Wave Polarization Research Articles

Evaluation of a digital stethoscope for electrocardiographic recording in donkeys: Preliminary results

The digital stethoscope (DS) is a cost-effective single-lead digital stethoscope that allows simultaneous electrocardiographic (ECG) and phonocardiographic recordings on a smartphone. Despite its application in small animals and horses, there are currently no studies on its use in donkeys. The aim of this study was to evaluate the use of a new smartphone-based DS device in recording ECG tracings in donkeys. Standard base-apex lead ECG (sECG) and single-lead DS ECG (dECG) were simultaneously recorded for at least 30 s. Both sECG and dECG tracings were analysed by the same operator, recording heart rate, ECG waves and intervals, and the presence and duration of artefacts. Thirty-seven donkeys were included. The dECG tracings were interpretable in all the animals (100 %). The results showed perfect agreement between the sECG and dECG data for the classification of heart rhythm and P-wave polarity. Strong agreement was found in the evaluation of heart rate calculated manually and automatically by the smartphone app, QRS complex polarity, T wave polarity, and duration of the PR interval. However, no agreement was found in the evaluation of P wave duration, QRS complex duration and amplitude, and T wave duration and amplitude. In conclusion, although this is only a preliminary study, the DS was a valid, practical, and easy to use electrocardiographic tool for recording good-quality ECG tracings to assess the ECGs of donkeys in the field.

Triple-broadband asymmetric transmission via cross-polarization conversion based on composite chiral metasurface structure

In this paper, a novel composite chiral metasurface (CCMS) was proposed and investigated, which is composed of a rectangle-patch-ring resonator (RPRR) structure sandwiched between two twisted sub-wavelength metal gratings separated by a dielectric substrate. Both simulation and experimental results demonstrate that the proposed CCMS achieves a significant asymmetric transmission (AT) effect and efficient cross-polarization conversion for the incident linear polarization wave in a triple-broadband range. Simulation results are in good agreement with the experiments. Specifically, the CCMS converts normal impinging y-/x-polarization wave along the forward/backward (-z/+z) direction into transmitted x-/y-polarization wave with the cross-polarization transmission coefficient over 0.7 at 4.15–5.5 GHz, 7.29–10.77 GHz and 12.49–16.59 GHz, respectively. In addition, the AT coefficient (Δlin) and the total transmittance (Tx) of the x-polarization wave propagation along the –z axis direction are both over 0.5 in the triple-broadband frequency range. The simulated polarization azimuth rotation and ellipticity angles, induced surface current, and electric field vector distributions further confirm the characteristics of the triple-broadband high-efficiency AT effect and cross-polarization conversion of the designed CCMS. The proposed CCMS design serves as an important reference for practical applications of microwave devices.

Terahertz polarimetric imaging of biological tissue: Monte Carlo modeling of signal contrast mechanisms due to Mie scattering.

Many promising biomedical applications have been proposed for terahertz (THz) spectroscopy and diagnostic imaging techniques. Polarimetric imaging systems are generally useful for enhancing imaging contrasts, yet the interplay between THz polarization changes and the random discrete structures in biological samples is not well understood. In this work, we performed Monte Carlo simulations of the propagation of polarized THz waves in skin and adipose tissues based on the Mie scattering from intrinsic structures, such as hair follicles or sweat glands. We show that the polarimetric contrasts are distinctly affected by concentration, size and dielectric properties of the scatterers, as well as the frequency and polarization of the incident THz waves. We describe the experimental requirements for observing and extracting these polarimetric signals due to the low energy and small angular spread of the back-scattered THz radiation. We analyzed the spatially integrated Mueller matrices of samples in the normal-incidence back-scattering geometry. We show that the frequency-dependent degree of polarization (DOP) can be used to infer the concentrations and dielectric contents of the scattering structures. Our modeling approach can be used to inform the design of the imaging modalities and the interpretation of the spectroscopic data in future terahertz biomedical imaging applications.

Wideband Polarization‐Insensitive 1‐Bit Phase Manipulations with Co‐and Cross‐Polarized Conversions for LP and CP Waves

AbstractPolarization and phase manipulations have gained widespread applications across various fields. The emergence of metasurface provides a new platform for the manipulation of electromagnetic waves. However, most metasurfaces are limited to modulating only one single property. Here, a coding metasurface that achieves 1‐bit phase manipulations of both co‐polarized and cross‐polarized reflected waves for linear and circular polarization wave excitations. Notably, the proposed metasurface enables co‐polarized 1‐bit phase responses for incident electric fields with arbitrary orientations and spin directions. The simulated results demonstrate an operating efficiency over −1.7 dB in the frequency range from 7 to 11.1 GHz, with a relative bandwidth of 45.3%. Furthermore, the introduction of space‐polarization coding strategies facilitates the beam and polarization manipulations. To verify the feasibility of the coding metasurface, a sample and conduct experiments are fabricated, revealing good agreement between the simulations and measurements. These findings hold significant promise for the advancement of radar, imaging, and wireless communication systems, with potential applications in various domains.

Absorption-transmission-diffusion-type radar cross section reduction metasurfaces based on frequency selective absorber and polarization conversion chessboard structure

Absorption-transmission-diffusion-type radar cross section (RCS) reduction metasurfaces based on the frequency selective absorber and the polarization conversion chessboard structure are proposed in this paper. The RCS reduction metasurfaces consist of three parts, i.e. the top layer realized by a cross-shaped meander line loaded with lumped resistors for low-frequency absorption and high-frequency transmission, the mid-layer constructed by angular ring vias frequency selective surface (FSS) sandwiched with two square patches FSS for broadband transmission in mid-frequency zone, and the bottom layer employed chessboard metasurfaces composed of dual-arrow type polarization conversion units for scattering cancellation in high frequency zone. The physical mechanisms of the RCS reduction metasurfaces with multifunctional electromagnetic controlling including wave absorption, bandpass transmission, polarization reversal, and phase cancellation are also discussed and analyzed. Simulation and experimental results demonstrate a reflection of lower than −10 dB for normal incident linearly polarized waves over a frequency range of 2.5 GHz–18 GHz, with a relative bandwidth of 151.22%; transmission passband properties with insertion loss better to −3 dB in the frequency range of 5.62 GHz–11.0 GHz, with a relative bandwidth of 64.74%. Due to the resonance-coupling-resonance principle, the broadband transmission with low insertion loss was obtained. The RCS properties of the proposed multi-functional metasurfaces were further analyzed by numerical simulation and experimental measurement, and the results showed that RCS reduction of over 10 dB was achieved for normal incident different polarized waves at the frequency range of 2.5 GHz to 17 GHz. Even in the case of normal incidence, there was a deviation between the reflection characteristics and RCS reduction. The RCS reduction metasurfaces are expected to be employed in applications where multi mechanism collaborative control of electromagnetic waves is of extreme importance.

Two‐Dimensional Hybrid Simulation of the Second‐Harmonic Generation of EMIC Waves in the Inner Magnetosphere

AbstractTwo‐dimensional (2‐D) hybrid model is developed to investigate the second harmonic (SH) generation of electromagnetic ion cyclotron (EMIC) waves. Applying the singular value decomposition method to simulated fields, we show that the SH exhibits wave properties analogous to typical EMIC waves generated by ion cyclotron instabilities, that is, left‐hand polarization and small wave normal angle. However, the bicoherence index inferred from simulated fields reflects a strong phase coupling between the fundamental wave (FW) and the SH, illustrating the nonlinear generation of the SH by the FW. The necessary conditions, especially for the wave vector relation, are further verified from a 2‐D perspective. The simulated amplitude ratios well meet the theoretical results only in the SH saturation stage, while the necessary conditions remain satisfied almost throughout the simulation. This study provides a comprehensive analysis of the SH excitation in a 2‐D simulation domain, contributing to a deeper understanding of EMIC wave nonlinear generation.

Measurements of mid-frequency acoustic signal variability in the Western Barents Sea

Two Vertical Line Arrays (VLAs) and one source moorings were deployed to study along- and cross-shelf acoustic propagation in the Western Barents Sea during October 12–19, 2022. An ITC 2010 source was used to transmit Linear Frequency-Modulated (LFM) acoustic signals covering a frequency band of 0.7–4.2 kHz. Several oceanographic moorings were also deployed to measure time series of ocean currents and sound-speed profiles. Analysis of the acoustic data showed strong effects of tidal currents and surface roughness on mid-frequency acoustic propagation. Along- and cross-shelf acoustic propagation data showed semidiurnal travel-time fluctuations due to tidal currents. In addition, a 15 dB increase in Transmission Loss was observed during the storm events due to rough surface scattering. Finally, an assessment of the Polar Front and its effects on acoustic propagation (both travel-time and amplitude) are studied. Several data and simulated examples are provided to demonstrate the 3D effects of a Polar front and nonlinear internal waves on mid-frequency acoustic propagation. [Work supported by the ONR.]

Anisotropic media with singular slowness surfaces

It is proved that if an anisotropic medium has an open set of singular directions, then this medium has two slowness surfaces that completely coincide. The coinciding slowness surfaces form one double singular slowness surface. The corresponding anisotropic medium is an elliptical orthorhombic (ORT) medium with equal stiffness coefficients c44=c55=c66 rotated to an arbitrary coordinate system. Based on the representation of the Christoffel matrix as a uniaxial tensor and considering that the elements of the Christoffel matrix are quadratic forms in the components of the slowness vector, a system of homogeneous polynomial equations was derived. Then, the identical equalities between homogeneous polynomials are replaced by the equalities between their coefficients. As a result, a new system of equations is obtained, the solution of which is the values of the reduced (density normalized) stiffness coefficients in a medium with a singular surface. Conditions for the positive definite of the obtained stiffness matrix are studied. For the defined medium, the Christoffel equations and equations of group velocity surfaces are derived. The orthogonal rotation matrix that transforms the medium with a singular surface into an elliptic ORT medium in the canonical coordinate system is determined. In the canonical coordinate system, the slowness surfaces S1 and S2 waves coincide and are given by a sphere with a radius . The slowness surface of qP waves in the canonical coordinate system is an ellipsoid with semi-axes , , . The polarization vectors of S1 and S2 waves can be arbitrarily selected in the plane orthogonal to the polarization vector of the qP wave. However, the qP wave polarization vector can be significantly different from the wave vector. This feature should be taken into account in the joint processing and modelling of S and qP waves. The results are illustrated in one example of an elliptical ORT medium.

Perfect circular polarization of elastic waves in solid media

Elastic waves involving mechanical particle motions of solid media can couple volumetric and shear deformations, making their manipulation more difficult than electromagnetic waves. Thereby, circularly polarized waves in the elastic regime have been little explored, unlike their counterparts in the electromagnetic regime, where their practical usage has been evidenced in various applications. Here, we explore generating perfect circular polarization of elastic waves in an isotropic solid medium. We devise a novel strategy for converting a linearly polarized wave into a circularly polarized wave by employing an anisotropic medium, which induces a so-far-unexplored coupled resonance phenomenon; it describes the simultaneous occurrence of the Fabry-Pérot resonance in one diagonal plane and the quarter-wave resonance in another diagonal plane orthogonal to the former with an exact 90° out-of-phase relation. We establish a theory explaining the involved physics and validate it numerically and experimentally. As a potential application of elastic circular polarization, we present simulation results demonstrating that a circularly polarized elastic wave can detect an arbitrarily oriented crack undetectable by a linearly polarized elastic wave.

Realization of Hadamard gate with twisted magnon modes in synthetic antiferromagnets

Manipulating the polarization of spin waves highlights the potential of antiferromagnetic magnonics in encoding and handling magnon information with high fidelity. Here, we propose a flexible approach to mutually convert polarization states (i.e., Hadamard gate) by incorporating a topological degree of freedom, intrinsic orbital angular momentum (OAM), into twisted spin wave modes within synthetic antiferromagnetic nanodisks. The polarization states of spin waves and the implementation of magnonic logic operations can be electrically read out through combined spin pumping and inverse spin Hall effect, as demonstrated by numerical micromagnetic simulations for CoFeB-based synthetic antiferromagnets. Our findings present an exciting possibility of parallel magnonic computing utilizing topologically protected and magnetic damping-resistance OAM of twisted magnons.

Genus Plasma-Based Self-Complementary Reconfigurable Intelligent Metasurfaces

AbstractThis paper investigates the radiation properties of plasma-based intelligent reflectarray surface (IRS) that utilizes self-complementary elements. The elements of the surface employ reconfigurable plasma conductivity to manipulate the polarization of electromagnetic waves in various Ka-band applications. The unit-cell of the IRS is composed of 2 × 2 sub-cells of 0.5 λo × 0.5 λo × 0.078 λo mm3. Each sub-cell contains three glass containers arranged in a triangle-rectangular-triangle configuration and filled with ionized plasma gas. When the IRS illuminated by a linearly polarized (LP) plane wave, the surface converts it into circularly polarized (CP) waves based on the plasma ionization state. It achieves dual-band polarization conversion at 7.3 GHz with bandwidth of 200 MHz and at 8.6 GHz with a bandwidth of 1.15 GHz. To enhance the polarization conversion bandwidth, a sequential arrangement of self-complementary unit-cell elements with different sizes is employed. An 8 × 8 genus reconfigurable IRS is utilized to generate both left-hand circularly polarized (LHCP) and right-hand circularly polarized (RHCP) waves from the upper and lower sides of the structure. The axial ratio (AR) bandwidth is maintained at 2.1 Hz for both surfaces. An LP horn antenna is used with the 8 × 8 genus reconfigurable IRS, resulting in a peak gain of 13.2 dBi, a side lobe level (SLL) of 6 dBi, and an AR bandwidth of 500 MHz. To further enhance the radiated gain, the ionized plasma within the self-complementary elements is controlled to focus the radiated power into a directive beam, resulting in an increased gain of 19.4 dBi.

Multipath-controlled bidirectional metasurface for multitasking polarization regulation and absorption.

In the design of metasurfaces, integrating multiple tasks into a single small unit cell and achieving regulation through various paths pose a serious challenge. In this paper, a multipath-controlled bidirectional metasurface (MCBM) is designed to achieve polarization regulation, perfect absorption and total reflection as multitasking functions. The findings demonstrate that under different excitation conditions, when co-planar polarized terahertz (THz) waves are incident normally on the metasurface, the MCBM can convert co-planar polarization to cross-polarization, co-planar polarization to circular polarization wave in reflection mode, and co-planar polarization to cross-polarization in transmission, respectively. When co-planar polarized THz waves are incident from the back side of the metasurface, the tasks of MCBM change to broadband perfect absorption, total reflection, and transmission co-planar polarization to cross-polarization conversion. Remarkably, all operating frequency bands of these tasks are very approximate. Additionally, the multitasking functions can be switched by altering the excitation conditions, and their performance can be regulated through multipath controls, such as the temperature, voltage, and polarization status. Our design provides an effective strategy for multipath-controlled multitasking integrated devices in the THz band.

Wind turbines as a metamaterial-like urban layer: an experimental investigation using a dense seismic array and complementary sensing technologies

The deflection and the control of the effects of the complex urban seismic wavefield on the built environment is a major challenge in earthquake engineering. The interactions between the soil and the structures and between the structures strongly modify the lateral variability of ground motion seen in connection to earthquake damage. Here we investigate the idea that flexural and compressional resonances of tall turbines in a wind farm strongly influence the propagation of the seismic wavefield. A large-scale geophysical experiment demonstrates that surface waves are strongly damped in several distinct frequency bands when interacting at the resonances of a set of wind turbines. The ground-anchored arrangement of these turbines produces unusual amplitude and phase patterns in the observed seismic wavefield, in the intensity ratio between stations inside and outside the wind farm and in surface wave polarization while there is no metamaterial-like complete extinction of the wavefield. This demonstration is done by setting up a dense grid of 400 geophones and another set of radial broadband stations outside the wind farm to study the properties of the seismic wavefield propagating through the wind farm. Additional geophysical equipment (e.g., an optical fiber, rotational and barometric sensors) was used to provide essential explanatory and complementary measurements. A numerical model of the turbine also confirms the mechanical resonances that are responsible for the strong coupling between the wind turbines and the seismic wavefield observed in certain frequency ranges of engineering interest.

A polarization insensitive terahertz tri-band graphene metasurface absorber

A graphene-based tri-band perfect metasurface absorber is presented for the far-infrared frequency range. A two-dimensional periodic pattern of graphene patches is created on a thin SiO2 dielectric layer, which is backed by a gold ground plane of sufficient thickness ( t ) ensuring zero transmission of the impinging electromagnetic (EM) wave. An array of graphene elliptical discs is placed around graphene square patches and thin graphene strips are used to connect these elliptical patches to the square patches. In terms of the lowest frequency of absorption, the proposed absorber seems to be a thin one as t∼λmax/13. It is also considered compact because the periodicity of the cells is on the order of λmax/22. The absorption spectrum gives three perfect absorption peaks with absorptivity greater than 99% at 5.76 and 9.06 THz. Further, the absorptivity is better than 96% at 7.86 THz. The performance of the proposed absorber remains invariant with change in polarization of the incident wave. It is revealed that this absorber performs satisfactorily throughout a broad angular range of incidence of about ± 60°.

Engineering Geological and Geophysical Investigations to Characterise the Unstable Rock Slope of the Sopu Promontory (Gozo, Malta)

Different engineering geological and geophysical investigations were performed at the Sopu promontory in the island of Gozo (Malta), involved in an impressive lateral spreading process due to the superimposition of a stiff limestone (ULC) on a ductile clay (BC). The applied techniques include: traditional geological and engineering geological surveys, unmanned aerial vehicles (UAV) survey, electrical resistivity tomography (ERT) survey, ground-penetrating radar (GPR) investigations, single-station seismic ambient noise measurements, and array seismic ambient noise measurements. The integration of the obtained results allowed us to reconstruct a subsoil model of the promontory that includes features related to the local geology of the slope and to the landslide process, as well as to define a conceptual model that describes the main evolution phases of the expansion process. The presence of back-tilted rock blocks with no features of polarization of Rayleigh waves evidenced the different failure mechanism of the rigid UCL plateau at the Sopu promontory with respect to the Selmun promontory, located in the close island of Malta, where the lateral spreading due to the same geological setting tends to produce unstable rock blocks with a toppling mechanism. This result encourages further future observations and analyses of this topic.

Polarization state conversion achieved by chiral mechanical metamaterial

Recently, metamaterials have driven advancements in wave propagation and polarization control. Chiral elastic metamaterials, in particular, have attracted considerable attention due to their distinctive properties, such as acoustical activity and auxeticity. Such characteristics arise from the additional degrees of freedom for tuning the embedded micro- and macro-rotations. In this study, we demonstrate an unusual energy exchange between longitudinal and in-plane shear waves in a 3D chiral mechanical metamaterial. The structural design is capable of inducing up to a 90∘ rotation in the plane of polarization. Additionally, this capacity for conversion is achieved by employing both an arrangement of chiral cells and a single meta-atom. This peculiar behavior enables a seamless switch between the three polarization states existing within a solid material, namely, the longitudinal state, the shear horizontal state, and the shear vertical state. Furthermore, a 2D discrete mono-atomic mass-spring model featuring inclined connectors is used to characterize the distinctive energy exchange between modes. This characterization is based on the retrieval of the pertinent elastic coefficients. The engineered chiral metamaterial polarization converter stands as a promising device for momentum conservation conversions and applications in elasto-dynamic polarimetry.

Three-in-one polarization detector enabled by metasurface

In the field of biomedicine, the detection of target analytes imposes requirements on polarization information detection and sensitivity. It necessitates non-destructive analysis of proteins and DNA. The combination of dielectric geometric metasurface with terahertz (THz) vortex beams present new opportunities for biomedical sensing. In this work, we employ simulation to demonstrate a novel methodology that integrates off-axis dual-focus with vector vortex beams (VVB). Moreover, the metasurface-based method provides potential solutions for miniaturized polarization detection. The capability of polarization detection is evaluated using different polarization states at three different focal planes. Specifically, the ellipticity and handedness of incident THz waves can be determined by the electric field intensity ratio of the two off-axis focused spots. The major axis direction of linear polarization waves can be determined by extracting the azimuth of the VVB. It is worth mentioning that higher-order VVB, benefiting from its high angular resolution characteristics, exhibits enhanced sensitivity in determining the direction of major axes. Our proposed scheme offers potential applications for THz communications, medical imaging, security detection and remote sensing.

A Self‐Powered Reconfigurable Intelligent Metasurface

AbstractIn recent years, reconfigurable intelligent metasurface (RIS) has gained much attention due to its flexibility in modulating the phase, amplitude, and polarization of electromagnetic waves. It is widely used for wireless power and information transmission. However, RIS requires an external wired power supply or batteries to activate their modulation capabilities, as they do not generate the required energy themselves. This paper presents a self‐powered RIS that does not require an external wired power supply or batteries. The modulation capability is activated by harvesting radio frequency (RF) energy. The self‐powered RIS consists of an energy‐metatom, which is loaded with a PIN diode for flexible phase modulation. The inclusion of an absorber/energy port on the energy‐metatom allows for independent RF energy harvesting. The RIS consisting of energy‐metatoms can cut out all wired power supplies and batteries so that it can dynamically regulate electromagnetic waves by self‐powered means. As a proof‐of‐concept, a prototype composed of 10 × 10 energy‐metatoms is fabricated. The measured results are in good agreement with the simulated. This self‐powered RIS can be used as an “infinite‐lifetime” power supply for dynamic electromagnetic modulation in the RIS.

Practical use of shear wave velocity measurements in UK clays (BGA Touring Lecture 2023)

This paper seeks to promote use of shear wave velocity (Vs) measurements in UK clays as a complement to more standard ground investigation techniques. Vs measurements seem to be repeatable and independent of the method of measurement used in isotropic soil conditions – for example, soft clays. However, in glacial tills, and especially in the overconsolidated clays of south-east UK, Vs measurements will differ depending on the direction of propagation and polarisation of the shear waves due to natural stress anisotropy and the fissured nature of the materials. Correlations between Vs and other in situ data and with a variety of soil properties can be very powerful and some have been proposed here for UK clays. However, these correlations should ideally be local and only applied to other soils and areas with great caution. Other uses of the techniques, beyond those of classical dynamic analyses, have been described together with some future challenges. Uncertainties in the methods have been well researched and several methods have been proposed to deal with these uncertainties. Nonetheless, advice from a geophysical colleague will enhance the geotechnical engineers’ use of Vs data.

Exactness of the First Born Approximation in Electromagnetic Scattering

Abstract For the scattering of plane electromagnetic waves by a general possibly anisotropic stationary linear medium in three dimensions, we give a condition on the permittivity and permeability tensors of the medium under which the first Born approximation yields the exact expression for the scattered wave whenever the incident wavenumber k does not exceed a preassigned value α. We also show that under this condition the medium is omnidirectionally invisible for k ≤ α/2, i.e. it displays broadband invisibility regardless of the polarization of the incident wave.