Lossy Structure Research Articles

“Cumulative effect” of second harmonic Lamb waves in a lossy plate

The second harmonic Lamb waves have high sensitivity to microstructural defects in materials and are therefore promising for incipient damage detection and monitoring of thin-walled structures. Existing studies have shown that the second harmonic Lamb waves can be cumulative with increasing propagation distance under the internal resonance conditions, which is conducive to nonlinear wave measurements in view of structural health monitoring. However, when propagating in a lossy structure with damping, the cumulative properties of the second harmonic Lamb waves are affected by energy dissipation and thus need to be re-examined. In this paper, a method for predicting the cumulative characteristics of second harmonic Lamb waves in damped plates is proposed. Instead of using material damping parameters which are difficult to obtain in practice, the proposed method relies on the attenuation patterns of Lamb waves at fundamental and double frequencies while taking into account the influence of the wave beam divergence. The proposed methodology is validated by finite element simulations and experiments. The results show that the cumulative second harmonic Lamb waves in the damped plate tend to increase and then decrease, and a “sweet” zone of relatively large amplitude can be predicted using the proposed method. The elucidation of the cumulative characteristics of the second harmonic Lamb waves provides guidance for effective system design for structural damage detection and monitoring applications.

Scattering exceptional point in the visible

Exceptional point (EP) is a special degeneracy of non-Hermitian systems. One-dimensional transmission systems operating at EPs are widely studied and applied to chiral conversion and sensing. Lately, two-dimensional systems at EPs have been exploited for their exotic scattering features, yet so far been limited to only the non-visible waveband. Here, we report a universal paradigm for achieving a high-efficiency EP in the visible by leveraging interlayer loss to accurately control the interplay between the lossy structure and scattering lightwaves. A bilayer framework is demonstrated to reflect back the incident light from the left side ( | r−1 | >0.999) and absorb the incident light from the right side ( | r+1 | < 10–4). As a proof of concept, a bilayer metasurface is demonstrated to reflect and absorb the incident light with experimental efficiencies of 88% and 85%, respectively, at 532 nm. Our results open the way for a new class of nanoscale devices and power up new opportunities for EP physics.

A Dual-Band Composite Frequency Selective Rasorber With Broadband Absorption Performance

In this paper, a dual-band frequency selective rasorber(FSR) is proposed based on a composite absorption structure. The FSR is a three-tier structure composed of lossy layer I, lossy layer II and a dual-band frequency selective surface (FSS). The lossy layer I is composed of a lossy cross-shaped structure in which, dual-passband resonators are added and constructed by parallel interdigital capacitors and meandering strip inductors(PLC). The lossy layer II is composed of a hybrid circular strip resonator loaded with lossy elements. FSS is implemented in two Jerusalem slots. By combining the lossy layers I and II as a composite lossy structure, the absorption bands of lossy layers I and II are successfully connected in the frequency response, and the bandwidth of S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> under -10dB is extended from 3.1 to 20.6GHz. The proposed FSR integrate dual-band, dual-polarized, and broadband absorption performance simultaneously. Finally, a prototype of the composite FSR is fabricated to validate the simulated results.

Foldcomp: a library and format for compressing and indexing large protein structure sets.

Highly accurate protein structure predictors have generated hundreds of millions of protein structures; these pose a challenge in terms of storage and processing. Here, we present Foldcomp, a novel lossy structure compression algorithm, and indexing system to address this challenge. By using a combination of internal and Cartesian coordinates and a bi-directional NeRF-based strategy, Foldcomp improves the compression ratio by a factor of three compared to the next best method. Its reconstruction error of 0.08 Å is comparable to the best lossy compressor. It is five times faster than the next fastest compressor and competes with the fastest decompressors. With its multi-threading implementation and a Python interface that allows for easy database downloads and efficient querying of protein structures by accession, Foldcomp is a powerful tool for managing and analysing large collections of protein structures. Foldcomp is a free open-source software (GPLv3) and available for Linux, macOS, and Windows at https://foldcomp.foldseek.com. Foldcomp provides the AlphaFold Swiss-Prot (2.9GB), TrEMBL (1.1TB), and ESMatlas HQ (114GB) database ready-for-download.

Design of a Terahertz Leaky-Wave Long-Slot Antenna Using Graphene

In this paper, a novel graphene-based leaky-wave antenna is presented. The frequency of the proposed antenna is in the terahertz range, and it is composed of a straight long slot covered with a graphene sheet. To tune the leakage constant along the slot, DC voltage biases are applied using gating pads. A transverse equivalent network model that includes the graphene slot structure is also presented. A design procedure for a lossy structure leaky-wave antenna with an unknown loss value is proposed. The antenna is designed and simulated in HFSS and CST software. An interesting feature of this antenna is the ability to control its radiation characteristics across its entire working frequency range through graphene conductivity tunability with DC voltage bias.

Design of broadband electromagnetic metamaterials absorption in double-layer resistance film

This paper designs a lossy double-layer resistance film absorption structure, which consists of two layers of resistance film and PFA medium alternately superimposed, the bottom of the Cu conductive film is used as the reflection bottom plate, and the metamaterials absorber thickness is 4.267mm. Simulation results show that an absorption rate greater than 90% is achieved in the 6.4-21.1GHz frequency interval, with a maximum absorption peak of 98.5%, a bandwidth reaching 14.6GHz, and a relative bandwidth of up to 106.2%. The wave absorption structure presents polarized insensitive absorption and has good wide-angle incident absorption characteristics. The single-layer resistance film realized periodic and multi-resonant peak narrow-band absorption, with a maximum absorption rate of 99.9%. Based on the software simulation results, the current density field distribution map and impedance characteristics, and the absorption mechanism were analysed.

Acoustic surface wave dispersion with strain gradient elasticity and micro-inertia effect in lossy polymer-coated piezoelectric structure

This research paper utilizes second-order gradient electroelasticity theory to develop a theoretical model of Love-type surface wave transference considering strain and inertia gradient effects in a Kelvin–Voigt type viscoelastic lossy polymer-coated piezoelectric substrate. Non-classical constitutive equations derived from variational principles are used to obtain electromechanically coupled field equations. Using proper analytical techniques and non-classical boundary conditions, dispersion relations for electrically open and short cases are derived. Numerical example has been provided and graphs have been plotted to visualize the dependency of velocity and attenuation traits of Love-type waves on considered strain gradient, inertia gradient, and other relevant parameters like viscosity and layer width. It has been observed that the phase velocity of the fundamental mode of surface wave is highly affected by strain gradient elasticity and micro-inertia and high gradient elasticity helps to achieve high phase velocity. The investigation can provide proper guidelines to optimize biological or chemical surface acoustic wave (SAW) sensors with nanocoating. Highlights Nanoscale strain gradient elasticity and micro-inertia effect are considered in the shear wave propagation model. Non-conductive Kelvin–Voigt model-based viscoelasticity is assumed for coating polymer. Attenuation is also characterized along with phase velocity on a logarithmic scale. Dispersion characteristics are investigated for electrically open and short boundary types.

Boosting Total Conversion Efficiency of Hybrid Photovoltaic–Thermal via a Spectral Splitter/Absorber Based on Lossy Periodic Structured Media

A spectral splitter plays a crucial role in the hybrid photovoltaic (PV)–thermal design for efficient solar energy harvesting. Herein, the lossy medium periodic structure of ITO/Si is utilized to construct a novel spectral splitter/absorber that selectively reflects the solar energy in PV band while absorbing the rest of solar energy. The as‐prepared spectral splitter/absorber demonstrates a weighted reflectance of 75.6% in the PV band, an absorptance of 83.1% in the photothermal (PT) band, as well as superior angular performance across a broad incident range of 0°–60°. The proposed hybrid photovoltaic–thermal (PVT) system assembled with the spectral splitter/absorber with 1100 nm long edge of the PV band enhances the total electrical efficiency to 26.5%, higher than that of the individual Si PV cell (19.1%), resulting from the precise modulation of solar spectrum via the splitter/absorber to realize efficient utilization of solar energy in different bands. The temperature of PV cells in a hybrid PVT system also declines by ≈25 °C compared with the single‐Si PV cell. The PV band can be easily shifted to match other narrow‐bandgap PV cells due to intrinsic properties of near‐zero k of both ITO and Si at near‐infrared wavelength range and the large difference in n between them.

Insertion Loss and Phase Compensation Using a Circular Slot Via-Hole in a Compact 5G Millimeter Wave (mmWave) Butler Matrix at 28 GHz.

Fifth generation (5G) technology aims to provide high peak data rates, increased bandwidth, and supports a 1 millisecond roundtrip latency at millimeter wave (mmWave). However, higher frequency bands in mmWave comes with challenges including poor propagation characteristics and lossy structure. The beamforming Butler matrix (BM) is an alternative design intended to overcome these limitations by controlling the phase and amplitude of the signal, which reduces the path loss and penetration losses. At the mmWave, the wavelength becomes smaller, and the BM planar structure is intricate and faces issues of insertion losses and size due to the complexity. To address these issues, a dual-layer substrate is connected through the via, and the hybrids are arranged side by side. The dual-layer structure circumvents the crossover elements, while the strip line, hybrids, and via-hole are carefully designed on each BM element. The internal design of BM features a compact size and low-profile structure, with dimensions of 23.26 mm × 28.92 mm (2.17 λ0 × 2.69 λ0), which is ideally suited for the 5G mmWave communication system. The designed BM measured results show return losses, Sii and Sjj, of less than −10 dB, transmission amplitude of −8 ± 2 dB, and an acceptable range of output phase at 28 GHz.

Resonant field enhancement in lossy periodic structures supporting complex bound states in the continuum

Resonant modes in a lossy periodic structure sandwiched between two lossless homogeneous media form bands that depend on the Bloch wave vector continuously and have a complex frequency due to radiation and absorption losses. A complex bound state in the continuum (cBIC) is a special state with a zero radiation loss in such a band. Plane waves incident upon the periodic structure can be resonantly enhanced. An existing scaling law states that the field enhancement η , defined as the ratio between the amplitudes of the local field and the incident wave, is proportional to Q r a d Q d i s / ( Q r a d + Q d i s ) , where Q r a d and Q d i s are the quality factors associated with radiation and absorption losses, respectively. In this paper, we show that this scaling law is not always valid when there are multiple radiation channels. Using a perturbation method, we derive a rigorous formula for field enhancement, and show the scaling law is valid for specific incident fields, but not valid for optimal incident fields that maximize the field enhancement.

Three‐dimensional lightweight metamaterial with ultra‐wideband microwave absorption

AbstractA three‐dimensional lightweight metamaterial (MM) with ultra‐wideband microwave absorption is proposed in this paper. Based on the concept of MMs, periodically arranged resistive films are introduced into the standing frame to form a standing lossy structure, which can introduce electric resonances in the gap between adjacent sub‐wavelength unit cells at lower frequencies. Several standing wave modes are also excited in the standing lossy structure at higher frequencies, resulting in an ultra‐wideband absorption for electromagnetic waves. The full‐wave simulation demonstrates that the MM can absorb over 90% of the incident wave energy in the range 1.46–40 GHz, with a relative bandwidth of 185.9%. The experimentally measured results correspond to the simulated results across the entire frequency range. In addition, the proposed MM is lightweight, easy to fabricate.

Structure and Magnetic Properties of Thermodynamically Predicted Rapidly Quenched Fe85-xCuxB15 Alloys.

In this work, based on the thermodynamic prediction, the comprehensive studies of the influence of Cu for Fe substitution on the crystal structure and magnetic properties of the rapidly quenched Fe85B15 alloy in the ribbon form are performed. Using thermodynamic calculations, the parabolic shape dependence of the ΔGamoprh with a minimum value at 0.6% of Cu was predicted. The ΔGamoprh from the Cu content dependence shape is also asymmetric, and, for Cu = 0% and Cu = 1.5%, the same ΔGamoprh value is observed. The heat treatment optimization process of all alloys showed that the least lossy (with a minimum value of core power losses) is the nanocomposite state of nanocrystals immersed in an amorphous matrix obtained by annealing in the temperature range of 300–330 °C for 20 min. The minimum value of core power losses P10/50 (core power losses at 1T@50Hz) of optimally annealed Fe85-xCuxB15 x = 0,0.6,1.2% alloys come from completely different crystallization states of nanocomposite materials, but it strongly correlates with Cu content and, thus, a number of nucleation sites. The TEM observations showed that, for the Cu-free alloy, the least lossy crystal structure is related to 2–3 nm short-ordered clusters; for the Cu = 0.6% alloy, only the limited value of several α-Fe nanograins are found, while for the Cu-rich alloy with Cu = 1.2%, the average diameter of nanograins is about 26 nm, and they are randomly distributed in the amorphous matrix. The only high number of nucleation sites in the Cu = 1.2% alloy allows for a sufficient level of grains’ coarsening of the α-Fe phase that strongly enhances the ferromagnetic exchange between the α-Fe nanocrystals, which is clearly seen with the increasing value of saturation induction up to 1.7T. The air-annealing process tested on studied alloys for optimal annealing conditions proves the possibility of its use for this type of material.

Earth-Effect Emulation Using Periodic Structures in Scaled-Model Characterization of HF Antennas

We present a technique for emulating the impact of the presence of earth on the responses of high-frequency (HF) antennas characterized using scaled-model measurements. Scaled-model measurements are routinely used to characterize the responses of HF antennas, particularly those mounted on physically large platforms. The presence of the ground in the vicinity of such antennas and its electrical parameters can significantly impact the antenna response. However, this impact is not usually taken into account in scaled-model measurements. In this letter, we demonstrate that the earth impact in such scaled-model measurements can be emulated by using a lossy periodic structure, with subwavelength unit-cell dimensions, designed to emulate the electrical properties of the ground at the scaled frequency of operation. The efficacy of this approach is demonstrated using a design example for a platform-based HF antenna operating in the vicinity of wet earth. A scaled-model prototype of the antenna and the ground-emulating periodic structure are fabricated and experimentally characterized. Measurement and simulation results demonstrate a good agreement and verify the capability of using this approach to perform more accurate, scaled-model measurement of HF antennas.

Loss Analysis of Plasmonic Metasurfaces Using Field-Network-Joint Method

Structured metallic surfaces, also called as plasmonic surfaces, have been proposed to support spoof surface plasmon polariton (SPP) modes, which can mimic natural SPPs at optical frequencies. Complex plasmonic surfaces can provide a large degree of freedom to engineer the characteristics of SPP dispersion. However, the analysis of complex plasmonic surfaces is difficult using the existing theoretical methods, such as the effective medium model and mode matching model. Recently, an accurate field-network-joint method has been introduced into the dispersion analysis of complex plasmonic surfaces, but it only works in the lossless case. In this communication, lossy complex plasmonic surfaces are investigated based on the modified field-network-joint method. The proposed method can predict not only the dispersion relationship between the frequency and the wavenumber, but also the lossy feature of the plasmonic surface with high efficiency and accuracy. As an example, a periodic structure with biforked slits filled by a lossy medium is fabricated and measured. The calculated attenuation constant (i.e., the imaginary part of the wavenumber) of the lossy structure is nearly identical to the measured result, demonstrating the superiority of the proposed analytical method.

Finite-element method for obtaining the regularized photon Green function in lossy material

The photon Green function (GF) is a vital and decisive factor in the field of quantum light-matter interaction. It is divergent with two equal space arguments in lossy structure and should be regularized. We introduce a finite-element method (FEM) for calculating the regularized GF in arbitrary-shaped lossy structure, which is expressed by the averaged radiation electric field over the finite size of a photon emitter. For an emitter located in a homogeneous lossy material, excellent agreement with the analytical results is found for both real and virtual cavity models. For an emitter located in a metal nano-sphere, the regularized scattered GF, which is the difference between the regularized GF and the analytical regularized one in homogeneous space, agrees well with the analytical scattered GF. Applying this method for an emitter located in a metal nano-rod where there is no analytical solution, we find that the scattering contribution to the enhancement of the spontaneous emission rate is nearly unrelated to the cavity radius while the homogeneous part depends heavily. Our results should be significant for novel photon sources within lossy structures.

Time-Domain Multimode Analysis of a Terahertz Gyro-TWT Amplifier

High-order mode employed as the operating mode of a terahertz (THz) gyrotron traveling-wave tube (gyro-TWT) amplifier shows strong dispersion, which deteriorates the device performance in amplifying picosecond THz pulses. Theoretical investigation of a THz gyro-TWT amplifier with a lossy cylindrical circuit is carried out to study the problems, such as lossy structure design, parameters selection, frequency-domain performance, and time-domain dynamics. The robust high growth rate low-kz amplification and the broadband high-kz amplification are two preferred operation conditions. The revealed characteristics are also useful for developing THz gyro-TWTs with other kinds of over-moded circuits.

Low windage resistance frequency‐selective rasorber based on windmill‐shape coupling line arrays

In this Letter, the authors report on the analysis, design, fabrication and measurement of a low windage resistance frequency selective rasorber (FSR) based on windmill-shape coupling slot line arrays. A lossy structure is added to the traditional frequency selective surface structure to make up a FSR with the characteristics of small insertion loss within the passband and weak reflection at frequencies below and above the passband meanwhile. In addition, the hollow structure can reduce windage resistance effectively. The measurements show that an insertion loss of 1.01 dB can be obtained at 7.71 GHz, while both lower and upper passband edges have at least 10 dB reflection reduction under the normal incidence. Both simulation and experiment results exhibit that the proposed FSR is insensitive to both transverse-electric (TE) and transverse-magnetic (TM) polarisation. The proposed FSR can be applied in the field of radome, cloaking and other microwave applications.

Uniqueness theorem and uniqueness of inverse problems for lossy anisotropic inhomogeneous structures with diagonal material tensors

The uniqueness theorem for lossy anisotropic inhomogeneous structures with diagonal material tensors is proven. For these materials, we prove that all the elements of the constitutive tensors must be lossy. Materials like cloaks and lenses designed based on transformation-optics (TO) could be examples of such materials. The uniqueness theorem is about the uniqueness of Maxwell's equations solutions for particular sets of boundary conditions. We prove the uniqueness theorem for three cases: Single medium, media composed of two lossy anisotropic inhomogeneous materials with diagonal constitutive parameters, and media composed of two materials, where a material with diagonal material tensors is surrounded by an isotropic material. The latter case can be considered for the TO-based materials like invisibility cloaks or hyper-lenses that usually have diagonal anisotropic inhomogeneous constitutive parameters and also because cloaks or hyper-lenses are usually surrounded by free space and the sources are usually outside. For the sake of our argument in the uniqueness theorem that loss is the main condition for the validity of this theory, for cloaks as an example case of our analysis, it is analytically and numerically proven that the ideal invisibility phenomenon is possible for a simple lossy structure. We also examine the uniqueness of the inverse problem for such structures. We prove that all these materials have the same surface field distribution on a surface enclosing the area of interest, while solutions to Maxwell's equations inside them are different. The uniqueness of the inverse problem suggests that within the surface, the same medium should exactly be present. However, for lossy anisotropic inhomogeneous structures with diagonal constitutive parameters, this paper illustrates that this might not be true, despite the result of a previous study that shows that uniqueness could be true for some anisotropic inhomogeneous structures. For the analysis, the transverse electric Z-polarization is used. The simulation results are obtained by using a commercial Finite-Element based simulator.

Study of scattering from a periodic grating structure using Lorentz–Drude model and Chandezon method

Presented study deals with the scattering from a dispersive, lossy and periodic grating structure using differential formalism of the Chandezon method also known as C-method. Losses and dispersion in the grating structure has been introduced through Lorentz–Drude model. For transverse magnetic and transverse electric excitation, efficiency behaviour of the diffracted orders in the optical regime has been analyzed and discussed.

Simulation and Experiment of a Ku-Band Gyro-TWT

Design techniques and experimental results are presented on a Ku-band TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> mode gyro-traveling wave tube. The hot test of this amplifier gives more than 153-kW output power, 2.3-GHz bandwidth (14%), 41-dB saturated gain, and 20% efficiency driven by a 63 kV, 12-A electron beam with a pitch angle (v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</sub> ) of 1.2, and velocity spread of 5%. A linear polarized TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> mode input coupler is used to introduce the input power. The stability of the amplifier from oscillation, including both the operating TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> mode and the backward wave TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> mode, has been investigated with linear codes, nonlinear selfconsistent theory, and 3-D PIC CHIPIC simulation. To suppress the potential gyro-backward wave oscillator interactions, the high frequency circuit is loaded with lossy ceramic rings. The lossy structure is optimized by nonlinear theory and 3-D PIC simulation. A low velocity spread magnetron injection gun is designed with a new structure.