Non-periodic Structures Research Articles

Investigation on Ultra-Compact 2DPC Coarse Wavelength Division Demultiplexer

A two dimensional Photonic Crystal (2DPC) based eight-channel wavelength division de-multiplexer is proposed and designed for Coarse Wavelength Division Multiplexing (CWDM) applications. The circular ring resonator, channel selector, circulator rod, L bend waveguide and linear bus waveguide are essential parts of the proposed system. The system’s functional parameters such as Transmission efficiency, resonant wavelength, spectral width, channel spacing, Quality factor and crosstalk investigated this paper. The eight different wavelengths of channels are filtered out by altering the size of channel selector rod, setting the radius of the circle shaped cavity and relative refractive index of circulator rod. Initially the Photonic Band gap (PBG) is manipulated by applying Plane Wave Expansion (PWE) method of the 2DPC structure. The functional parameters are analysed by Finite Difference Time Domain (FDTD) method in periodic and non-periodic structure of the proposed system to arrive normalized transmission spectrum. The resonant wavelengths of designed eight paths of the device are varying from 1420nm to 1460nm with average spectral width and channel spacing are 5.8nm, 5.6nm respectively. The footprint of the device is 286.84μm2. Hence, this small device can be implemented for CWDM systems in Photonic Integrated Circuits (PIC)

Electronic Quantum Materials Simulated with Artificial Model Lattices.

The band structure and electronic properties of a material are defined by the sort of elements, the atomic registry in the crystal, the dimensions, the presence of spin–orbit coupling, and the electronic interactions. In natural crystals, the interplay of these factors is difficult to unravel, since it is usually not possible to vary one of these factors in an independent way, keeping the others constant. In other words, a complete understanding of complex electronic materials remains challenging to date. The geometry of two- and one-dimensional crystals can be mimicked in artificial lattices. Moreover, geometries that do not exist in nature can be created for the sake of further insight. Such engineered artificial lattices can be better controlled and fine-tuned than natural crystals. This makes it easier to vary the lattice geometry, dimensions, spin–orbit coupling, and interactions independently from each other. Thus, engineering and characterization of artificial lattices can provide unique insights. In this Review, we focus on artificial lattices that are built atom-by-atom on atomically flat metals, using atomic manipulation in a scanning tunneling microscope. Cryogenic scanning tunneling microscopy allows for consecutive creation, microscopic characterization, and band-structure analysis by tunneling spectroscopy, amounting in the analogue quantum simulation of a given lattice type. We first review the physical elements of this method. We then discuss the creation and characterization of artificial atoms and molecules. For the lattices, we review works on honeycomb and Lieb lattices and lattices that result in crystalline topological insulators, such as the Kekulé and “breathing” kagome lattice. Geometric but nonperiodic structures such as electronic quasi-crystals and fractals are discussed as well. Finally, we consider the option to transfer the knowledge gained back to real materials, engineered by geometric patterning of semiconductor quantum wells.

Beam–Wave Resynchronization Method of the Nonperiodic Meander Line and Folded-Waveguide SWS for TWTs

A beam-wave resynchronization (BWRS) method is proposed by changing the structural parameters of the nonperiodic (NP) slow wave structure (SWS) to reduce the phase velocity of the wave for traveling-wave tubes (TWT) application. The mathematical model and analysis for the BWRS method are introduced to investigate what condition the NP meander line SWS (ML-SWS) and the NP folded-waveguide SWS (FW-SWS) should satisfy so that the corresponding TWT can have a good output performance. As the applications, the NP zigzag ML-SWS and the concentric arc FW-SWS are designed according to the method, which is also compared with that designed according to the phase velocity synchronization (PVS). The simulated results show that, by using the BWRS method, the max output power (efficiency) of the zigzag ML-SWS TWT can be improved from 56.7 (18.9%) to 76.26 W (25.4%), and the bandwidth can be expanded from 6 to 10 GHz in the Ka-band. Furthermore, the max output power (efficiency) of the concentric arc FW-SWS TWT can be improved from 199 (27.6%) to 237 W (32.9%), and the bandwidth can be expanded more than twice in the W-band as before. These results indicate that the BWRS method is beneficial for the improvement of the output power and electron efficiency and may be beneficial for wider bandwidth.

Cold-chain cooked rice with different water contents: Retarded starch digestion by refrigeration

Cold-chain cooked rice is a widely-consumed instant food. While the quality of cooked rice as affected by processing has been widely studied, it remains largely unexplored how concurrent cold-chain conditions (e.g., refrigeration time with specific water contents) tailor the structure and starch digestibility of cooked rice. Here, as shown by combined techniques (e.g., scanning electron microscopy and small angle X-ray scattering), the cold storage (1 to 3 days) of cooked rice at 1.1:1 w/w water-to-rice ratio increased the uniformity of the rice matrix, strengthened the nonperiodic structure, and allowed more B-type starch crystallites and short-range orders. This induced an increase in the slowly digestible starch (SDS) content (from ca. 33.7% to 38.5%) as the refrigeration time rose. In contrast, for cooked rice with 1.5:1 w/w water-to-rice ratio, the cold storage (mainly 1 day) strengthened the matrix uniformity and the nonperiodic structure, and eventually increased the resistant starch (RS) content from ca. 10.3% to 17.7%. The present data could facilitate the design of cold-chain cooked rice with tailored starch digestibility.

Increasing the pH value during thermal processing suppresses the starch digestion of the resulting starch-protein-lipid complexes

To date, how the pH conditions of thermal processing tailor the structure and digestibility of resulting starch-based complexes remains largely unclear. Here, indica rice starch (IRS), stearic acid (SA), and a whey protein isolate (WPI) were used as materials. Increasing the pH value from 4 to 8 during thermal processing (pasting) mainly suppressed the starch digestion of starch-WPI-SA complexes rather than starch-SA counterparts. The starch-SA complexes showed moderate structural changes as the pH value rose, and there was less rapidly digestible starch (RDS) only at pH 8. For the starch-WPI-SA complexes, an increased pH value allowed larger nonperiodic structures and more V-type starch crystallites, with almost unchanged short-range orders but apparently collapsed networks at pH 8. Such ternary complexes displayed more resistant starch (RS) as the pH value rose. The ternary sample at pH 8 contained ca. 29.87% of the RS fractions.

Vibration Localization and Anti-Localization of Nonlinear Multi-Support Beams with Support Periodicity Defect

A response analysis method for nonlinear beams with spatial distribution parameters and non-periodic supports was developed. The proposed method is implemented in four steps: first, the nonlinear partial differential equation of the beams is transformed into linear partial differential equations with space-varying parameters by using a perturbation method; second, the space-varying parameters are separated into a periodic part and a non-periodic part describing the periodicity defect, and the linear partial differential equations are separated into equations for the periodic and non-periodic parts; third, the equations are converted into ordinary differential equations with multiple modes coupling by using the Galerkin method; fourth, the equations are solved by using a harmonic balance method to obtain vibration responses, which are used to discover dynamic characteristics including the amplitude–frequency relation and spatial mode. The proposed method considers multiple vibration modes in the response analysis of nonlinear non-periodic structures and accounts for mode-coupling effects resulting from structural nonlinearity and parametric non-periodicity. Thus, it can handle nonlinear non-periodic structures with a high parameter-varying wave in wide frequency vibration. In numerical studies, a nonlinear beam with non-periodic supports (resulting in non-periodic distribution parameters or periodicity defect) under harmonic excitations was explored using the proposed method, which revealed some new dynamic response characteristics of this kind of structure and the influences of non-periodic parameters. The characteristics include remarkable variation in frequency response and spatial mode, and in particular, vibration localization and anti-localization. The results have potential applications in vibration control and the support damage detection of nonlinear structures with non-periodic supports.

Functionally Graded Non-Periodic Cellular Structure Design and Optimization

Abstract Topological tailoring of materials at a micro-scale can achieve a diverse range of exotic physical and mechanical properties that are not usually found in nature. Modification of material properties through customizing the structural pattern paves an avenue for novel functional products design. This paper explores a non-periodic microstructure design framework for functional parts design with high-strength and lightweight. To address the geometric frustration problem commonly found in non-periodic microstructure designing, we employ a smooth transition layer to connect distinct structural patterns and thus achieve functional gradation among adjacent microstructures. The concept of spatial control points is introduced for the interpolation of this transition layer. To achieve a high-strength macro-structural performance for designing functional parts, we formulate the control points as the design variables and encapsulate them into a macro-structural design optimization problem. Given that our objective function involves expensive finite element (FE) simulations, a Bayesian optimization scheme is exploited to address the computational challenge brought by the FE simulation. Experimental results demonstrate that the proposed design framework can yield both functionally graded lightweight structures and high-strength macro-mechanical performance for the designing parts. The compatibility issue of non-periodic microstructure design is well-addressed. Comparative studies reveal that the proposed framework is robust and can achieve superior mechanical performance to design functional parts with spatially varying properties.

Design Method of the Non-Periodic Meander Line and Folded-Waveguide SWS for TWTs Based on the Effective Phase Velocity Synchronization

A design method of the non-periodic meander line slow-wave structure (ML-SWS) and folded-waveguide slow-wave structure (FW-SWS) for traveling-wave tubes (TWTs) is proposed based on effective phase velocity synchronization. For the non-periodic ML-SWS and FW-SWS, the unified effective axial phase velocity seen by electrons, the dispersion equation, and the ideal beam-wave synchronization voltage are derived. The theoretical results demonstrate that the non-periodic ML-SWS and FW-SWS reported to date are consistent with the proposed method. Furthermore, two examples are presented as applications for the proposed method to the non-periodic SWS design, including the concentric arc FW-SWS and the zigzag ML-SWS. In addition, the simulated results of the two examples are in good agreement with the theoretical ones.

Acoustical properties of spinodoid porous structures

Recently, Kumar et al. [1] have proposed a new class of non-periodic cellular structures called spinodoid (spinodal-like) metamaterials. Spinodal decomposition is a diffusion-driven phase transformation mechanism by which a multiphase solution separates into distinct phases, separated by surfaces with smooth, non-intersecting, and non-periodic topologies. Modifying the classical linear Cahn-Hillard equation—which describes this spinodal decomposition process—by biasing the sampled phase field in a controlled manner, Kumar et al. have provided a numerically efficient method for designing spinodal-like structures. Here, we use this method to generate porous structures with spinodoid topologies and investigate their acoustical properties. The designed spinodoid structures are fabricated using an additive manufacturing process and experimentally studied using a normal incidence impedance tube setup. We investigate the effect of various topological parameters on the acoustical properties, including the effects of through-thickness gradients with finite transition layers. Our results show that the sound absorption and transmission loss properties of such structures can be conveniently altered by altering their cellular anisotropy, making them a suitable candidate for multifunctional applications that require simultaneous high stiffness, high sound absorption performance. [1] S. Kumar, S. Tan, L., Zheng, and D. M. Kochmann, “Inverse-designed spinodoid metamaterials,” npj Comput. Mater. 6(1), 1–10 (2020).

Effect of coordination on topological phases on self-similar structures

Topologically nontrivial phases have recently been reported on self-similar structures. Here we investigate the effect of local structure, specifically the role of the coordination number, on the topological phases on self-similar structures embedded in two dimensions. We study a geometry dependent model on two self-similar structures having different coordination numbers, constructed from the Sierpinski gasket. For different nonspatial symmetries present in the system, we numerically study and compare the phases on both structures. We characterize these phases by the localization properties of the single-particle states, their robustness to disorder, and by using a real-space topological index. We find that both structures host topologically nontrivial phases and the phase diagrams are different on the two structures. This suggests that, in order to extend the present classification scheme of topological phases to nonperiodic structures, one should use a framework which explicitly takes the coordination of sites into account.

Monte Carlo study of multiple scattering of steady-state waves in a one-dimensional random multi-layer medium

An analytical solution of the one-dimensional wave propagation is proposed to predict the steady-state SH wave behaviour in a one-dimensional random multi-layer medium with both periodic and non-periodic structures. A finite element model sandwiched by non-reflective boundary is also proposed and used to verify the analytical solution. Monte Carlo studies are conducted to demonstrate the influence of randomness of segment length on the wave propagation behaviour and the band structure. A macroscopic elastic model with structural damping is proposed to describe the homogenized steady-state wave behaviour in the multi-layer medium.

Carrier localization and miniband modeling of InAs/GaSb based type-II superlattice infrared detectors

Microscopic features of carrier localization, minibands, and spectral currents of InAs/GaSb based type-II superlattice (T2SL) mid-infrared detector structures are studied and investigated in detail. In the presence of momentum and phase-relaxed elastic scattering processes, we show that a self-consistent non-equilibrium Green’s function method within the effective mass approximation can be an effective tool to fairly predict the miniband and spectral transport properties and their dependence on the design parameters such as layer thickness, superlattice periods, temperature, and built-in potential. To benchmark this model, we first evaluate the band properties of an infinite T2SL with periodic boundary conditions, employing the envelope function approximation with a finite-difference discretization within the perturbative eight-band framework. The strong dependence of the constituent material layer thicknesses on the band-edge positions and effective masses, offers a primary guideline to design performance-specific detectors for a wide range of operation. Moving forward, we demonstrate that using a finite T2SL structure in the Green’s function framework, one can estimate the bandgap, band-offsets, density of states and spatial overlap which comply well with the results and the experimental data. Finally, the superiority of this method is illustrated via a reasonable estimation of the band alignments in barrier-based multi-color non-periodic complex T2SL structures. This study, therefore, provides deep physical insights into the carrier confinements in broken-gap heterostructures and sets a perfect stage to perform transport calculations in a full-quantum picture.

Inverse-design of non-Hermitian potentials for on-demand asymmetric reflectivity.

We propose a genetic algorithm-assisted inverse design approach to achieve 'on- demand' light transport in periodic and non-periodic planar structures containing dielectric and gain-loss layers. The optimization algorithm efficiently produces non-Hermitian potentials from any arbitrarily given real (or imaginary) permittivity distribution for the desired frequency selective and broadband asymmetric reflectivity. Indeed, we show that the asymmetric response is directly related to the area occupied by the obtained permittivity distribution in the complex plane. In particular, unidirectional light reflection can be designed in such a way that it switches from left to right (or vice versa) depending on the operating frequency. Moreover, such controllable unidirectional reflectivity is realized using a stack of dielectric layers while keeping the refractive index and gain-loss within realistic values. We believe this proposal will benefit the integrated photonics with frequency selective one-way communication.

Research on Structure Data Link Detection Method Based on Internet of Things System

In order to improve the fault diagnosis and analysis capabilities of the cyber-physical system in the Internet of Things, it is necessary to detect and identify the vicious data chain of the cyber-physical system in the Internet of Things. Moreover, to realize the detection and identification of the vicious data link in the cyber-physical system in the Internet of Things, a fast extraction algorithm for the vicious data link of the cyber-physical system in the Internet of Things is proposed in the paper to collect vicious data from the cyber-physical system in the Internet of Things. In addition, the malignant data generated during the factor adjustment of the Internet of Things can be divided into periodic structure and non-periodic structure in structure, and the malignant data sensor network model of the cyber-physical system in the Internet of Things is constructed. Therefore, through simulation experiment analysis, the method proposed in the paper has superior performance in the detection and identification of vicious data links in the cyber-physical system of the Internet of Things.

Examination of structure and optical properties of Ce3+-doped strontium borate glass by regression analysis

Amorphous materials with non-periodic structures are commonly evaluated based on their chemical composition, which is not always the best parameter to evaluate physical properties, and an alternative parameter more suitable for performance evaluation must be considered. Herein, we quantified various structural and physical properties of Ce-doped strontium borate glasses and studied their correlations by principal component analysis. We found that the density-driven molar volume is suitable for the evaluation of structural data, while chemical composition is better for the evaluation of optical and luminescent data. Furthermore, the borate-rich glasses exhibited a stronger luminescence due to Ce3+, indicating a higher fraction of BO3/2 ring and larger cavity. Moreover, the internal quantum efficiency was found to originate from the local coordination states of the Ce3+ centres, independent of composition or molar volume. The comparison of numerical data of the matrix is useful not only for ensuring the homogenous doping of amorphous materials by activators, but also for determining the origin of physical properties.

Using Atom Dynamics to Map the Defect Structure Around an Impurity in Nano-Hematite.

The bulk behavior of materials is often controlled by minor impurities that create nonperiodic localized defect structures due to ionic size, symmetry, and charge balance mismatches. Here, we used transmission electron microscopy (TEM) of atom-resolved dynamics to directly map the topology of Fe vacancy clusters surrounding structurally incorporated U6+ in nanohematite (α-Fe2O3). Ab initio molecular dynamic simulations provided additional independent constraints on coupled U, Fe, and vacancy mobility in the solid. A clearer understanding of how such an apparently incompatible element can be accommodated by hematite emerged. The results were readily interpretable without the need for sophisticated data reconstruction methods, model structures, or ultrathin samples, and with the proliferation of aberration-corrected TEM facilities, the approach is accessible. Given sufficient z-contrast, the ability to observe impurity-vacancy structures by means of atom hopping can be used to directly probe the association of impurities and such defects in other materials, with promising applications across a broad range of disciplines.

Study on Local Chemical Orderings in Li3NbO4-Based Positive Electrode Materials with a Disordered Rock-Salt Structure By a Reverse Monte Carlo Modeling

1. Introduction As a positive electrode material for a lithium-ion battery, lithium-rich transition-metal oxides have drawn much attention due to the high discharge capacity. Among the oxides, we have focused on Li1+xM1-xO2 with a disordered rock-salt structure such as Li3NbO4-based solid solution [1]. It is well known that all the cations occupy a same crystallographic site in the crystals. Due to different ionic radii and electronic states of the cations, however, it can be predicted that local structures (non-periodic structures) around different cation species must be distinguishable, and such hidden strucures must affect Li+ behavior in charge and discharge processes. Therefore, some previous works tried to uncover the local structures [2], but the structures are still under debete. This is because the structures without periodicity do not show prominent Bragg peaks in a diffraction profile.From such background, this work investigated local structures of the Li3NbO4-based materials, which can be also denoted as Li1+x(Nb, M)1-xO2, with the disordered rocksalt structure by a reverse Monte Carlo (RMC) modeling using total scattering data. Especially, we paid special attention on Li1.3Nb0.3Fe0.4O2 and Li1.3Nb0.43Ni0.27O2 as typical materials. Based on the modeling results, we discuss effects of the cation species on the local structures.2. Experimental The Li3NbO4-based materials were synthesized by a conventional solid-state reaction method, according to literature [1]. Metal compositions of the samples were analyzed by an inductively coupled plasma atomic emission spectroscopy, and valences of cations were estimated by X-ray absorption near edge structures (BL14B2, SPring-8). Synchrotron X-ray Bragg profiles were collected with BL19B2 (SPring-8), and the crystal structures (average structures) were refined by a Rietveld method. We also measured neutron and synchrotron X-ray total scattering data at NOVA (J-PARC) and BL04B2 (SPring-8), respectively. By using structure factors S(Q) and reduced pair distribution functions G(r) in addition to the Bragg profiles, we performed the RMC modeling with an RMCProfile software under a bond-valence-sum (BVS) constraint. In the modeling, we optimized cation distributions by swapping the cations.3. Results We made a supercell with 1000 atoms from a refined unit cell, and then performed RMC modeling on Li1.3Nb0.3Fe0.4O2 with the disordered rock-salt structure. As a result, we could obtain an atomic configuration which could reproduce X-ray Bragg profile, neutron G(r), neutron and X-ray S(Q). From the atomic configuration, it was found that a local distortion of NbO6 was larger than those of the others (LiO6 and FeO6) in the material. The large distortion should be induced by a smaller ionic radius of Nb5+ and/or a mismatch of electronic state of Nb5+. Furthermore, it was clarified from a coordination-number analysis that Li tended to be surrounded by Nb considerably in the disordered rock-salt structure. Such a tendency was also observed in an atomic configuration of Li1.3Nb0.43Ni0.27O2. From the results, it can be concluded that Li diffusion is disturbed by a cation with a higher valence in the disordered structure.

Hierarchical combinatorial design and optimization of non-periodic metamaterial structures

Abstract As advanced manufacturing techniques like additive manufacturing have enabled the fabrication of complex geometries, it is worthwhile to explore potential techniques that can be used to design metamaterial structures to achieve desired behaviors or physical properties. Designing the fine-scale geometry of a metamaterial structure to perform a specific behavior defines an inverse problem that is often intractable. To make this inverse problem tractable, most researchers represent metamaterial structures relying on a single-type periodic unit cell that is repeated in regular grids. Such a representation simplifies modeling and simulation tasks but at the cost of conceivably limiting the spectrum of physical properties that can be achieved by using more than one type of unit cells. This paper defines a non-periodic representation via implicit functions that generate two types of unit cells in pairs, which cope with the geometric frustration of interface connection in non-periodic metamaterial design. To solve the inverse problem of non-periodic metamaterial design, we propose a genetic algorithm-based combinatorial optimization scheme to design light-weight and high-strength functional parts by optimizing relevant parameters in the generating functions of non-periodic unit cells.

Electromagnetic metamaterials—from effective media to field programmable systems

Electromagnetic metamaterials are periodic or nonperiodic artificial structures comprising basic elements in subwavelength scales. They can be used to control electromagnetic waves by designing the basic elements and their spatial arrangements. Metamaterial has been a research hotspot in physical and information sciences. In this review, I present the history and main progresses achieved in metamaterial studies conducted in the State Key Laboratory of Millimeter Waves at Southeast University, which were performed in three stages. In the first stage, we improved the developments in effective-medium metamaterials. In particular, we proposed a general theory to accurately describe the effective permittivity and permeability of metamaterials. Based on the general theory, we designed and realized a three-dimensional (3D) wideband ground-plane invisibility cloak, free-space electrostatic invisibility cloak, electromagnetic black hole, electromagnetic illusions, and radially anisotropic zero-index metamaterial for omni-directional radiations and nearly perfect power combinations of source arrays. Moreover, we undertook significant efforts in engineering applications of microwave metamaterials, such as broadband and low-loss 3D transformation-optic lens for wide-angle scanning, 3D planar gradient-index lens for high-gain radiations, and random metasurfaces to reduce radar cross sections. In the second stage, we opened up a new direction of plasmonic metamaterials from the viewpoint of microwave transmission lines. We proposed an ultrathin, narrow, and flexible corrugated metallic strip to guide spoof surface plasmon polaritons (SPPs) with small bending and radiation losses, based on which we designed and realized a series of passive SPP devices (such as power divider, coupler, filter, and resonator), active SPP devices (such as amplifier and duplexer) and SPP antennas in the microwave bands. We demonstrated a significant feature of the ultrathin SPP transmission line in overcoming challenges of signal integrity existing in the traditional integrated devices and circuits. We built up a full-SPP wireless communication system with unique properties to remotely transmit two movies in two deep-subwavelength-distance channels exhibiting excellent performance. In the third stage, we established a new area of metamaterial-information metamaterial. We proposed to describe the metamaterial using digital coding from the information science viewpoint, which directly resulted in a new kind of metamaterial-digital coding metamaterial. We theoretically and experimentally illustrated that the digital coding metamaterials comprising digital units can be controlled using different coding sequences to achieve different functions. When the digital state of the coding unit was controlled using a field programmable gate array (FPGA), we realized a field programmable metamaterial that can manipulate electromagnetic waves in real time and generate numerous different functions using a single piece of hardware. The digital coding metamaterial provides a platform to establish a digital space on the physical space and enriches the methodology and theory of metamaterials. We proposed measuring the information capacity of metamaterials using Shannon entropy and presented the convolution and addition theorems on metamaterial platforms, which can be used to simultaneously control the physical properties and digital information. Based on information metamaterials, we established new-architecture microwave imaging systems, wireless communication systems, and other intelligent information systems.

Observation of an Unidentified Phonon Peak in SiGe Alloys and Superlattices Using Molecular Dynamics Simulation

Molecular dynamics was applied to a variety of SiGe alloys and superlattice structures in order to investigate the production of an unidentified phonon mode. The mode in question, observed in the resulting phonon dispersion relations, was produced around 2.5 – 5 THz. The corresponding spectral energy density data for the SiGe alloys produced varying intensities around this region, with a higher intensity tending towards lower thermal conductivity values. The unidentified mode was produced by the alloy and the non-periodic superlattice structures, whereas in the case of the periodic superlattices, the mode is less conspicuous. Regarding the SiGe compound, the mode was not produced. It is concluded that the atomic disorder within the alloys and non-periodic superlattices induce a stronger intensity of the unidentified mode.