the TTI media are sampled at primary nodes, and the group velocity values at secondary nodes are obtained by tri-linear interpolation of the primary nodes across each cell, from which the group velocities of the three wave modes (qP, qSV and qSH) are calculated. Finally, we conduct grid-/cell-based wave front expansion to trace multiple transmitted/reflected/converted arrivals from one region to the next. The results of calculations in uniform anisotropic media indicate that the numerical results agree with the analytical solutions except in directions of SV-wave triplications, at which only the lowest velocity value is selected at the singularity points by the multistage irregular shortest-path anisotropic ray tracing method. This verifies the accuracy of the methodology. Several simulation results show that the new method is able to efficiently and accurately approximate situations involving continuous velocity variations and undulating discontinuities, and that it is suitable for any combination of multiple transmitted/reflected/converted arrival tracking in TTI media of arbitrary strength and tilt. Crosshole synthetic traveltime tomographic tests have been performed, which highlight the importance of using such code when the medium is distinctly anisotropic.

Slow/fast light using a very short Er^3+/Yb^3+ co-doped fiber

A slow/fast light device with a sealed size of 130 mm×30 mm×3 mm has been demonstrated. Ultraslow propagation and superluminal propagation with group velocity values from 8.4 to -14.7 m/s are observed in a 3.86 cm long Er3+/Yb3+ co-doped single-mode phosphate glass fiber. The dependence of pump power, modulation frequency, and wavelength on the slow/fast light effect in this fiber is investigated in detail. These results suggest that this compact slow/fast device is more suitable for all-fiber applications than those made by traditional methods.

Edge magnetoplasmons in a wide armchair graphene ribbon with a weak superlattice potential: Finite-frequency gaps and zero group velocity

We show strong effects of a weak and smooth, on the magnetic length, superlattice potential upon edge magnetoplasmons (EMPs) at the armchair edge, with a smooth steplike electrostatic lateral confining potential, of a wide graphene channel in the $\nu=2$ quantum Hall effect regime. The superlattice potential leads to essential enlargement of a number of EMPs, descend from two fundamental EMPs in the absence of superlattice. For the wave vector $k_{x}$ within the first Brillouin zone, the EMPs show as the regions of acoustical or quasi-acoustical dispersion, with a finite value of group velocity, so the regions with frequency gaps, where a group velocity is nullified at some $k_{x}$. We obtain that for $k_{x} \to 0$ only for two EMPs the frequency tends to zero as for other EMPs it obtains finite values. Strong dependence of dispersion relations of the EMPs from the period of the superlattice $a_{0}$ and the distance $d$ from a metallic gate is shown; in particular, for typical size of a gap, for characteristic value of the frequency and $k_{x}$ at which the group velocity is reduced to zero. At the frequency that corresponds to zero group velocity of pertinent fundamental EMP branch the response of the system should present a strong resonance.