Adjacent Unit Cells Research Articles

Metal-graphene hybridized plasmon induced transparency in the terahertz frequencies.

In this work, metal-graphene hybridized plasmon induced transparency (PIT) is systematically studied in the proposed simple metal/dielectric/graphene system. The PIT effect is the result of the coupling between the bright dipolar modes excited in the graphene regions under the shorter metallic bars and the dark quadrupolar modes excited in the graphene regions under the longer metallic bars. The coupled Lorentz oscillator model is used to help explain the physical origin of the PIT effect. Other than being tuned by the distance and the lateral displacement of the orthogonal metallic bars, the coupling efficiency can be further enhanced by the in-phase coupling or quenched by the out-of-phase coupling between the adjacent unit cells. Reduced barrier thickness will result in the enhancement of the coupling strengths and the scaling down of the device. Finally, we show that the PIT window can be actively tuned by changing the Fermi energy of graphene. The proposed structure has potential applications in actively tunable THz modulators, sensors and filters.

Resolving Rotational Stacking Disorder and Electronic Level Alignment in a 2D Oligothiophene-Based Lead Iodide Perovskite

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) represent diverse quantum well heterostructures composed of alternating inorganic and organic layers. While 2D HOIPs are nominally periodic in three dimensions for X-ray scattering, the inorganic layers can orient quasi-randomly, leading to rotational stacking disorder (RSD). RSD manifests as poorly resolved, diffuse X-ray scattering along the stacking direction, limiting the structural description to an apparently disordered subcell. However, local ordering preferences can still exist between adjacent unit cells and can considerably impact the properties, particularly the electronic structure. Here, we elucidate RSD and determine the preferred local ordering in the 2D [AE2T]PbI4 HOIP (AE2T: 5,5′-bis(ethylammonium)-[2,2′-bithiophene]). We use first-principles calculations to determine energy differences between a set of systematically generated supercells with different order patterns. We show that interlayer ordering tendencies are weak, explaining the observed RSD in terms of differing in-plane rotation of PbI6 octahedra in neighboring inorganic planes. In contrast, the ordering preference within a given organic layer is strong, favoring a herringbone-type arrangement of adjacent AE2T cations. The calculated electronic level alignments of proximal organic and inorganic frontier orbitals in the valence band vary significantly with the local arrangement of AE2T cations; only the most stable AE2T configuration leads to an interfacial type-Ib band alignment consistent with observed optical properties. The present study underscores the importance of resolving local structure arrangements in 2D HOIPs for reliable structure-property prediction. (Less)

Broadband and wide-angle reflective linear polarization converter for terahertz waves

Polarization control of electromagnetic waves has wide applications in the field of communications, imaging, and remote sensing. Recent designs of periodic two-dimensional devices or metasurfaces employed for polarization control are limited in efficiency, bandwidth, and allowable incidence angle. This is attributed to high dissipation in the dielectric material used and to less-optimal device configuration. We propose and experimentally validate a reflective linear polarization converter metasurface with high efficiency, wide bandwidth, and wide acceptance angle in the terahertz regime. Our device is composed of three layers: an array of oriented metallic T-shaped resonators, cyclic olefin copolymer (COC) as a low loss dielectric layer, and a ground plane. For the normal and 45° incidence angles, a fabricated sample shows a bandwidth of 95% and 100%, with the average polarization conversion ratio above 80%, covering a frequency range of 0.38–1.07 and 0.36–1.08 THz, respectively. The wide-angle stability is attributed to a phase difference between a single resonance along the T-shaped resonator and a smooth phase response in the low-loss COC dielectric layer. For broad bandwidth performance, a resonator arm extending to adjacent unit cells introduces the fundamental resonance at a lower frequency, while the packed unit cell size shifts the grating lobe onset to a higher frequency. These design aspects can significantly improve the performance of other metasurfaces operating in any frequency range.

Phase transformations in reduced-activation duplex alloy Fe52Mn30Cr18 under isothermal heat treatment

Reduced-activation Fe-Mn-Cr alloys have been regarded as the candidate structural materials for advanced nuclear systems. The phase stability of one alloy at high temperature is critically required for its application. In this work, a series of phase transformations in a new type reduced-activation duplex alloy Fe52Mn30Cr18 under isothermal heat treatment were investigated. The ferrite phase in the alloy converted to χ phase after the alloy treated at 550°C for 3 h. The cube-on-cube orientation relationship was found between χ phase and the original ferrite phase with the lattice constant ratio of 3:1. The χ phase was eutectoid decomposed into austenite and σ phases with lamellar structures due to the diffusion of Cr atoms at temperatures above 800 °C. According to the evolution of the crystal structure and magnetic properties, the transformation from ferrite to χ phase was considered as atomic rearrangement in the adjacent unit cells, accompanied by the disappearance of ferromagnetism in the alloy. The microhardness results indicated that the χ phase formation could be one of the potential reasons for the intermediate temperature embrittlement in the high Cr Fe-based alloys.

Ultrasensitive tunable terahertz sensor based on five-band perfect absorber with Dirac semimetal.

For non-invasive detection, terahertz (THz) sensing shows more promising performance compared to visible and infrared regions. But so far, figure of merit (FOM) of THz sensor has been exceeding low due to weak radiation and absorption loss. Here, we propose an easily implemented THz sensor based on bulk Dirac semimetal (BDS). The presented structure not only achieves narrowband absorption and dynamic tunability at five perfect absorption bands, but also exhibits excellent sensing performance with a FOM of 813. These fascinating properties can be explained by the combination of the classical magnetic resonance induced by the anti-parallel current, the electric resonance of adjacent unit cells resulting from the air slots at both ends of the absorber, and Mie resonance supported by coating, respectively. Our work can provide a new avenue for the design of multi-band photodetectors and sensors in the future.

Multi-displacement continuum modelling of the metamaterial plate with periodical arranged resonators

• The wave propagation in a metamaterial plate with periodically arranged resonators is studied. • The additional displacement fields are introduced to characterize the response of resonators. • The function variation principle is used to derive the governing equations of Bloch waves. • The propagation along symmetric direction and any oblique direction are both considered. • The influences of interface conditions on the dispersion feature under the oblique propagation are first discussed. Multi-displacement continuum modelling of a two-dimensional (2D) elastic metamaterials plate with periodically arranged local resonator over the surface of the plate is studied in this paper. The additional displacement fields are introduced to model the response of the local resonators. The continuous conditions between the adjacent unit-cells are used to reflect the periodicity of the microstructured continuum and resultantly turned into the constraint conditions between the additional displacement field and the other continuous field. The dispersion features of the multiple-displacement coupled wave propagating along the high symmetrical direction and any oblique direction are both studied numerically. It is found that the multi-displacement coupled waves can be divided into the coupled longitudinal wave and the coupled transversal wave when propagating along the highly symmetric direction but cannot be divided into the coupled longitudinal wave and the coupled transversal wave when propagating along any oblique direction. The effects of boundary conditions on the dispersion of acoustic and optical branches of coupled waves are discussed in detail. At last, the influences of the parameters of resonator on the dispersion feature of the multi-displacement coupled waves are investigated numerically.

Topological plasmonic modes in graphene-coated nanowire arrays

We investigate the topological edge modes of surface plasmonic polaritons (SPPs) in a two-dimensional waveguide array composed of graphene-coated nanowires. The system experiences topological phase transition by tuning the inter-layer spacing between adjacent unit cells. The topological modes emerge at the edge of topological non-trivial nanowire arrays when the intra-layer spacing is larger than the inter-layer spacing. Thanks to the strong confinement of graphene SPPs, the modal wavelength of topological edge modes can be squeezed as small as 1/20 of incident wavelength. The study provides a promising approach to realizing robust light transport beyond diffraction limit.

Tunable Anisotropic Absorption in Hyperbolic Metamaterials Based on Black Phosphorous/Dielectric Multilayer Structures

Black phosphorus (BP) as a promising two-dimensional material with extraordinary optical properties constitutes an excellent building block in multilayer hyperbolic metamaterials. In this work, we design a multilayer structure composed of BP/dielectric layer stacking unit cells patterned on a gold mirror and theoretically demonstrate the tunable anisotropic absorption in the infrared regime. The electric dipole resonance between the adjacent unit cells drives the structure in the critical coupling state, and impedance of the structure matches to that of free space, showing the perfect absorption for one polarization direction, while the impedance mismatch for the other polarization direction leads to only 8.2$\%$ absorption at the same wavelength. The anisotropic absorption response of the proposed structure can be attributed to the intrinsic anisotropy of BP, which exhibits few dependence on the incident angle. Furthermore, we investigate the tunable optical absorption of the proposed structure with the electron doping of BP and the geometric parameters. These results demonstrate great potentials of BP in constituting multilayer hyperbolic metamaterials, and open up avenues in designing anisotropic metadevices with tunable spectral and polarization selectivity in the infrared regime.

New Accurate Subentire-Domain Basis Functions Method for the Analysis of Large-Scale Finite Periodic Structures with Electrically Connected Cells

Accurate subentire-domain (ASED) basis functions method have been proved efficient for solving large-scale finite periodic structures (LFPSs) with electrically isolated elements, based on utilizing ASED basis functions. These basis functions, defined on macrodomains (blocks) including a relatively large number of Rao–Wilton–Glisson (RWG) basis functions, lead to a great reduction in the number of the unknowns and result in the substantial size reduction of the original method of moment matrix. However, the ASED basis functions method cannot obtain an accurate analysis of the LFPSs with electrically connected cells. In this communication, a new ASED basis functions method, based on the ASED basis functions, is proposed to solve the electromagnetic scattering problem of the LFPSs with electrically connected cells. In the new method, the multiple plane waves are employed to excite the $3\times 3$ array and construct more ASED basis functions. Due to the intrinsic continuity of the first $3\times 3$ array problem, the new ASED basis functions method do not request any overlap between adjacent unit cells, or any “connection” basis functions. In addition, the new ASED basis functions method merges the advantage of the multiple PWs on the efficient computation of monostatic radar cross section (RCS). Numerical examples are given to prove the accuracy and efficiency of the new ASED basis functions method including the computation of both bistatic and monostatic RCSs.

Compatibility in microstructural optimization for additive manufacturing

Microstructures with spatially-varying properties such as trabecular bone are widely seen in nature. These functionally graded materials possess smoothly changing microstructural topologies that enable excellent micro and macroscale performance. The fabrication of such microstructural materials is now enabled by additive manufacturing (AM). A challenging aspect in the computational design of such materials is ensuring compatibility between adjacent microstructures. Existing works address this problem by ensuring geometric connectivity between adjacent microstructural unit cells. In this paper, we aim to find the optimal connectivity between topology optimized microstructures. Recognizing the fact that the optimality of connectivity can be evaluated by the resulting physical properties of the assemblies, we propose to consider the assembly of adjacent cells together with the optimization of individual cells. In particular, our method simultaneously optimizes the physical properties of the individual cells as well as those of neighbouring pairs, to ensure material connectivity and smoothly varying physical properties. We demonstrate the application of our method in the design of functionally graded materials for implant design (including an implant prototype made by AM), and in the multiscale optimization of structures.

Enhanced ferro-photocatalytic performance for ANbO<sub>3</sub> (A = Na, K) nanoparticles

In this study, NaNbO₃ with average grain size of ~50 nm and KNbO₃ with average grain size of ~300 nm nanocrystals are prepared by the water-based citrate precursor sol-gel process. However, the KNbO₃ sample exhibits better photocatalytic performance than that of the NaNbO₃ sample by Rh B degradation experiment. By Rietveld refinements and piezoelectric displacement measurements, the KNbO₃ with the space group of Bmm2 is ferroelectric while the NaNbO₃ with the space group of Pbma is antiferroelectric. The polarization-modulated built-in electric fields in the ferroelectric KNbO₃ nanoparticles can efficiently enhance the separation of photo-generated charge carries and thus improve the photocatalytic activity. However, there is no internal electric field in the antiferroelectric grain because of the antiparallel spontaneous polarization in the adjacent unit cell. Therefore, KNbO₃ exhibits better oxidizing ability of organic dyes than NaNbO₃. The ferroelectric KNbO₃ nanoparticles exhibit an optimum photocatalytic performance for a complete degradation of Rh B in 100 min under UV-Vis light irradiation with auxiliary ultrasonic excitation. This study demonstrates that the perovskite-type ferroelectric nanocrystals are potentially to design high-performance catalysts for degradation of contaminant.

Enhanced ferro-photocatalytic performance for ANbO3 (A = Na, K) nanoparticles.

In this study, NaNbO3 with average grain size of ~50 nm and KNbO3 with average grain size of ~300 nm nanocrystals are prepared by the water-based citrate precursor sol-gel process. However, the KNbO3 sample exhibits better photocatalytic performance than that of the NaNbO3 sample by Rh B degradation experiment. By Rietveld refinements and piezoelectric displacement measurements, the KNbO3 with the space group of Bmm2 is ferroelectric while the NaNbO3 with the space group of Pbma is antiferroelectric. The polarization-modulated built-in electric fields in the ferroelectric KNbO3 nanoparticles can efficiently enhance the separation of photo-generated charge carries and thus improve the photocatalytic activity. However, there is no internal electric field in the antiferroelectric grain because of the antiparallel spontaneous polarization in the adjacent unit cell. Therefore, KNbO3 exhibits better oxidizing ability of organic dyes than NaNbO3. The ferroelectric KNbO3 nanoparticles exhibit an optimum photocatalytic performance for a complete degradation of Rh B in 100 min under UV-Vis light irradiation with auxiliary ultrasonic excitation. This study demonstrates that the perovskite-type ferroelectric nanocrystals are potentially to design high-performance catalysts for degradation of contaminant.

Broadband and High-Gain Circularly-Polarized Antenna With Low RCS

In this paper, a wideband circularly polarized (CP) antenna with a low radar cross section (RCS) and high gain properties is investigated. The proposed antenna is based on a combination of the Fabry-Perot cavity (FPC) and sequential feeding technique. The purpose of this antenna is to produce CP with high directive gain over a wide bandwidth while preserving low RCS. The principle of the FPC and resonance is achieved by applying one frequency selective surface (FSS) metasurface. A microstrip slot array operating at the Ka-band, excited by a sequentially rotated feeding network, is designed and fabricated. It is indicated that all the merits mentioned above can be obtained over a broad frequency band by designing a suitable FSS metasurface and modifying the feeding lengths to adjust desirable phase. RCS reduction is realized by 180° ±37° reflection phase variations between adjacent FSS unit cells on the metasurface. The experimental results show that the gain of the antenna with the metasurface is at least 7 dB greater than that of the primary antenna with a peak value approximately 20 dB at 28.5 GHz. In addition, bandwidths of 3-dB gain, impedance (|S11| ≤ -10 dB), and axial ratio ≤3 dB are ranged from 27.5 to 33.5 GHz (19.7%), 26.7 to 34.2 GHz (24.6%), and 26.8 to 33.1 GHz (21%), respectively. The monostatic RCS reduction for a normal incidence is effectively suppressed from 28 to 48 GHz (52%).

A Way to Design the Miniaturized Dual-Stopbands FSS Based on the Topology Structure

The miniaturized frequency selective surface (FSS) obtained by connecting a number of limbs with two flexible adjustable transmission zeros is proposed in this paper. The proposed FSS has two transmission zeros on both sides of the passband. The effects of the geometrical parameters on the bandwidth of transmission zeros and pole are analyzed. Adjusting the numbers of the limbs in the FSS can make a great difference to the ratio of the frequencies of the two working transmission zeros, and the coupling between the adjacent FSS unit cells is used to adjust the ratio of the frequencies of the two working transmission zeros slightly. The simulated results show that the ratio of the frequencies of the two working transmission zeros has wide adjustable range. The effects of the ways to connect the limbs on the ratio of the frequencies of the two working transmission zeros are analyzed. The prototype of the FSS with two limbs is fabricated and tested to verify the feasibility of the proposed method. The measured results show that the FSS designed by the proposed method has good polarization and angel stabilities, and this proves the proposed way is effective. This FSS is a good candidate to isolate the communication systems with two working bands.

Computational description of key spectroscopic features of zeolite SSZ-13.

The catalytic properties of zeolites are intimately linked to the distribution and relative positions of Al atoms and defects in the pore network. However, characterizing this distribution is challenging, in particular when different local Al arrangements are considered. In this contribution we use a combination of first principles calculations and experimental measurements to develop a model for the Al-distribution in protonated SSZ-13. We furthermore apply this model to understand trends in OH-IR, 27Al-NMR and 29Si-NMR spectra. We use a Boltzmann distribution to predict the proton position for a given local Al configuration and show that for each configuration several H positions are occupied. Therefore a multi-peak spectrum in OH-IR vibrational spectroscopy is observed for all Al configurations, which is in line with experimentally measured spectra for zeolites at different Si/Al ratios. From NMR spectroscopy we find that the proton position leads to significant shifts in 27Al-NMR and 29Si-NMR spectra due to the modification of the local strain, which is lost when a uniform background charge is introduced. These findings are supported by experimental measurements. Finally we discuss the shortcomings of the presented model in terms of unit cell size and the impact of adjacent unit cells.

Structure and luminescence of extraframework TiCl62− in Cs+-containing zeolite LTA

Octahedral TiCl62− ions were introduced into zeolite LTA at extraframework positions by allowing TiCl4(g) to react with dehydrated Cs,Na-A. The structure of the product, Ti,Cl,Cs,Na-A, was determined by single-crystal crystallographic technique in the space group $$Pm\bar {3}m$$ (a = 12.228(1) A) and refined to the error index R1 = 0.046. Its composition was confirmed by energy-dispersive X-ray analysis. Octahedral TiCl62− ions center 8% of the large cavities. The TiCl62− ion is stabilized by eight surrounding Cs+ ions to form a Cs8TiCl66+ cluster. Each of these Cl− ions bonds further to an 8-ring Cs+ ion. These 8-ring Cs+ ions bridge between TiCl62− ions of adjacent unit cells to form a Cs11TiCl69+ continuum in the surface volume of the crystal. The X-ray induced luminescence spectrum of the product is a broad band from 360 to 700 nm, peaking at 475 nm. The emission is red-shifted and less intense than those of M,Cl,Cs,Na-A, M = Zr and Hf, but appears to occur by the same self-trapped exciton (STE) mechanism.

On the Computation of the Dispersion Diagram of Symmetric One-Dimensionally Periodic Structures

A critical discussion on the computation of the dispersion diagram of electromagnetic guiding/radiating structures with one-dimensional periodicity using general-purpose electromagnetic simulation software is presented in this work. In these methods, full-wave simulations of finite sections of the periodic structure are combined with appropriate simplifying network models. In particular, we analyze the advantages and limitations of two different combined methods, with emphasis on the determination of their range of validity. Our discussion is complemented with several selected numerical examples in order to show the most relevant aspects that a potential user of these methods should be aware of. Special attention is paid to the relevant role played by the high-order coupling between adjacent unit cells and between the two halves of unit cells exhibiting reflection, inversion, and glide symmetries.

SAR effect reduction using Reject Band Filter arrays for Wi-Fi portable devices

ABSTRACTThis paper suggests a novel 1D Reject Band Filter (RBF) array to prevent the radiation leakage from portable devices around 2.45 GHz. The proposed RBF array is constructed from eight-unit cell elements mounted on the same side of an FR-4 substrate of 1.5-mm thickness. Each two adjacent unit cells are separated with 2 mm. The individual unit cell design is based on fractal-based open-loop resonators of a modified first iteration Minkowski configuration. Such configuration is coupled with an open-stub transmission line to increase the selectivity of the filter stopband response at the frequency of interest. The unit cell performance is evaluated numerically and analytically, and then compared against measurements. Next, the optimal RBF array is fabricated and simulated to evaluate the Specific Absorption Rate (SAR) at 2.45 GHz. The SAR effect on the human thigh tissue is studied numerically to show an excellent reduction from 0.912 to 0.352 W/kg. Finally, the field strength before and after introducing the proposed array underneath a laptop computer, just below the Wi-Fi antenna, is measured and found to be reduced from 497.1 to 171.8 mV/m.

An efficient method for the dynamic responses of periodic structures based on the physical features of the structure and group theory

This paper proposes an efficient numerical method for solving the dynamic responses of one-dimensional and two-dimensional periodic structures. Efficiently solving a system of linear equations is a crucial issue for computation of the dynamic responses of large-scale periodic structures. By using condensation technology and based on the periodic properties of a structure, the scale of the system of linear equations that corresponds to the entire structure is reduced. Based on the properties of the periodic structure, it is strictly proved in mathematics that, within a time step, the external force applied on a local unit cell can cause only the dynamic responses of a limited number of adjacent unit cells. By using this conclusion, the dynamic responses of periodic structures can be converted into the response analysis of some small-scale structures. Then, the dynamic responses of the small-scale structures are converted into the response analysis of the cyclic periodic structures, which can be efficiently solved by using the group theory. Outstanding efficiency and low computational resources are achieved because the dynamic responses for the entire structure are converted into that of some small-scale structures. Numerical examples are presented to illustrate the high efficiency and low computational resources of the proposed method.

Atom-replaced pins in a Y-based superconductor—single-crystalline perovskite structure including both PrBa2Cu3O7−x and YBa2Cu3O7−y

Metal organic deposition using trifluoroacetates (TFA-MOD) provides many uniform superconductors on long metal tapes. The large numbers of long wires have been applied for power grids or superconducting fault current limiters. The related applications worked for a long time without fatal trouble. The quasi-liquid produced during the firing process assisted the perfectly uniform structure in TFA-MOD. On the other hand, when it was desired to introduce artificial pinning centers, the quasi-liquid also enlarged the diameter of the artificial pinning centers to several tens of nanometers. In other words, due to the nature of TFA-MOD, there is very little chance of using TFA-MOD to prepare several nm-sized artificial pinning centers. By proposing atom-replaced pins (ARPs), we aim to overcome the impasse. ARPs are realized by replacing yttrium (Y) with praseodymium (Pr) whose valence number changes from 3+ to approximately 4+. Analytical results suggested that Pr makes pinning centers on a PrBa2Cu3O7−x (PrBCO) unit cell, and the weak-linked superconductivity derived from PrBCO extends to the adjacent unit cells in the a/b-plane. Jc decrease by Pr is five times as large as the volume fraction of Pr in the Y-site of the perovskite structure. On the other hand, Tc does not show large degradation in YBCO including 10% PrBCO. These results suggest that PrBCO unit cells are fully dispersed in YBa2Cu3O7−y matrix. With regard to Jc in the magnetic field, (Y0.98, Pr0.02)BCO has been slightly improved compared with pure YBCO only in the region of high temperature and low magnetic field of less than 1 T.