Kinds Of Unit Cells Research Articles

Harnessing chiral buckling structure to design tunable local resonance metamaterial for low-frequency vibration isolation

This paper proposes a tunable local resonance metamaterial with chiral buckling structures for low-frequency vibration isolation. The unit cell consists of the inner and outer buckling stiffness structure, basic structure, and local resonator. The bistability of three kinds of unit cells with different initial shapes of beams is investigated and compared via the elliptic integral method and finite element analysis. Then, their axial stiffness in different states is studied. The dynamic responses of the metamaterial with simple unit cells and hybrid supercells are theoretically derived and analyzed by Bloch's theorem and transfer coefficient method. Theoretical, numerical, and experimental results demonstrate that the unit cell with initially convex downward beams has strong bistability and determinate deformation modes. The axial stiffness of the unit cell in two stable states differs by an order of magnitude. Changing some specific parameters broadens the gap in the axial stiffness between the two stable states. The axial negative stiffness characteristics and rotation effect of the unit cell have the potential for energy dissipation and tensile-torsion material. The metamaterial with simple unit cells has tunable and wide band gap and low-frequency vibration isolation via the stable states switching of the inner and outer buckling stiffness structures. The metamaterial with hybrid supercells can further broaden the band gap width and lower the band gap frequency, which has up to 16 different band gap characteristics.

Dual-Band Aperture-Shared High Gain Antenna for Millimeter-Wave Multi-Beam and Sub-6 GHz Communication Applications

This communication presents a dual-band aperture-shared high gain antenna for millimeter-wave (mm-wave) multi-beam and sub-6 GHz applications by integrating transmitarray into Fresnel zone plate (FZP) lens. The transmistarray consists of two sets of unit cells (UCs), which can provide high transmission magnitude and [ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$-\pi $ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula> ] phase tuning at the mm-wave frequency band. Meanwhile, they show distinct transmission features at the sub-6 GHz band (one set of UC allows the electromagnetic (EM) wave to pass while the other prohibits the EM wave from passing). Then, by properly arranging the two kinds of UCs into the transparent and opaque region of the FZP lens respectively, the sub-6 GHz FZP lens and the mm-waveband transmitarray are integrated into the same aperture seamlessly. A waveguide-integrated patch antenna is adopted as the feed. The patch antenna operates at the sub-6 GHz band and three open waveguide antennas are used for the mm-wave multi-beam radiation. The proposed antenna achieves high gain at both bands with a reused radiating aperture. Mm-wave multi-beam radiation is obtained without an extra feeding network. For demonstration purposes, the central frequencies of the sub-6 GHz and the mm-wave bands are selected at 6 and 27 GHz, respectively. An antenna prototype is fabricated and experimentally verified.

Seismic metamaterials: Generating low-frequency bandgaps induced by inertial amplification

In the last two decades, seismic metamaterials have attracted significant attention of researchers because of the characteristic of the bandgap which can be used to control seismic surface waves. The inertial amplification mechanism has been used to design metamaterials capable of isolating elastic waves in rods, beams, and plates at low frequencies. In this work, we propose an alternative type of seismic metamaterial with a low-frequency bandgap induced by inertial amplification for isolating seismic surface waves. The characteristics of the bandgap induced by inertial amplification are experimentally demonstrated by using a metamaterial plate composed of 25 unit cells. The propagation of the flexural waves imaged by scanning laser Doppler vibrometer shows strong attenuation effects induced by the metamaterial plate in the bandgap. The broadband attenuation is investigated by using two kinds of unit cells. Finally, similar structures with inertial amplification are introduced to design the seismic metamaterials to isolate seismic surface waves at low frequencies.

A tunable metasurface based on Vanadium dioxide for Broadband RCS reduction

In this paper, a novel chessboard-like of metasurface based on two kinds of unit cell, using Vanadium dioxide (VO2) is proposed, designed, and demonstrated for radar cross-section (RCS) reduction. By accurately designing the geometric parameters of two units, the reflection phase difference between the reflected waves can be obtained over a broadband region, thereby causing the interference cancellation and leading to the RCS reduction. In addition, the utilization of VO2 enables this metasurface to realize the reconfiguration of the working band while under different temperature conditions. Based on the above points, simulation results show that the proposed metasurface is provided with a −10 dB RCS reduction in 6.40–7.23 GHz and 14.97–23.97 GHz at a low-temperature state (300 K), and can be shifted to 7.10–14.13 GHz and 24.10–25.87 GHz at a high-temperature state (350 K), actualizing full coverage of the working frequency band from 6.40–25.87 GHz. Besides, the influences brought by different periodic arrangements of the unit cell and dielectric layer thickness on the reduction of RCS are also discussed in detail.

Generating Dual-Polarized Vortex Beam by Detour Phase: From Phase Gradient Metasurfaces to Metagratings

Traditional methods of generating vortex beams based on metasurfaces consist mainly in modulating propagation phase or geometric phase. Here, by introducing detour phase, we propose the construction of dual-polarized vortex beam generators in the form of metasurface and metagrating (MG). The phase is modulated through moving the position of meta-atoms instead of varying the geometrical parameters or rotating the unit cells. To use detour phase, two kinds of unit cells are designed to achieve specific diffraction order. Each unit can arbitrarily and independently adjust the operation frequency and diffraction angle of transverse electric (TE) and transverse magnetic (TM) polarizations. Two vortex beam generators are designed and fabricated with different topological charges carried by orthogonal polarizations. To demonstrate the ability to independently manipulate, two polarizations of the generator based on MG are designed in different frequency bands. Both the simulation and experimental results validate the proposed method, showing great potential for polarization division multiplexing in orbital angular momentum (OAM) communication systems.

Simulation study of acoustic refraction wave manipulation based on sub-wavelength artificial periodic structure

We proposed a kind of unit cell composed of simple steel plate structures in this work. A variety of acoustic phenomena including anomalous refraction, asymmetric transmission, acoustic splitting and acoustic focusing were realized by coding the unit cells with different splicing modes. The transformation from plane acoustic wave to vortex acoustic wave was also realized by using the coding method of three-dimensional rotation. This work increased the functionality of the unit cell and provided a method for the design of sub-wavelength acoustic devices.

Metasurfaces for efficient digital noise absorption

We numerically demonstrate two types of metasurface absorbers to efficiently absorb digital signals. First, we show that the digital waveforms used in this study contain not only a fundamental wave but also nonnegligible harmonic waves, which limits the absorption performance of a conventional metasurface absorber operating in only a single, finite frequency band. The first type of the proposed absorbers is designed using two kinds of unit cells, each of which absorbs either a fundamental frequency or third harmonic of an incident digital waveform. This dual-band metasurface absorber exhibits absorption performance exceeding that of the conventional metasurface absorber and more strongly dissipates the energy of a digital waveform. In addition, the second type of absorber exploits the concept of nonlinear analogous circuits to convert an incoming wave to a different waveform, specifically, a triangular waveform that has a larger magnitude at a fundamental frequency. Therefore, the incoming waveform is more effectively absorbed by this waveform-conversion metasurface absorber as well. Although still there remain some issues to put these digital signal absorbers into practice, including experimental validation, our results contribute to mitigating electromagnetic interference issues caused by digital noise and realising physically smaller, lighter digital signal processing products for the next generation.

Compact dual band circularly polarized meta‐structured antenna for GPS application

AbstractIn this letter, a dual band circularly polarized (CP) meta‐structured antenna is designed by −1 mode for L1 band and mu‐zero resonance mode for L2 band, respectively. The antenna consists of two kinds of unit cells which have four unit elements for −1 mode and mu‐zero resonance mode, respectively, in order to utilize the limited space efficiently, and to maximize the gain. Since two modes of −1 mode and mu‐zero resonance mode operate like as loop antenna, these are fed by magnetic coupling of an aperture, simultaneously. In order to reduce mutual coupling between two modes, the orientation of −1 mode antennas are tilted by 45° from that of MZR mode antennas. In this arrangement, magnetic fields with two directions are needed to excite all unit elements, simultaneously. Thus, a cross shaped asymmetric aperture is employed. The RHCP gain of the antenna is measured to be 0.59 dBic at L1 band and −0.02 dBic at L2 band, respectively. The size of the antenna is only 0.14 λ0 by 0.14 λ0 (krground = 1.28) at L2 band. The measured results show a good agreement with those of simulations.

Trifunctional metasurface based on spoof surface plasmon polaritons.

In this paper, we propose the design of a metasurface that can achieve three functions in different frequency bands. The proposed metasurface is composed of two kinds of unit cells which are designed on the basis of the spatial k-dispersion engineering of spoof surface plasmon polaritons (SSPPs). By arranging these two kinds of unit cells in the chessboard configuration, the three functions of transmission, anomalous refraction and absorption can be integrated into one metasurface. High transmission and strong absorption can be achieved in 2.0-9.0 GHz and 12.6-20.0 GHz, respectively. Meanwhile, anomalous refraction can be achieved in 10-11.7 GHz due to forward scattering cancellation of two unit cells. To verify the design, a prototype was fabricated and measured. The measured results are consistent with the simulation ones. The metasurface can integrate multiple functions into one aperture and therefore has potential application values in multifunctional microwave devices such as shared-aperture antennas, etc.

Superfocusing plate of terahertz waves based on a gradient refractive index metasurface

A new scenario to realize superfocusing of terahertz waves based on a gradient index (GRIN) metasurface is proposed. To design the GRIN material, a robust algorithm is presented and numerically demonstrated to retrieve the constitutive effective parameters (permittivity and permeability) of the metasurface from the measurement of S parameters. In addition, a new kind of unit cell with a simple structure is designed to verify the theory of effective parameters. By computing the effective refractive index of the unit cell using the algorithm, we design a plate with a thickness of approximately 0.11λ0 (free-space wavelength) at 0.967 THz to superfocus terahertz waves in one dimension. We also revise the theory of superfocusing to make full use of the coupling effect among the cells in the plate and are able to demonstrate it. Comprehensive simulations of focusing are provided, and the full width at half-maximum beam width reaches up to 0.167λ0 at a distance of 0.2λ0. The focusing depth can be further enhanced by decreasing the cell size of the metasurface. This work is beneficial for metasurface design, super-resolution imaging, and other applications in the near field.

The ice-like vortex states in a nanostructured superconducting film with a dice lattice of elongated antidots

We study the degeneracy of the superconducting vortex ground state in a dice lattice of elongated antidots within the Ginzburg-Landau (GL) theory. It is found there are two kinds of unit cells in this dice lattice, namely z6 unit cell where six antibars meet and z3 unit cell where three antibars meet. This can result in more complicated vortex arrangements in the unit cells than that in the kagome lattice. We demonstrate that the vortex patterns at specific fractional magnetic fields can be determined by the local constraints, which presented the ice-like vortex configuration based on the degeneracy rules. This is the reason that the vortex system follows the rule of “two vortices in, one vortex out” or “two vortices out, one vortex in” at each vertex of z3 unit cell. The so-called degeneracy rules are explored by considering nearest-neighbor interaction. Thus, it is possible to realize a rich variety of ice-like vortex configurations in a large area of the sample.

Progressive tensile damage simulation and strength analysis of three-dimensional braided composites based on three unit-cells models

In order to explore the micro-failure mechanism and predict tensile strength of three-dimensional braided composites, the three unit-cells models, namely interior cell, surface cell and corner cell, are established to simulate progressive damage of these materials. Macro model is firstly created and divided into three kinds of unit cells by their periodical distributions. A criterion is approached to determine damage and its pattern of each element, and stiffness degradation is implemented for the damaged elements with geometric damage theory. Periodical boundary conditions are applied on the models to calculate micro-stress and damage propagation is simulated with the increase of load. Each type of damage and its percentage is obtained by simulation and micro-failure mechanism is analyzed. Furthermore, the tensile strengths are predicted from calculated stress–strain curves. From simulation, composites with large braiding angle have more complicated micro-failure mechanism than composites with small braiding angle. It is also observed that there are more damages in surface cell than in interior cell and the damage types in the surface cell are various. The predicted results on the three unit-cells models agree well with the experimental data and are more accurate than only using an interior-cell model.

Plasmonic Luneburg lens antenna synthesized by metasurfaces with hexagonal lattices

The metasurface lens composing of 2D periodic metallic patches on a grounded dielectric slab possesses several advantages such as being light, low-profile, compact, and also cheap to manufacture. In this paper, a Luneburg lens synthesized by a metasurface and designated for use as a surface wave antenna is proposed. Two types of unit cell will be compared, one whose surface wave modal dispersion varies significantly with the grazing direction and another that does not. In the context of being applied as surface wave antennas, it will be shown that the Luneburg lens synthesized by the latter kind of unit cell provides improved performance as compared to the former. Several aperture sub-efficiencies of the metasurface-based Luneburg-lens antenna shall be used for the characterization of the radiation. A prototype of the designed lens antenna has also been manufactured. Measurement results agree well with theoretical predictions and the efficacy of this device over a fairly wide bandwidth has been experimentally demonstrated.

One-Bit Digital Coding Broadband Reflectarray Based on Fuzzy Phase Control

In this letter, we propose a new 1-bit digital coding broadband reflectarray design based on fuzzy phase control. To realize the coding reflectarray, two kinds of unit cells with 0 and π phase responses to mimic “0” and “1” elements for 1-bit digital coding are introduced. By coding the sequence of “0” and “1” elements, the electromagnetic waves can be manipulated to reach different functionalities. Theoretical rules of fuzzy phase control and coding are deduced and used to design a broadband reflectarray antenna. A prototype of a practical configuration is designed, fabricated, and measured to validate the theoretical analysis in X-band. Full-wave simulations and experiments clearly demonstrate the proposed antenna has the −1-dB bandwidth of 25% from 8.4 to 10.8 GHz. The measured results are in good agreement with the theoretical designs and simulated results, which verify the feasibility and correctness of the proposed approach in this letter.

Microstructural modeling of three-dimensionally full five-directional braided composites based on a new multiunit cell scheme

In this study, a new multiunit cell model is proposed based on the microstructure analysis of three-dimensionally full five-directional braided composites produced by four-step 1 × 1 procedures. The new multiunit cell model consists of five kinds of unit cells, namely interior, exterior surface, interior surface, exterior corner, and interior corner unit cells, which are developed by control volume method to characterize the unique microstructure features for different regions of three-dimensionally full five-directional braided composites. The relationships between the microstructure parameters of unit cells and the braiding process parameters are analyzed in detail and the structural parameters of the preform are derived. Especially, the squeezing condition of the yarns in the interior region is studied. Finally, the main microstructure parameters of braided specimens are calculated to validate the effectiveness of the multiunit cell model. Good agreement has been obtained between the predicted values and the available experimental data. In addition, the variation of the volume proportion of five kinds of unit cells to the overall specimen with the number of yarn carriers is discussed, respectively. The effect of braiding angle on the squeezing factor of braiding yarn is analyzed. Results indicate that the presented multiunit cell model can be adopted to effectively predict the microstructure and structural parameters of three-dimensionally full five-directional braided composites.

Coding metamaterials, digital metamaterials and programmable metamaterials

Smart materials offering great freedom in manipulating electromagnetic radiation have been developed. This exciting new concept was realized by Tie Jun Cui and co-workers at the Southeast University, China, who developed digital metamaterials consisting of two kinds of unit cells whose different phase responses allow them to act as ‘0’ and ‘1’ bits. These cells can be judiciously arranged in sequences to enable controlled manipulation of electromagnetic waves. This is one-bit coding; higher-bit coding is possible by employing more kinds of unit cells. The researchers developed a metamaterial cell whose binary response can be controlled by a biased diode. By using a field-programmable gate array, they demonstrated that this digital metamaterial can be programmed. Such metamaterials are attractive for controlling radiation beams in antennas and for realizing other ‘smart’ metamaterials.

Microstructure Analysis and Multi-Unit Cell Model of Three Dimensionally Four-Directional Braided Composites

In this paper, a new multi-unit cell model of three dimensionally braided composites is presented on the basis of the microstructure analysis of 3D braided preforms produced by four-step 1 × 1 method. According to a new unit cell partition scheme, the multi-unit cell model possesses five kinds of unit cells, namely interior, exterior surface, interior surface, exterior corner and interior corner unit cells. Each type of the representative volume cell has unique microstructure and volume fraction in braided composites. On the basis of these five unit cell models, the structural geometry parameters of the preforms are analyzed and the relationship between the structural parameters and the braiding parameters in different regions are derived in detail, such as the braiding angles, fiber volume fraction, yarn packing factor, braiding pitch and so on. Finally, by using the multi-unit cell model, the main structural parameters of braided composites specimens are calculated to validate the effectiveness of the model. The results are in good agreement with the available experimental data. In addition, the effect of braiding angle on the squeezing condition of braiding yarn is analyzed. The variations of the volume proportion of five unit cells to the whole specimen with rows and columns are discussed, respectively. The presented multi-unit cell model can be adopted to design 3D braided composites and predict their mechanical properties.

Diffraction by nanocrystals

X-ray femtosecond nanocrystallography is a new, potentially powerful technique for imaging biological macromolecules that uses ensemble-averaged measurements of diffraction of x-ray free-electron laser pulses from nanocrytalline specimens. Nanocrystals have some diffraction characteristics that are distinct from those of macroscopic crystals, due to the presence of different kinds of unit cell in the crystal and of truncated unit cells on the crystal surface. Expressions are derived for diffraction by nanocrystals with variable and incomplete unit cells, averaged over a distribution of crystal sizes and shapes. The diffraction contains differently modulated Bragg components that are due to interference effects within and between the full and incomplete unit cells. Estimates are obtained for the relative magnitudes of the components. The nature of the diffraction is illustrated by two-dimensional simulations. Implications for molecular imaging are discussed.

A new analytical model for calculation of stiffness of three-dimensional four-directional braided composites

The present paper represents a new analytical method for calculation of the stiffness of three-dimensional four-directional braided composites. In most previous works, the analytical approach had been largely neglected in the favor of the finite element model. Among those who have used the analytical model, the braided preform has been considered as made of one, or three, types of representative unit cells, while microscopic evidence of the microstructure of preforms reveals different configurations of the yarns in the interior, surface and corner regions of a braided preform. This paper presents a Multi-Unit Cell Model in which four kinds of unit cells, namely interior, interior surface, exterior surface and corner unit cells have been introduced as representative cells. Each type of unit cell in a braided composite possesses unique mechanical properties and has been considered as a uni-directional composite. Using rotation matrices, the angle between yarns and longitudinal direction has been incorporated in general coordinates of the model. Finally, using a volume averaging method, the total stiffness of the braided composite is calculated. The results are in good agreement with the available experimental data. The effect of braiding angle on the stiffness of braided composites is also examined.

Theoretical Analysis of the Influence of the Electron Pulse Modification on Al-Cu-Mn Alloy Initial Aging Stage with Valence Electron Structure

Statistical values of valence electron structure parameter for phase structures of super saturation solid solutions in the initial aging stage of Al-Cu-Mn alloy were calculated by using the Empirical Electronic Theory in solid and molecules (EET), then the influence of various kinds of unit cells on the aging initial stage of the Al-Cu-Mn alloy subjected to the electron pulse modification was analyzed. The results show that the a-Al-Cu-Mn phase structure unit (cell) with larger atomic distortion provides the driving force for the saturated Al-Cu-Mn alloy to form G.P quickly and results in the increase in the primary nucleus of Al-Cu-Mn alloy; thus the a-Al-Cu-Mn phase structure unit (cell) playes an important role in the improvement of alloy aging. After the application of pulsed electric field into Al-Cu-Mn alloy melt, the movement of atoms is accelerated, and the number of segregation units of Al-Cu-Mn is increased, as a result the number of the second phase is increased; generating the remarkable aging strengthening.