Propagation Axis Research Articles

Quantitative Assessment of Boiling Histotripsy Progression Based on Color Doppler Measurements.

Boiling histotripsy (BH) is a mechanical tissue liquefaction method that uses sequences of millisecond-long high intensity focused ultrasound (HIFU) pulses with shock fronts. The BH treatment generates bubbles that move within the sonicated volume due to acoustic radiation force. Since the velocity of the bubbles and tissue debris is expected to depend on the lesion size and liquefaction completeness, it could provide a quantitative metric of the treatment progression. In this study, the motion of bubble remnants and tissue debris immediately following BH pulses was investigated using high-pulse repetition frequency (PRF) plane-wave color Doppler ultrasound in ex vivo myocardium tissue. A 256-element 1.5 MHz spiral HIFU array with a coaxially integrated ultrasound imaging probe (ATL P4-2) produced 10 ms BH pulses to form volumetric lesions with electronic beam steering. Prior to performing volumetric BH treatments, the motion of intact myocardium tissue and anticoagulated bovine blood following isolated BH pulses was assessed as two limiting cases. In the liquid blood the velocity of BH-induced streaming at the focus reached over 200 cm/s, whereas the intact tissue was observed to move toward the HIFU array consistent with elastic rebound of tissue. Over the course of volumetric BH treatments tissue motion at the focus locations was dependent on the axial size of the forming lesion relative to the corresponding size of the HIFU focal area. For axially small lesions, the maximum velocity after the BH pulse was directed toward the HIFU transducer and monotonically increased over time from about 20-100 cm/s as liquefaction progressed, then saturated when tissue was fully liquefied. For larger lesions obtained by merging multiple smaller lesions in the axial direction, the high-speed streaming away from the HIFU transducer was observed at the point of full liquefaction. Based on these observations, the maximum directional velocity and its location along the HIFU propagation axis were proposed and evaluated as candidate metrics of BH treatment completeness.

Reverse circular Bragg phenomenon

The problem of axial propagation of circularly polarized light in a circularly birefringent structurally chiral medium is exactly solved via full electromagnetic analysis. Underlying symmetries of the system's characteristic matrix reveal interesting insights, which are confirmed by coupled wave theory. For extreme values of chirality, a reverse circular Bragg resonance arises in the negative refraction regime where handedness reversal of counterpart modes occurs. A condition is identified under which circular birefringence precisely offsets structural chirality, rendering the medium simply linearly birefringent. Manufacturing such a medium is feasible via current metamedia and inorganic materials technology and has applications in optics, optoelectronics, and sensing.

Structure transitions in arrays of point-vortices upon free space propagation

We study the free space propagation of arrays of N × N point vortices with equal unitary topological charge embedded in a Gaussian beam, initially distributed into either a square lattice or a disordered array with the same mean vortex density and separation. The far-field patterns and the near-field evolution are analyzed as a function of different control parameters, such as the size of the carrier beam, the number of vortices, the mean separation distance among them P, and a geometrical parameter S p , defined as the ratio of the size of the array to the size of the host beam. The value of S p turns out to be determinant for the final spatial distribution of the optical vortices in the far-field pattern and also plays a very relevant role in the near-field evolution. While the initial array structure is basically preserved, rotated and scaled up when , the vortices redistribute forming a circumference around the propagation axis for , depleting most of the central intensity, even if the vortex density is far lower than the previously predicted limit for depletion. In the near field, we find an expression for the rotation angle of the whole structure when preserved, as a function of the propagation distance, which depends on P, but it is around eight-fold larger than the Gouy phase shift of the host. Also, we observe the formation of a high intensity ring pattern, whose radius is independent of the size of the carrier and the initial distribution of vortices, but it is only determined by the number of vortices. We verified our numerical results with experiments, finding a very good agreement between them.

Anti-blast analysis and design of a sacrificial cladding with graded foam-filled tubes

Foam-filled tubes are excellent energy absorption structures for impact resistance, and graded foam may have superiority for the design of anti-blast sacrificial cladding. The anti-blast response of density-graded foam-filled tubes as a core layer is investigated numerically and theoretically. Mesoscopic finite element models for graded foam-filled tubes are constructed, and their constant-velocity compression and anti-blast performance are simulated. One-dimensional analytical shock models based on an empirical force–displacement relationship are developed to analyze the axial collapse wave propagation in graded foam-filled circular tubes. Single-/double-wave deformation patterns are observed for graded foam-filled tubes under blast loading, especially the negatively graded foam-filled tubes with gradient parameters close to zero may also show a single-wave deformation pattern. The results demonstrate that the negatively graded foam-filled tube with a small density gradient is a good choice to design anti-blast sacrificial cladding with a small critical length and a low transmitted force. A contour map of the critical length required to absorb the impulse of blast loading for different gradient parameters and diameter-thickness ratios of the outer tube is established. This study proposes a new design guideline for superior blast resistance structures in engineering applications.

Vertically Printable Evanescent Mode Filters

This work presents a new topology of evanescent mode filter (EMF) that allows for its efficient additive manufacturing (AM). Chamfered ridges which avoid the appearance of undesired supports when printed vertically (i.e., along the propagation axis of the filter) are considered. This strategy ensures accurate tolerances, reduced cost and it also enables monolithic manufacturing of the EMF as part of a feed chain. A circuit model is developed to explain the behavior of the new ridges as well as to speed up the overall filter design. The proposed concept and methodology are validated experimentally through the manufacturing and measurement of a prototype.

Elastic-plastic analysis with pre-integrated beam finite element based on state diagrams: Elastic-perfectly plastic flow

The behaviour of a structure beyond its elastic limit involves a non-linear analysis, commonly conducted in engineering design with the benefit of Finite Element Analysis. Many structures are an assembly of one-dimensional components (beams) to form a frame. There are two main FE approaches to studying axial plastic propagation and the distribution within the cross-section of a beam: solid elements or beam elements with cross-section sampling (fiber section). The first requires a considerable number of d.o.f.’s to obtain reliable results; therefore, it is not generally used for extensive size modelling; the second, although requiring less computational efforts, necessitates an extensive sampling of cross-sections to evaluate the plastic distribution at every fiber. Furthermore, a high number of elements are needed to assess the plastic propagation. This paper provides a new approach that allows to considerably reduce the numerical heaviness by providing two ideas: i) elastic-plastic State Diagrams that allow the pre-integration of the cross-section response (that overcame the sampling within the section) ii) section-slices subdivision, that enable to build-up the tangent stiffness matrix of an entire partially plasticized beam (whatever is the plastic distribution and the section shape), with the minimum number of d.o.f.’s. The pre-integrated approach here developed assume planar bending and an elastic-perfectly plastic flow. The proposed element is validated by three test cases, comparing with the solutions obtained through FEA using solid elements and fiber-section beams. A significant enhancement in the overall efforts emerges, obtaining very similar results among the three approaches but with a considerable reduction of the computational time when using the proposed element.

Nonreciprocal vortex isolator via topology-selective stimulated Brillouin scattering

Optical nonreciprocity, which breaks the symmetry between forward and backward propagating optical waves, has become vital in photonic systems and enables many key applications. So far, all the existing nonreciprocal systems are implemented for linearly or randomly polarized fundamental modes. Optical vortex modes, with wavefronts that spiral around the central axis of propagation, have been extensively studied over the past decades and offer an additional degree of freedom useful in many applications. Here, we report a light-driven nonreciprocal isolation system for optical vortex modes based on topology-selective stimulated Brillouin scattering (SBS) in chiral photonic crystal fiber. The device can be reconfigured as an amplifier or an isolator by adjusting the frequency of the control signal. The experimental results show vortex isolation of 22 decibels (dB), which is at the state of the art in fundamental mode isolators using SBS. This device may find applications in optical communications, fiber lasers, quantum information processing, and optical tweezers.

Strain rate effect on compressive damages of thermo-oxidative aged 3D braided composites

Revealing mechanical behaviors of three-dimensional five-directional (3D5d) braided composites at different strain rates is a key point to its durability design and structural optimization. Here we report compressive responses, surface deformations and failure behaviors of aged 3D5d braided composites under different strain rates with digital image correlation (DIC) technique, X-ray micro-computed (micro-CT) tomography and finite element analysis (FEA). The compressive rigidity, peak stress, maximum strain, energy absorption, surface strain and damages exhibit strain rate strengthening effects and ageing weakening behaviors. The strain concentration zone transfers from X-shaped to transverse V-shaped at the square cross-section with the increasing of strain rate. Moreover, ageing effect does not change the internal shear mode, while it changes the near-surface crack distribution along axial direction and crack propagation path under impact loading. The failure modes show strain rate correlation. The FEA results reveal the failure mechanisms of the aged 3D5d braided composites under different strain rates.

Basin-scale propagation modeling of MLS signals between Kauai and Monterey

Long distance underwater acoustic propagation is of interest for a variety of applications including underwater navigation, yet modeling such propagation is challenging due to the large degree of environmental uncertainty. In this presentation, the propagation of 75 Hz center frequency maximum length sequence (MLS) signals emitted from a submerged source near Kauai and received at the Monterey Accelerated Research System (MARS) observatory are modeled with the Bellhop ray tracing and the Monterey-Newport Parabolic Equation (MNPE) models. The range-dependent sound speed environment is based on historical profiles from the World Ocean Atlas database and reanalysis data from the hybrid coordinate ocean model (HYCOM). Bathymetry profiles along the axis of propagation are interpolated from the General Bathymetric Chart of the Oceans (GEBCO) 2021 database and the Naval Oceanographic Office Digital Bathymetric Data Base Variable Resolution (DBDB-V) data product. Simulated channel impulse responses are compared to observations for transmission loss and signal coherence.

Langevin’s contributions to pulse echo piezoelectric transducers

Paul Langevin made fundamental discoveries which laid the foundations of modern pulse echo ultrasound over a hundred years ago. He was the first to employ piezoelectric crystals, X-cut quartz, with the propagation and electric axes aligned. In his design, the crystal thickness was a half-wavelength longitudinal wave resonator for both a transmitter and receiver which is the most popular configuration in use today. He invented the Langevin transducer which added two quarter wave layers on either side of the piezoelectric crystal to increase overall efficiency. A mosaic pattern embodied smaller pieces of scarce quartz. His original designs will be demonstrated through simulator models. By selecting ultrasound frequency of 40 KHz, he was able to make more directive beams and extend range for pulse echo detection. By adding triode amplifiers, the sensitivity of reception was increased greatly, and transmission powers of a kilowatt were achieved. With special circuits for making short pulses, long range, high resolution pulse echo ranging became a practical reality. Langevin’s innovations led to sonar and eventually to ultrasound imaging.

Femtosecond filamentation of optical vortices for the generation of optical air waveguides.

We study the filamentation in air of multi-millijoule optical vortices and compare them with the classical filamentation regime. The femtosecond vortex beam generates multiple plasma filaments organized in a cylindrical geometry. This plasma configuration evolves into a meter-scale tubular neutral gas column that can be used as a waveguide for nanosecond laser pulses at 532 nm. It appears that optical vortices produce a more uniform heating along the propagation axis, when compared with Gaussian or super-Gaussian beams, and that the resulting low-density channel is poorly sensitive to the laser input power thanks to the combination of filamentation intensity clamping and phase vorticity.

Molecular sensors producing circularly polarized luminescence responses

Circularly polarized luminescence (CPL) refers to spontaneous emission with the oscillation axis of the emission's electric dipole rotating either right handedly or left handedly along the propagation axis. CPL provides a unique principle in luminescence visualization of target analytes: both the direction and magnitude of light polarization are sensitive to an asymmetric environment around a luminophore, which enables chirality sensing. In addition, CPL spectra are usually less complex than electric circular dichroism spectra because they emerge only from the lowest excited state. To exploit these benefits, researchers have vigorously developed CPL probes. This minireview provides the basic principles for creating molecular CPL activity, including asymmetric exciton coupling and helical intramolecular charge transfer. Emphasis is placed on molecular design. Analyte interactions can perturb the magnitude, reverse the polarization direction, or shift the peak wavelength of CPL, thereby enabling tractable strategies for creating CPL sensors. This minireview outlines selected examples of CPL sensors for physical stimuli, including changes in external magnetic fields, solvent polarity, and temperature. Probes capable of detecting chemical species, such as protons, metal ions, anions, amino acids, nucleosides and DNA, reactive oxygen species, and humidity, are also highlighted. These examples demonstrate the unique sensing utility of CPL sensors. We hope that this minireview will stimulate future research interest toward developing advanced CPL sensors with applications in bioimaging.

Fundamental Studies on Electron Dynamics in Exact Paraxial Beams with Angular Momentum

Classical electromagnetic radiation with orbital angular momentum (OAM), described by nonvanishing vector and scalar potentials (namely, Lorentz gauge) and under Lorentz condition, is considered. They are employed to describe paraxial laser beams, thereby including non-vanishing longitudinal components of electric and magnetic fields. The relevance of the latter on electron dynamics is investigated in the reported numerical experiments. The lowest corrections to the paraxial approximation appear to have a negligeable influence in the regimes treated here. Incoherent Thomson scattering (TS) from a sample of free electrons moving subject to the paraxial fields is studied and investigated as a beam diagnosis tool. Numerical computations elucidate the nature and conditions for the so called trapped solutions (electron motions bounded in the transverse plane of the laser and drifting along the propagation direction) in long quasi-steady laser beams. The influence of laser parameters, in particular, the laser beam size and the non-vanishing longitudinal field components, essential for the paraxial approximation to hold, are studied. When the initial conditions of the electrons are sufficiently close to the origin, a simplified model Hamiltonian to the full relativistic one is introduced. It yields results comparing quite well quantitatively with the observed amplitudes, phase relationships and frequencies of oscillation of trapped solutions (at least for wide laser beam sizes). Genuine pulsed paraxial fields with OAM and their features, modeling true ultra-short pulses are also studied for two cases, one of wide laser beam spot (100 μm) and other with narrow beam size of 6.4 μm. To this regard, the asymptotic distribution of the kinetic energy of the electrons as a function of their initial position over the transverse section is analyzed. The relative importance of the transverse structure effects and the role of longitudinal fields is addressed. By including the full paraxial fields, the asymptotic distribution of kinetic energy of an electron population distributed across the laser beam section, has a nontrivial and unexpected rotational symmetry along the optical propagation axis.

On growth, buckling, and rupture of aneurysms: Cylindrical tube analogy

Aneurysms are localized bulges of arteries; and they can rupture with fatal consequences. Complex mechanobiological factors preclude in vivo testing to assess the rupture risk of an aneurysm, and size based criteria are often used in clinical practice to guide surgical interventions. It is often found that tortuous and buckled aneurysms can exceed the size recommended for surgical intervention, and yet do not rupture. This study addresses why buckled aneurysms exhibit this intriguing behavior by combining in vitro inflation experiments on hyperelastic cylindrical tubes with finite element calculations. Using a biologically relevant material model for an arterial wall, we show that buckled aneurysms can grow in size without rupture under favorable arterial pre-tensions. Stretch reversal phenomenon exhibited by arteries governs whether buckling or bulging occurs first. Exponential stiffening favors the axial propagation of an aneurysm instead of radial growth or size. The choice of failure criteria based on Ogden’s strain energy function, Gent’s first stretch invariant, and Cauchy stress are discussed. Failure maps incorporating post-bifurcation (bulging and buckling) response are constructed to delineate the regimes of growth, buckling and rupture of an aneurysm.

Jet Characteristics of a Narrow Throat Pre-Chamber and Influence on the Main-Chamber Combustion

<div class="section abstract"><div class="htmlview paragraph">Lean combustion is one of the most applied methods to increase engine efficiency and maintain a good trade-off with engine emissions. The pre-chamber combustion (PCC) is one of the most promising combustion concepts to extend the lean operating limits of the engine. The Narrow throat pre-chamber has shown better lean limit extension compared to other ignition sources. The pre-chamber jets and the main-chamber combustion were studied in a Heavy-Duty optical engine using methane fuel. The tested conditions covered global excess air ratios (λ), between 1.9 to 2.3. The combustion process was recorded using three collection systems: (a) Natural Flame Luminosity (NFL) with a temporal resolution of 0.1 CAD; (b) OH* Chemiluminescence, and (c) CH* Chemiluminescence with a temporal resolution of 0.2 CAD for both. The propagating velocity of the reacting jets was studied using Combustion Image Velocimetry (CIV) based on bottom view images of the main chamber. After the pre-chamber jets ignite the main-chamber charge, a two-stage heat release rate (HRR) was observed for all the tested conditions. None of the three combustion visualizations exhibited a linear correlation with the HRR. The CIV reported high-speed jets penetrating around 400 m/s into the main chamber. The richest case of λ 1.9 presented a higher axial and radial propagation speed than the other cases. The individual vector fields have significant differences with the ensemble average fields, behavior characteristic of a high turbulent process. Finally, the ultra-lean case showed higher vorticity during the pre-chamber discharge, a synonym of high air-fuel entrainment that generates heavy waviness on the flame front. This heavy waviness generates local/partial or total extinction, resulting in abnormal combustion cycles.</div></div>

An experimental study of coherent swirling vortex rings in a turbulent pulsed jet

This experimental study examines the coherent swirling vortex rings in a turbulent pulsed jet at a Reynolds number Re=1.0×104 using planar particle image velocimetry. The coherent vortex rings are produced by a stepper-motor-driven piston successively injecting fluid into a background jet flow. The swirl is generated by axial swirlers which are made by three-dimensional printing technology and mounted behind the nozzle. The swirl number S generated in the experiments ranges from 0 to 1.10, which covers the critical swirl number of a swirling jet. The stroke-to-nozzle diameter ratio L/d=1.56 produces compact swirling vortex rings in a single pulsed jet without a background jet flow. Two pulsing frequencies (St=0.16 and 0.32) that generate coherent vortex rings are investigated, with the free jet also measured at the same Re for comparison. The combination of vortex rings and swirling flows is found to enhance the turbulent fluctuation intensity; this effect is positively related to the compactness of the vortex rings (St) and central recirculation zone (S). At the early stage, the generated swirling vortex ring develops until it reaches its limit, which decreases as S increases but increases with St. The axial distance at which this limit is achieved decreases with both increasing S and St. Then, the ring decays and is eventually dissipated at a decay rate that grows larger in proportion to S and St. Moreover, the swirling vortex rings evolve at their corresponding constant axial and radial velocities in the near field. The axial velocity decreases as S increases and increases with St, while the radial velocity increases with S and is independent of St. In addition, the axial distance between two adjacent swirling vortex rings decreases with increased S and St. Furthermore, the temporal variation of the axial distance also decreases at higher S and St, indicating that the axial propagation velocity decreases downstream. Finally, detailed structures of the swirling vortex ring cores and the corresponding structures induced by the rolling-up process are presented.

Tunable Compact Metamaterial-Based Double-Negative/Near-Zero Index Resonator for 6G Terahertz Wireless Applications.

This paper introduces the tunability performance, concept, and analysis of a unique and miniaturized metamaterial (MTM) unit cell covering the upcoming 6G applications. The proposed metamaterial consists of two metallic star-shaped split-ring resonators (SRR). It has a line segment placed in the middle of the structure, which can feature tunable characteristics. The proposed design provides dual resonances of transmission coefficient S21 at 0.248 and 0.383 THz with a significant operating frequency span of 0.207–0.277 and 0.382–0.390 THz, respectively. Moreover, wide-range achievement, negative permittivity, double-negative (DNG) refractive index, and near-zero permeability characteristics have been exhibited in two (z and y) principal wave propagation axes. The resonance frequencies are selective and modified by adjusting the central slotted-strip line length. Furthermore, the metamaterial is constituted on a polyimide substrate while the overall dimensions are 160 × 160 μm2. A numerical simulation of the proposed design is executed in CST microwave studio and has been compared with advanced design software (ADS) to generate the proposed MTM’s equivalent circuit, which exhibits a similar transmission coefficient (S21).

Nanoscale active tuning of the second harmonic generation efficiency in semiconductors from super-low to gigantic values

Second harmonic generation efficiency (SHGE) strongly depends on the length of the interaction material along the beam propagation axis. Since a nanoscale interaction length is considered too short even in the optical wavelength scale, the attained SHGE through nanomaterials is usually too low to be of practical use. In this study, it will be shown that by properly adjusting the conduction-band electron density in a semiconductor nanomaterial under a certain optical pumping rate (active tuning), the SHGE can be effectively tuned from being super-low to being ultra-high. Such sharp tunability is only valid for small-scale materials as their density of conduction-band electrons can be rapidly switched between high and low under moderate optical pumping. Using an experimentally verified computational model, we have observed that at a given frequency, for a certain range of conduction-band electron densities, the SHGE can reach up to 1080% for Ga-As and 230% for silicon nanomaterials under active tuning, with respect to the intensity of the first harmonic of the input signal. Such SHGEs are unprecedented, which is very promising for generating higher harmonics via cascaded second harmonic generation performed via adaptive tuning of the conduction band electron density at each stage.

Effect of small amount of CF4 addition on He and Ar atmospheric pressure plasma jet and influence of relative permittivity of downstream material on plasma–surface interaction

CF4 is an important source of fluorine groups in atmospheric pressure plasma jet (APPJ). In order to obtain reactive fluorine species under atmospheric pressure, noble gas (Ar or He) and CF4 are usually mixed and used as the working gas of APPJ. In this paper, the differences in the discharge dynamics on He/CF4 APPJ and Ar/CF4 APPJ are investigated experimentally. Meanwhile, combined with simulation, the effects of downstream targets with different relative permittivity on the radial propagation range of the plasma plume and the distribution of F-containing reactive species are studied. It is discovered that the addition of a small amount of CF4 (20 sccm) will increase the intensity of He/CF4 APPJ due to the contribution of Penning ionization of metastable He with CF4. Differently, the addition of CF4 will continuously lead to a significant decrease in the intensity of Ar/CF4 APPJ. The radial propagation range of He/CF4 APPJ on the target surface decreases with the increase in the relative permittivity of the downstream target. The smaller relative permittivity inhibits the axial propagation speed of APPJ, but it increases the radial propagation range of reactive species. The larger relative permittivity promotes the production of F-containing reactive species and their flux intensity on the target surface.

Intensity-surged and bandwidth-extended terahertz radiation in two-foci cascading plasmas.

The two-color strong-field mixing in gas medium is a widely used approach to generate bright broadband terahertz (THz) radiation. Here, we present a new, to the best of our knowledge, and counterintuitive method to promote THz performance in the two-color scheme. Beyond our knowledge that the maximum THz generation occurs with two-color foci overlapped, we found that, when the foci of two-color beams are noticeably separated along the propagation axis resulting in cascading plasmas, the THz conversion efficiency is surged by one order of magnitude and the bandwidth is stretched by more than two times, achieving 10-3 conversion efficiency and >100 THz bandwidth under the condition of 800/400 nm, ∼35 fs driving lasers. With the help of the pulse propagation equation and photocurrent model, the observations can be partially understood by the compromise between THz generation and absorption due to the spatial redistribution of laser energy in cascading plasmas. The present method can be extended to a mid-infrared driving laser, and new records of THz peak power and conversion efficiency are expected.