Athanasios N. Papadimopoulos, Stamatios A. Amanatiadis, Nikolaos V. Kantartzis, Theodoros T. Zygiridis and Theodoros D. Tsiboukis
Important statistical variations are likely to appear in the propagation of surface plasmon polariton waves atop the surface of graphene sheets, degrading the expected performance…
Abstract
Purpose
Important statistical variations are likely to appear in the propagation of surface plasmon polariton waves atop the surface of graphene sheets, degrading the expected performance of real-life THz applications. This paper aims to introduce an efficient numerical algorithm that is able to accurately and rapidly predict the influence of material-based uncertainties for diverse graphene configurations.
Design/methodology/approach
Initially, the surface conductivity of graphene is described at the far infrared spectrum and the uncertainties of its main parameters, namely, the chemical potential and the relaxation time, on the propagation properties of the surface waves are investigated, unveiling a considerable impact. Furthermore, the demanding two-dimensional material is numerically modeled as a surface boundary through a frequency-dependent finite-difference time-domain scheme, while a robust stochastic realization is accordingly developed.
Findings
The mean value and standard deviation of the propagating surface waves are extracted through a single-pass simulation in contrast to the laborious Monte Carlo technique, proving the accomplished high efficiency. Moreover, numerical results, including graphene’s surface current density and electric field distribution, indicate the notable precision, stability and convergence of the new graphene-based stochastic time-domain method in terms of the mean value and the order of magnitude of the standard deviation.
Originality/value
The combined uncertainties of the main parameters in graphene layers are modeled through a high-performance stochastic numerical algorithm, based on the finite-difference time-domain method. The significant accuracy of the numerical results, compared to the cumbersome Monte Carlo analysis, renders the featured technique a flexible computational tool that is able to enhance the design of graphene THz devices due to the uncertainty prediction.
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Christos Salis, Nikolaos Kantartzis and Theodoros Zygiridis
Random media uncertainties exhibit a significant impact on the properties of electromagnetic fields that usually deterministic models tend to neglect. As a result, these models…
Abstract
Purpose
Random media uncertainties exhibit a significant impact on the properties of electromagnetic fields that usually deterministic models tend to neglect. As a result, these models fail to quantify the variation in the calculated electromagnetic fields, leading to inaccurate outcomes. This paper aims to introduce an unconditionally stable finite-difference time-domain (FDTD) method for assessing two-dimensional random media uncertainties in one simulation.
Design/methodology/approach
The proposed technique is an extension of the stochastic FDTD (S-FDTD) scheme, which approximates the variance of a given field component using the Delta method. Specifically in this paper, the Delta method is applied to the locally one-dimensional (LOD) FDTD scheme (hence named S-LOD-FDTD), to achieve unconditional stability. The validity of this algorithm is tested by solving two-dimensional random media problems and comparing the results with other methods, such as the Monte-Carlo (MC) and the S-FDTD techniques.
Findings
This paper provides numerical results that prove the unconditional stability of the S-LOD-FDTD technique. Also, the comparison with the MC and the S-FDTD methods shows that reliable outcomes can be extracted even with larger time steps, thus making this technique more efficient than the other two aforementioned schemes.
Research limitations/implications
The S-LOD-FDTD method requires the proper quantification of various correlation coefficients between the calculated fields and the electrical parameters, to achieve reliable results. This cannot be known beforehand and the only known way to calculate them is to run a fraction of MC simulations.
Originality/value
This paper introduces a new unconditional stable technique for measuring material uncertainties in one realization.
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Christos P. Exadaktylos, Dimitrios I. Karatzidis, Theodoros T. Zygiridis and Nikolaos V. Kantartzis
A class of robust and efficient beamforming methods is developed in this paper for the optimised design of realistic microstrip antennas on arbitrarily curved substrates. More…
Abstract
Purpose
A class of robust and efficient beamforming methods is developed in this paper for the optimised design of realistic microstrip antennas on arbitrarily curved substrates. More specifically, this paper aims to focus on the formulation of an effective and computationally light beamforming algorithm and its implementation on a novel realistic cylindrical-substrate microstrip array antenna with significantly decreased size, wideband operation and enhanced radiation characteristics.
Design/methodology/approach
The proposed multi-parametric schemes introduce an efficient null-steering concept, which drastically annihilates the undesired beamformer waveform artefacts, while retaining the real output signal undistorted. In particular, the key objective is the accurate calculation of the appropriate complex feeding weights, required to set nulls along the propagation directions of the unwanted signals and a maximum towards the propagation direction of the desired incoming signal. The featured technique, combined with a modified finite element method, is applied to the design of a new family of cylindrical-substrate microstrip array antennas.
Findings
Numerical results, mainly concerning customisable three-dimensional radiation patterns and attributes, certify the merits of the algorithm and its limited system demands. The introduced beamforming algorithms are applied to a variety of different inputs (desired radiation patterns), which indicate that the designed cylindrical-substrate antenna overwhelms existing designs in terms of computational cost for the beamforming algorithm, while retaining acceptable values for radiation characteristics, such as gain, directivity and side-lobe suppression. In this manner, the effectiveness of the prior methodology and the benefits of this newly shaped array antenna are comprehensively revealed and substantiated.
Originality/value
Rigorous beamforming techniques in conjunction with a class of contemporary array antennas are developed for potential use in high-end communication systems, such as 5G configurations. The proposed cylindrical-shaped structures are systematically designed, with an emphasis on space efficiency and wideband radiation effectiveness to offer fully adjustable setups. To this aim, the cylindrical-substrate microstrip antenna, because of its inherent azimuthal symmetry and confined overall dimensions, provides reliable operation and promising performance.
Details
Keywords
Dimitrios I. Karatzidis, Theodoros T. Zygiridis and Nikolaos V. Kantartzis
The purpose of this paper is to present a family of robust metasurface-oriented wireless power transfer systems with improved efficiency and size compactness. The effect of…
Abstract
Purpose
The purpose of this paper is to present a family of robust metasurface-oriented wireless power transfer systems with improved efficiency and size compactness. The effect of geometric and structural features on the overall efficiency and miniaturisation is elaborately studied, while the presence of substrate losses is, also, considered. Moreover, to further enhance the performance, possible means for reducing the operating frequency, without comprising the unit-cell size, are proposed.
Design/methodology/approach
The key element of the design technique is the edge-coupled split-ring resonators patterned in various metasurface configurations and optimally placed to increase the total efficiency. To this goal, a rigorous three-dimensional algorithm, launching a new high-order prism macroelement, is developed in this paper for the fast evaluation of the required quantities. The featured scheme can host diverse approximation orders, while it is drastically more economical than existing methods. Hence, the demanding wireless power transfer systems are precisely modelled via reduced degrees of freedom, without the need to conduct large-scale simulations.
Findings
Numerical results, compared with measured data from fabricated prototypes, validate the design methodology and prove its competence to provide enhanced metasurface wireless power transfer systems. An assortment of optimized 3 x 3 and 5 x 5 metamaterial setups is investigated, and interesting deductions, regarding the impact of the inter-element gaps, the distance between the transmitting and receiving components and the substrate losses, are derived. Also, the proposed vector macroelement technique overwhelms typical implementations in terms of computational burden, particularly when combined with the relevant commercial software packages.
Originality/value
Systematic design of advanced real-world wireless power transfer structures through optimally selected metasurfaces with fully controllable electromagnetic properties is presented. The analysis is performed by means of a rapid prism macroelement methodology, which leads to very confined meshes, accurate results and significantly reduced overhead. The selected metamaterial resonators are found to be very flexible and reconfigurable, even in the case of large substrate conductivity losses, whereas their contribution to the system’s total efficiency is decisive.
Details
Keywords
Theodoros Zygiridis and Nikolaos Kantartzis
The computational accuracy and performance of finite-difference time-domain (FDTD) methods are affected by the implementation of approximating derivative formulae in diverse ways…
Abstract
Purpose
The computational accuracy and performance of finite-difference time-domain (FDTD) methods are affected by the implementation of approximating derivative formulae in diverse ways. This study aims to focus on FDTD models featuring material dispersion with negligible losses and investigates two specific aspects that, until today, are usually examined in the context of non-dispersive media only. These aspects pertain to certain abnormal characteristics of coarsely resolved electromagnetic waves and the selection of the proper time-step size, in the case of a high-order discretization scheme.
Design/methodology/approach
Considering a Lorentz medium with negligible losses, the propagation characteristics of coarsely resolved waves is examined first, by investigating thoroughly the numerical dispersion relation of a typical discretization scheme. The second part of the study is related to the unbalanced space-time errors in FDTD schemes with dissimilar space-time approximation orders. The authors propose a remedy via the suitable choice of the time-step size, based on the single-frequency minimization of an error expression extracted, again, from the scheme’s numerical dispersion formula.
Findings
Unlike wave propagation in free space, there exist two parts of the frequency spectrum where waves in a Lorentz medium experience non-physical attenuation and display non-changing propagation constants, due to coarse discretization. The authors also show that an optimum time-step size can be determined, in the case of the (2,4) FDTD scheme, which minimizes the selected error formula at a specific frequency point, promoting more efficient implementations.
Originality/value
Unique characteristics displayed by discretized waves, which have been known for non-dispersive media, are examined and verified for the first time in the case of dispersive materials, thus completing the comprehension of the space-time discretization impact on simulated quantities. In addition, the closed-form formula of the optimum time-step enables the efficient implementation of the (2,4) FDTD method, minimizing the detrimental influence of the low-order temporal integration.
Details
Keywords
Christos Salis, Nikolaos V. Kantartzis and Theodoros Zygiridis
The fabrication of electromagnetic (EM) components may induce randomness in several design parameters. In such cases, an uncertainty assessment is of high importance, as…
Abstract
Purpose
The fabrication of electromagnetic (EM) components may induce randomness in several design parameters. In such cases, an uncertainty assessment is of high importance, as simulating the performance of those devices via deterministic approaches may lead to a misinterpretation of the extracted outcomes. This paper aims to present a novel heuristic for the sparse representation of the polynomial chaos (PC) expansion of the output of interest, aiming at calculating the involved coefficients with a small computational cost.
Design/methodology/approach
This paper presents a novel heuristic that aims to develop a sparse PC technique based on anisotropic index sets. Specifically, this study’s approach generates those indices by using the mean elementary effect of each input. Accurate outcomes are extracted in low computational times, by constructing design of experiments (DoE) which satisfy the D-optimality criterion.
Findings
The method proposed in this study is tested on three test problems; the first one involves a transmission line that exhibits several random dielectrics, while the second and the third cases examine the effects of various random design parameters to the transmission coefficient of microwave filters. Comparisons with the Monte Carlo technique and other PC approaches prove that accurate outcomes are obtained in a smaller computational cost, thus the efficiency of the PC scheme is enhanced.
Originality/value
This paper introduces a new sparse PC technique based on anisotropic indices. The proposed method manages to accurately extract the expansion coefficients by locating D-optimal DoE.
Details
Keywords
Theodosios Karamanos, Stamatis A. Amanatiadis, Theodoros Zygiridis and Nikolaos V. Kantartzis
The majority of first-principle, homogenisation techniques makes use of the dipole terms of a small particle radiation, and, consequently, the respective dipole polarisabilities…
Abstract
Purpose
The majority of first-principle, homogenisation techniques makes use of the dipole terms of a small particle radiation, and, consequently, the respective dipole polarisabilities. This paper aims to take the next step and propose a new systematic technique for extracting the quadrupolarisability of planar metamaterial scatterers.
Design/methodology/approach
Firstly, it is assumed that the particle, under study, can be modelled as a set of dipole and quadrupole moments, and by utilising the respective polarisabilities, the far-field response of the scatterer is calculated. Then, the far-field scattering field of the particle is constructed in terms of the dipole and quadrupole moments, which, in turn, are expressed as a function of the unknown polarisabilities. Finally, the desired polarisabilities are retrieved by a system of equations, which involves numerically derived electric field values at specific positions around the scatterer.
Findings
The quadrupolarisability of planar metamaterial particles is extracted, through an easy to use, yet very accurate and efficient methodology. Moreover, the proposed technique is verified via comprehensive comparisons of consequently computed and simulated total radiated power values, which reveal its advantages and applicability limits. Finally, the total radiation power contribution of each calculated, individual multipole is provided, to further investigate the radiation mechanism of all nano-particles under study.
Originality/value
The initial and most important step of extracting a single quadrupolarisability of a planar realistic nano-particle has been performed, herein, for the first time. The addition of the respective quadrupole in the scattering model, shifts the multipole approximation limit upwards in terms of frequency, and, therefore, nano-particles with quadrupole resonances can, now, be precisely represented via polarisabilities for various metamaterial or metasurface applications.
Details
Keywords
Theodoros Zygiridis, Stamatis A. Amanatiadis, Theodosios Karamanos and Nikolaos V. Kantartzis
The extraordinary properties of graphene render it ideal for diverse contemporary and future applications. Aiming at the investigation of certain aspects commonly overlooked in…
Abstract
Purpose
The extraordinary properties of graphene render it ideal for diverse contemporary and future applications. Aiming at the investigation of certain aspects commonly overlooked in pertinent works, the authors study wave-propagation phenomena supported by graphene layers within a stochastic framework, i.e. when uncertainty in various factors affects the graphene’s surface conductivity. Given that the consideration of an increasing number of graphene sheets may increase the stochastic dimensionality of the corresponding problem, efficient surrogates with reasonable computational cost need to be developed.
Design/methodology/approach
The authors exploit the potential of generalized Polynomial Chaos (PC) expansions and develop low-cost surrogates that enable the efficient extraction of the necessary statistical properties displayed by stochastic graphene-related quantities of interest (QoI). A key step is the incorporation of an initial variance estimation, which unveils the significance of each input parameter and facilitates the selection of the most appropriate basis functions, by favoring anisotropic formulae. In addition, the impact of controlling the allowable input interactions in the expansion terms is investigated, aiming at further PC-basis elimination.
Findings
The proposed stochastic methodology is assessed via comparisons with reference Monte-Carlo results, and the developed reduced basis models are shown to be sufficiently reliable, being at the same time computationally cheaper than standard PC expansions. In this context, different graphene configurations with varying numbers of random inputs are modeled, and interesting conclusions are drawn regarding their stochastic responses.
Originality/value
The statistical properties of surface-plasmon polaritons and other QoIs are predicted reliably in diverse graphene configurations, when the surface conductivity displays non-trivial uncertainty levels. The suggested PC methodology features simple implementation and low complexity, yet its performance is not compromised, compared to other standard approaches, and it is shown to be capable of delivering valid results.
Details
Keywords
Stamatis A. Amanatiadis, Theodoros Zygiridis and Nikolaos V. Kantartzis
The coupling characteristics between adjacent circuits are crucial for their efficient design in terms of electromagnetic compatibility features. Specifically, either the wireless…
Abstract
Purpose
The coupling characteristics between adjacent circuits are crucial for their efficient design in terms of electromagnetic compatibility features. Specifically, either the wireless power transfer can be enhanced or the interference can be limited. This paper aims to the extraction of the coupling characteristics of surface plasmon polariton waves propagating onto graphene layers to facilitate the telecommunication system design for advanced THz applications.
Design/methodology/approach
The surface conductivity of graphene is described at the far-infrared spectrum and modelled accurately by means of a properly modified finite-difference time-domain) scheme. Then, a series of numerical simulations for different coupling setups is conducted to extract an accurate generalised parametric coupling model that is dependent explicitly on the fundamental propagation features of graphene.
Findings
The coupling coefficients of two basic waveguiding setups are examined thoroughly. The initial one includes two parallel graphene layers of infinite dimensions, and it is observed that the coupling is influenced via the ratio between their distances to the confinement of the surface wave. The second scenario is composed of graphene microstrips that are parallel to their small edge, namely, microstrip width. The extracted numerical results indicate that the coupling coefficient depends on the ratio between widths to wavelength.
Originality/value
The accurate extraction of the generalised coupling coefficients for graphene surface wave circuits is conducted in this work via an adjustable numerical technique for a novel family of plasmonic couplers. It is derived that only the fundamental propagation features of graphene, such as the wavelength and the confinement of the surface waves, have an effect on the coupling calculation, thus enabling a consistent electromagnetic compatibility study.
Details
Keywords
Theodoros Zygiridis, Georgios Pyrialakos, Nikolaos Kantartzis and Theodoros Tsiboukis
The locally one-dimensional (LOD) finite-difference time-domain (FDTD) method features unconditional stability, yet its low accuracy in time can potentially become detrimental…
Abstract
Purpose
The locally one-dimensional (LOD) finite-difference time-domain (FDTD) method features unconditional stability, yet its low accuracy in time can potentially become detrimental. Regarding the improvement of the method’s reliability, existing solutions introduce high-order spatial operators, which nevertheless cannot deal with the augmented temporal errors. The purpose of the paper is to describe a systematic procedure that enables the efficient implementation of extended spatial stencils in the context of the LOD-FDTD scheme, capable of reducing the combined space-time flaws without additional computational cost.
Design/methodology/approach
To accomplish the goal, the authors introduce spatial derivative approximations in parametric form, and then construct error formulae from the update equations, once they are represented as a one-stage process. The unknown operators are determined with the aid of two error-minimization procedures, which equally suppress errors both in space and time. Furthermore, accelerated implementation of the scheme is accomplished via parallelization on a graphics-processing-unit (GPU), which greatly shortens the duration of implicit updates.
Findings
It is shown that the performance of the LOD-FDTD method can be improved significantly, if it is properly modified according to accuracy-preserving principles. In addition, the numerical results verify that a GPU implementation of the implicit solver can result in up to 100× acceleration. Overall, the formulation developed herein describes a fast, unconditionally stable technique that remains reliable, even at coarse temporal resolutions.
Originality/value
Dispersion-relation-preserving optimization is applied to an unconditionally stable FDTD technique. In addition, parallel cyclic reduction is adapted to hepta-diagonal systems, and it is proven that GPU parallelization can offer non-trivial benefits to implicit FDTD approaches as well.