Symmetric Unit Cell Research Articles

Incorporation of expanded organic cations in dysprosium(III) borohydrides for achieving luminescent molecular nanomagnets

Luminescent single-molecule magnets (SMMs) constitute a class of molecular materials offering optical insight into magnetic anisotropy, magnetic switching of emission, and magnetic luminescent thermometry. They are accessible using lanthanide(III) complexes with advanced organic ligands or metalloligands. We present a simple route to luminescent SMMs realized by the insertion of well-known organic cations, tetrabutylammonium and tetraphenylphosphonium, into dysprosium(III) borohydrides, the representatives of metal borohydrides investigated due to their hydrogen storage properties. We report two novel compounds, [n-Bu4N][DyIII(BH4)4] (1) and [Ph4P][DyIII(BH4)4] (2), involving DyIII centers surrounded by four pseudo-tetrahedrally arranged BH4– ions. While 2 has higher symmetry and adopts a tetragonal unit cell (I41/a), 1 crystallizes in a less symmetric monoclinic unit cell (P21/c). They exhibit yellow room-temperature photoluminescence related to the f–f electronic transitions. Moreover, they reveal DyIII-centered magnetic anisotropy generated by the distorted arrangement of four borohydride anions. It leads to field-induced slow magnetic relaxation, well-observed for the magnetically diluted samples, [n-Bu4N][YIII0.9DyIII0.1(BH4)4] (1@Y) and [Ph4P][YIII0.9DyIII0.1(BH4)4] (2@Y). 1@Y exhibits an Orbach-type relaxation with an energy barrier of 26.4(5) K while only the onset of SMM features was found in 2@Y. The more pronounced single-ion anisotropy of DyIII complexes of 1 was confirmed by the results of the ab initio calculations performed for both 1–2 and the highly symmetrical inorganic DyIII borohydrides, α/β-Dy(BH4)3, 3 and 4. The magneto-luminescent character was achieved by the implementation of large organic cations that lower the symmetry of DyIII centers inducing single-ion anisotropy and separate them in the crystal lattice enabling the emission property. These findings are supported by the comparison with 3 and 4, crystalizing in cubic unit cells, which are not emissive and do not exhibit SMM behavior.

Wide-angle monostatic RCS enhancement using symmetrical periodic structures

In this paper, first the basic principles of monostatic RCS enhancement (RCS ENH) for a structure are established. Then it is shown that for a periodic structure with periodicity larger than half wavelength, according to Fluquet’s theory, the larger the periodicity, the greater the number of incidence angles for which energy is reflected in the same direction, that is, monostatic RCS ENH. Then for a symmetrical patch unit cell, the optimum dimensions are obtained. Finally several symmetrical patch unit cells with different periodicities are arranged symmetrically with respect to the center of structure to achieve monostatic RCS ENH in a wide angle.

Triple-band tunable terahertz perfect absorber based on graphene metamaterial

To further reduce the structural complexity of high-performance modulation devices in the terahertz region, we propose and demonstrate a triple-band tunable metamaterial perfect absorber, which consists of a patterned graphene and Au ground plane spaced by Si dielectric layer. The graphene surface plasmon resonance at the terahertz region is used to couple the patterned graphene with the electric field and provide electric dipole resonance to form multiple absorption peaks. Numerical results indicate that the amplitude of the absorption peaks is large than 99.9% at 0.489 THz, 1.492 THz, and 2.437 THz, respectively. In addition, due to the amplitude of the absorption peaks that can be controlled by the Fermi level via externally applied bias voltage, it means that we are able to switch the structure between reflector and absorber at their corresponding absorption bands. However, the symmetric unit cell structure of the proposed absorber is the inherent reason for the excellent polarization-insensitive, and the broadband absorption of the absorber maintains excellent absorption performance over a wide range of incident angles. Therefore, the designed graphene-based terahertz metamaterial function device has great potential in modulation and sensing.

Equivalent Circuit Models for Metasurfaces Using Floquet Modal Expansion of Surface Current Distributions

This work presents a fully analytical method for obtaining equivalent circuit models (ECMs) that predict the magnitude and phase of scattering parameters for reflectarray (RA) and/or metasurface (MTS) unit cells. The proposed ECM is derived from a Floquet modal expansion of surface currents and does not rely on full-wave simulations. Analytical functions are used to represent the current distribution on the unit cell, and the proposed ECM is validated against full-wave simulations for several RA/MTS element types including loaded loops, dogbone and symmetric cross unit cells, and is shown to be accurate under normal and oblique incidence conditions, thus providing a fast and efficient design tool for deriving the scattering parameters of patch-like unit cells in the initial design process. As this approach allows for modeling unit cells loaded using lumped components such as varactor diodes, it can also be used to derive the dynamic response of tunable RA and MTS unit cells.

Conformal Wireframe Nets for Trimmed Symmetric Unit Cells in Functionally Graded Lattice Materials

Tessellating a periodic unit cell of lattice material to fill a design space in complex geometries has many challenges arising from their computer-aided design (CAD) modeling intricacy. A solution to this difficulty is the use of trimmed micro-truss lattice structures with a conformal net. This paper presents a novel algorithm for constructing conformal lattice net as wireframe of one-dimensional line segments suitable for Bravais cubic symmetric truss-based topologies. The novel algorithm is an excellent candidate when dealing with lattice structures using cubic, body-centered cubic (BCC), face-centered cubic (FCC), and/or diamond unit cell configurations. The wireframe structure is easily transferred into one-dimensional beam elements for microscale optimizations to obtain a functionally graded lattice material. It is shown that introduction of the lattice net resulted in a significant reduction in the mass of the optimized design.

A Multiband Antenna Stacked with Novel Metamaterial SCSRR and CSSRR for WiMAX/WLAN Applications.

This paper presents an innovative method for the design of a triple band meta-mode antenna. This unique design of antenna finds application in a particular frequency band of WLAN and WiMAX. This antenna comprises of a square complimentary split ring resonator (SCSRR), a coaxial feed, and two symmetrical comb shaped split ring resonators (CSSRR). The metamaterial unit cell SCSRR independently gains control in the band range 3.15–3.25 GHz (WiMAX), whereas two symmetrical CSSRR unit cell controls the band in the ranges 3.91–4.01 GHz and 5.79–5.94 GHz (WLAN). This design methodology and the study of the suggested unit cells structure are reviewed in classical waveguide medium theory. The antenna has a miniaturized size of only 0.213 × 0.192 × 0.0271 (20 × 18 × 2.54 mm3, where is the free space wavelength at 3.2 GHz). The detailed dimension analysis of the proposed antenna and its radiation efficiency are also presented in this paper. All the necessary simulations are carried out in High Frequency Structure Simulator (HFSS) 13.0 tool.

Chiral Bilayer All-Dielectric Metasurfaces

Three-dimensional chiral plasmonic metasurfaces were demonstrated to offer enormous potential for ultrathin circular polarizers and applications in chiral sensing. However, the large absorption losses in the metallic systems generally limit their applicability for high-efficiency devices. In this work, we experimentally and numerically demonstrate three-dimensional chiral dielectric metasurfaces exhibiting multipolar resonances and examine their chiro-optical properties. In particular, we demonstrate that record high circular dichroism of 0.7 and optical activity of 2.67 × 105 degree/mm can be achieved based on the excitation of electric and magnetic dipolar resonances inside the chiral structures. These large values are facilitated by a small amount of dissipative loss present in the dielectric nanoresonator material and the formation of a chiral supermode in a 4-fold symmetric metasurface unit cell. Our results highlight the mechanisms for maximizing the chiral response of photonic nanostructures and offer important opportunities for high-efficiency, ultrathin polarizing elements, which can be used in miniaturized devices, for example, integrated circuits.

Design and modeling of the 2D auxetic metamaterials with hyperelastic properties using topology optimization approach

In this paper, the modified SIMP topology optimization (TO) method was used, to introduce diverse auxetic unit cells with different volume fractions, composing of the symmetric, chiral and re-entrant unit cells. The output results suggested that the geometry of the 2D auxetic structures were extremely dependent on the values of SIMP method parameters, including initial designs and volume fractions. The brilliant modes of deformation in the auxetic structures, whose unit cells are symmetric, are based on either buckling or bending. To validate the TO results, four structures containing symmetric unit cells were chosen, called structure 1, 2, 3 and 4. The auxetic structures were produced by using 3D printing, with TPU as a hyperelastic material as well. Under Compression, the dominating mode of deformation in structures 1 and 2 is based on bending, caused by the movement of the oblique struts between the unit cells. There is also a linear relationship between the vertical and horizontal displacements in structure 1, the structure with 50 % volume fraction and -0.55 Poisson’s ratio value. However, the deformation mechanisms in structures 3 and 4 are based on buckling, called shearing deformation, which is the result of the buckling of the vertical struts between the unit cells.

Fabry–Pérot-Based Compound All-Dielectric Huygens’ Structure for Circularly Polarized Millimeter-Wave Beamforming

An all-dielectric Fabry–Perot compound Huygens’ structure is proposed for beam-forming at the millimeter-wave (mm-wave) band for circularly polarized (CP) waves. The proposed structure is based on a symmetric unit cell in the plane of the structure, which allows for CP wave transmission, and is demonstrated in the near field of a 60 GHz right-hand CP slot array antenna. Full-wave demonstration of a uniform structure shows a negligible effect on the reflection and radiation of the antenna from 59 to 62 GHz. Full-wave simulations were further used to demonstrate a phase-gradient structure for beam-steering from 59 to 61 GHz. Finally, a difference-pattern structure was fabricated and experimentally demonstrated for operation from 61 to 62 GHz, exhibiting low reflection and good axial ratio (AR) performance across the bandwidth.

4.2 V Stack of metaloxide‐polypyrrole‐based composite electrodes and their power management

Polypyrrole (PPy)-based composites containing nanostructured VO x, PbO x, and MnO x are galvanostatically synthesized on exfoliated graphite papers (1.0 and 6.25 cm2) formed as a result of the intercalation with their metals. Both intercalation and synthesis are performed in the synthesis solution containing metal(II)tetrafluoroborate salts, pyrrole monomer, HBF4, H2O, Triton-X 100 (TX100), and carboxymethyl cellulose (CMC). The mass loading of the composite at the electrode is maintained at 10 mg cm−2. Symmetric and asymmetric unit cells are prepared in polyvinyl alcohol (PVA)/H2SO4 (1:1) gel electrolyte using composites synthesized on the treated graphite paper electrode. The electrical properties of these cells are examined by electrochemical impedance spectroscopy and their capacitive properties by cyclic voltammetry and galvanostatic-charge discharge test. The asymmetrical cell, prepared with PPy/PbO x/CMC composite having energy density and cycle life better than others, exhibits 7.77 Wh kg−1 and 0.51 kW kg−1 at a working voltage of 1.4 V, considering the total mass of all sub-components. The three-cell asymmetric stack design, which is prepared using these composite-coated electrodes, achieves a coulombic efficiency of 76% at 4.2 V. Finally, an electronic control system (DC/DC buck-boost converter) is designed and implemented to manage the charge and discharge cycles of single-cell and bipolarly connected stack to operate the asymmetric supercapacitor arrays with high performance.

Optical states in a 1D superlattice with multiple photonic crystal interfaces

Interface states in a 1D photonic crystal heterostructure with multiple interfaces are examined. The heterostructure is a periodic network consisting of two different photonic crystals. In addition, the two crystals themselves are periodic, with one being made of alternating binary layers and the other being a quaternary crystal with a tunable layer. The second crystal can thus be smoothly transformed from one binary crystal to another. All individual photonic crystals in the superstructure have symmetric unit cells, as well as identical periods and optical path lengths. Therefore, as the tunable layer in the quaternary crystal expands, other layers will shrink. It is found that the behavior of the localized modes in the band gaps is dependent on whether there is an even or odd number of interfaces in the heterostructure. With certain sequences of all dielectric photonic crystals, topological states are shown to split in two, whereas for other heterostructures they are shown to vanish. Additional resonant Tamm states appear depending on how many crystals are in the heterostructure. If the tunable layer is frequency dependent, the band gap can still support topological/resonant modes with some band gaps even supporting two separate groups.

Giant Goos-Hänchen shift with a high reflectance assisted by interface states in photonic heterostructures

We achieve interface states with a high reflectance in photonic heterostructures composed of two kinds of all-dielectric one-dimensional photonic crystals (1DPCs) with symmetric unit cells. The reflectance of interface states can be controlled flexibly by tuning the mismatch degree between the imaginary phases of the two kinds of 1DPCs. Assisted by the interface state with a high reflectance as well as a high-$Q$ factor, the Goos-H\"anchen (GH) shift can be greatly enhanced to larger than three orders of wavelengths. Our work provides a route, completely under the current experimental conditions, to design high-performance sensitive sensors, wavelength division demultiplexers, and polarization beam splitters based on this giant GH shift together with a high reflectance.

X Wave Radiator Implemented With 3-D Printed Metamaterials

A radiator is presented, capable of generating paraxial X waves over a 50% fractional bandwidth (7.5–12.5 GHz) in its radiative near-field. X waves are localized pulses formed by the superposition of Bessel beams with a common cone angle. Quasiconformal transformation optics, a 2-D variant of transformation optics, is employed to find the inhomogeneous, isotropic dielectric constant profile needed to convert the radiation of a monopole into a paraxial Bessel beam. An impedance matching layer is applied to reduce reflections at the air-dielectric interface of the device. The transformation and impedance matching regions are implemented using rotationally symmetric metamaterial unit cells that yield a spatially varying effective dielectric constant. The resulting design is fabricated through 3-D printing, by combining parts made from three low-loss dielectric filaments. The device is experimentally measured, and shows good agreement with simulations. Its ability to generate paraxial X waves when excited by a broadband pulse is verified.

Dynamically tunable multi-channel and polarization-independent electromagnetically induced transparency in terahertz metasurfaces

Switchable multi-channel electromagnetic induced transparency (EIT) is achieved in designed terahertz metasurfaces. The underlying physics of the EIT effect is analyzed. The influence of structure parameters on the EIT effect is investigated. A metasurface with a symmetrical unit cell is proposed, by which polarization-independent EIT is achieved. In addition, photosensitive silicon is integrated into the metal layer aiming for active modulation. Tunable EIT with adjustable peak frequency and amplitude is obtained. Moreover, the dynamic control is not limited to a single EIT transparency window. We show that the number of EIT transparency windows can be tailored, and switchable multi-channel EIT are successively realized. The results would provide significant guidance in multifunctional active devices, such as modulators and switches in nanophotonics optics.

Design of 3D broad-band and wide-angle absorber based on resistive metamaterial and magnetic absorbing material

A 3D broad-band and wide-angle absorber (3D BWA) was designed through a 3D resistive metamaterial in which a traditional magnetic absorbing material was added on top of the metallic backplane. Owing to the better magnetic loss performance of the magnetic absorber at lower frequencies and superior impedance matching effect and ohmic loss property of the resistive films at higher frequencies, the absorption can be extended to a wider band through their combination. Simulation result shows that the absorption of the 3D BWA is more than 90% in the frequency range of 3.7–40 GHz at normal incidence, and it is in accordance with the experimental result. The absorption performance for each part of the 3D BWA was analyzed using electric field and magnetic field distributions, surface current distributions and Smith chart. For TE polarized waves of 3.7–40 GHz, the absorption with the incident angles of 0°–60° is more than 77%. The absorption higher than 80% can be achieved under TM polarized waves of 4.2–40 GHz as the incident angle changes from 0° to 60°. When the incident angle is 70°, the absorption is greater than 85% in the frequency range of 5.7–40 GHz. Owing to the four-fold rotationally symmetric unit cell structure, the absorption spectrum under different polarization angles remains unchanged. These two characteristics make the designed absorber a good candidate in stealth applications.

Surface acoustic waves in one-dimensional piezoelectric phononic crystals with symmetric unit cell

The paper studies the existence of surface acoustic waves in half-infinite one-dimensional piezoelectric phononic crystals consisting of perfectly bonded layers, which are arranged so that the unit cell is symmetric, i.e., is invariant with respect to inversion about its midplane. An example is a bilayered structure with exterior layer being half thinner than the interior layers of the same material. The layers may be generally anisotropic. The maximum possible number of surface acoustoelectric waves referred to a fixed wave number and a given full stop band is established for different types of electric boundary conditions at the mechanically free or clamped surface. In particular, it is proved that the phononic crystal-vacuum interface can support two surface waves in any full stop band. The same statement holds true in the case of a metallized surface of the crystal. This number is greater than that in a purely elastic case. In the presence of crystallographic symmetry, which decouples the sagittally and horizontally polarized surface waves, their separate admissible numbers are obtained. It is shown that the propagation along the normal to the surface is a special case, where the maximum number of surface waves is less than that along oblique directions.

Interfacial acoustic waves in one-dimensional anisotropic phononic bicrystals with a symmetric unit cell.

The paper is concerned with the interfacial acoustic waves localized at the internal boundary of two different perfectly bonded semi-infinite one-dimensional phononic crystals represented by periodically layered or functionally graded elastic structures. The unit cell is assumed symmetric relative to its midplane, whereas the constituent materials may be of arbitrary anisotropy. The issue of the maximum possible number of interfacial waves per full stop band of a phononic bicrystal is investigated. It is proved that, given a fixed tangential wavenumber, the lowest stop band admits at most one interfacial wave, while an upper stop band admits up to three interfacial waves. The results obtained for the case of generally anisotropic bicrystals are specialized for the case of a symmetric sagittal plane.

A Low-Profile Design of Polarization Rotation Reflective Surface for Wideband RCS Reduction

A novel compact ultrawideband polarization rotation reflective surface (PRRS) with a high polarization conversion ratio (PCR) is proposed. The 90° polarization rotation (PR) is achieved by employing a PRRS-based diagonally symmetric unit cell including two L-shaped strips with two vias and a centered via-less square patch. The air-based substrate of the structure and its overall dimensions equal to 6 × 6 × 3.3 mm3 (i.e., $0.128{\boldsymbol{\lambda \times 0.128{\boldsymbol{\lambda \times 0.07{\boldsymbol{\lambda }_0} $ based on the lowest frequency of the band) provide a low-profile PRRS with a considerable reduction in thickness. The PR bandwidth of the proposed PRRS is 111% with a PCR over 90%. This PRRS is also applied for radar cross section reduction (RCSR), which provides a measured 10 dB RCSR in an ultrawide bandwidth of about 114.4% (6.4∼23.5 GHz, covering C-, X-, Ku-, and K-bands). A good agreement between the simulated and measured results and a fair comparison with the state-of-the-art approaches prove the accuracy and efficiency of the proposed design.

Topological design of 3D phononic crystals for ultra-wide omnidirectional bandgaps

As a unique kind of artificial periodic composite materials, phononic crystals have attracted the wide attention in recent years. Unremitting efforts have been made to investigate the potential existence of phononic bandgaps, which can be used to manipulate the propagation of acoustic/elastic waves. This paper proposes a new topology optimization algorithm to achieve the optimised bandgap structures of 3D phononic crystals based on fixed-grid finite element method (FEM) and evolutionary procedure. The maximum normalised gap ratio obtained is as large as 97% between the 12th and 13th order of bands, with the optimised structure of tungsten carbide spheres embraced in epoxy matrix. Symmetric unit cells for simple cubic (SC) lattice are presented for the specified bandgaps to validate the effectiveness of the proposed optimization algorithm for designing 3D phononic bandgap crystals. Meanwhile, the proposed topology optimization represents structures with clear topologies and smooth boundaries.

Wide incidence angle and polarization insensitive dual broad-band metamaterial absorber based on concentric split and continuous rings resonator structure

A new design of a metamaterial absorber (MMA) with two broad-bands is simulated, fabricated and experimentally investigated in this paper. The MMA structure unit cell consists of combination of eight rings with two pair of concentric split rings and two pair of concentric continuous rings placed diagonally opposite. The proposed MMA has x-y symmetrical copper unit cells with middle layer of FR-4 substrate and copper laminated ground plane. The simulated and measured absorption peaks as well as the absorptance of the proposed MMA exhibits insensitivity to change in polarization and incidence angle of electromagnetic waves. The metamaterial behavior of the studied MMA is confirmed by extracting effective permeability, μeff(f), effective permittivity, εeff(f), and effective refractive index, ηeff(f), from the measured S-parameters. The proposed MMA has resonance peaks at 7.6 GHz, 8.9 GHz, 12.3 GHz, and 12.8 GHz with measured absorptance of 94%, 99.9%, 96%, and 96% at respective frequency bands. The first absorption broad-band extends from 7.36 GHz to 9.36 GHz and the second broad-band extends from 11.9 GHz to 13 GHz. The high electromagnetic wave absorptance from 7.36 GHz–9.36 GHz and 11.9 GHz–13 GHz makes this MMA suitable for satellite, mobile and radar applications.