Resonant Electromagnetic Modes Research Articles

Ultrafast modulable 2DEG Huygens metasurface

Huygens metasurfaces have demonstrated remarkable potential in perfect transmission and precise wavefront modulation through the synergistic integration of electric resonance and magnetic resonance. However, prevailing active or reconfigurable Huygens metasurfaces, based on all-optical systems, encounter formidable challenges associated with the intricate control of bulk dielectric using laser equipment and the presence of residual thermal effects, leading to limitations in continuous modulation speeds. Here, we present an ultrafast electrically driven terahertz Huygens metasurface that comprises an artificial microstructure layer featuring a two-dimensional electron gas (2DEG) provided by an AlGaN/GaN heterojunction, as well as a passive microstructure layer. Through precise manipulation of the carrier concentration within the 2DEG layer, we effectively govern the current distribution on the metasurfaces, inducing variations in electromagnetic resonance modes to modulate terahertz waves. This modulation mechanism achieves high efficiency and contrast for terahertz wave manipulation. Experimental investigations demonstrate continuous modulation capabilities of up to 6 GHz, a modulation efficiency of 90%, a transmission of 91%, and a remarkable relative operating bandwidth of 55.5%. These significant advancements substantially enhance the performance of terahertz metasurface modulators. Importantly, our work not only enables efficient amplitude modulation but also introduces an approach for the development of high-speed and efficient intelligent transmissive metasurfaces.

Experimental implementation for near-field displaying application of bound states in continuum supported by terahertz metasurfaces.

Optical bound states in continuum (BICs) are the special lossless electromagnetic resonance modes found in optical transmission media. Optical BICs supported by metasurfaces can bring a series of image-based applications, which can be classified into far-field image-based applications and near-field image-based applications according to imaging distance. The far field image-based applications of optical BICs supported by metasurfaces have been proved theoretically and experimentally. However, the near-field image-based applications are still in the stage of theoretical demonstration, and its practical feasibility is still questioned. In this letter, we have experimentally demonstrated the feasibility of near-field image-based applications of optical BICs. An all-silicon terahertz metasurface that supports near-field displaying based on quasi-BIC is designed. Its unit cell is based on the classic asymmetric elliptical strip structure, and one unit cell of the metasurface corresponds to one display pixel. By observing the electric field distribution of the metasurface in the near-field region, the display of the given image is successfully constructed. The near-field image-based application of BICs may be beneficial for the information encryption, the hidden item detection, matter analysis, etc.

Pushing and pulling optical pressure control with plasmonic surface waves

The basis for pulling a passive nanostructured material using laser light by establishing a transverse surface plasmon resonance on the back of a membrane is presented and supported with both theory and numerical simulations. The total force magnitude and direction can be regulated by electromagnetic resonant modes interacting with materials that can be adjusted using the material and geometry variables, and with plane-wave illumination under sinusoidal steady-state conditions. In the situations treated, the structured material is fixed in position and the force density and hence total force are determined numerically. Simulations indicate that a silicon nitride film coated with gold and patterned to produce a unit cell having a slot passing through both materials allows satisfactory surface wave control to facilitate a pulling force with visible wavelengths. When the plasmon surface wave on the back dominates, pulling occurs, and when the mode on the front has more energy, the result is pushing. It is also shown that pushing or pulling, regulated by varying the wavelength of the incident light, is possible, and an example is presented. This work motivates an experimental effort to investigate pulling with nanostructured media and offers a different paradigm in optomechanics.

Sensitivity Analysis of GaAs-Metamaterial Nanocavity as a Tunable Surface Plasmon Resonance Sensor

Background: The paper reports on typical characteristics of resonant electromagnetic modes propagation through interfaces of a multilayer device. Methods: Using the transfer matrix method, p-reflectance is analyzed in angular interrogation for a symmetrical cavity performed with left-handed metamaterial layer mediated with GaAs. Results: An advantage of SPR sensor is demonstrated in terms of optimal performances by controlling thicknesses, refractive indices and dielectric gap layers of the media involved. Conclusion: The functionality of the proposed design, as a tunable filter, has been also identified.

Giant low-field tunability of THz transmission in patterned magnetic split-ring metastructures.

Mirror-asymmetric split-ring metamaterials with high quality factor in the terahertz (THz) band, consisting of patterned high magnetic permeability and low coercivity FeNHf films deposited on high resistivity silicon substrates, were studied for their magnetic field tunable response in frequency and transmission. Dynamic tuning of terahertz transmission and electromagnetic resonance modes were investigated theoretically and experimentally as a function of magnetization of the FeNHf film. Experimental results indicate that the metamaterial structure provides a giant tunability of resonance frequency (Δfr/fr=3.3%) and transmittivity (21%) at a frequency of 0.665 THz under a low magnetic field of H=100 Oe. Remarkable tuning coefficients of frequency and transmittivity, 0.23 GHz/Oe and 0.21%/Oe, respectively, were measured. Finite difference time domain simulations reveal that the incredible tunability stems predominately from the response of the THz dynamic magnetic field to magnetization. As a result, the metamaterial, consisting of a simple magnetic split-ring microstructure, provides previously unimagined paths to tunable devices for potential use in emerging THz technologies including 6G communication systems and networks.

Enhance of the absorption and bandwidth based on a ultra-thin tungsten structure metamaterial absorber in 400–1500 nm range

In this paper, an ultra-thin metamaterial absorber with simple structure strategy is proposed and verified in the 400–1500 nm band. A 0.2μm thick metamaterial absorber with silver disk array is proposed and measured. Three absorption peaks (The absorption amplitudes are: 90%, 75%, and 85%) are obtained at resonance wavelengths 612nm, 1100nm, and 1228nm, respectively. It should be noted that these achieved absorption bandwidths are relatively small. In order to enhance the absorption rate and bandwidth, an ultra-thin tungsten structure absorber with the same structural parameters is also verified and measured. Four absorption peaks (The absorption amplitudes are: 98%, 88%, 92%, and 95%) are obtained at resonance wavelengths 614nm, 741nm, 1072nm,and 1312nm, respectively. The average absorbance of the tungsten structure absorber is also higher than that of the silver structure absorber. These absorption peaks are excited by the electromagnetic resonance modes of the absorber and the higher average absorbance is due to the field dissipation effect of the tungsten layer. The proposed ultra-thin tungsten structure absorber reveals both high absorption rate and large absorption bandwidth.

Characteristic Mode Analysis applied to antennas

Abstract This paper introduces the Characteristic Mode Analysis in the computation of electromagnetic resonant modes in conducting bodies. Canonical cases (thin wire and a rectangular plate) are analyzed, along with real-world complex structures, a mobile phone chassis and an unmanned aerial vehicle. Given the analogy to other areas, such as modal analysis in Differential Equations, this method can be another tool when teaching Electromagnetic Radiation and antenna related topics.

Surface plasmon induced enhancement in selective laser melting processes

PurposeSelective laser melting (SLM) is an advanced rapid prototyping or additive manufacturing technology that uses high power density laser to fabricate metal/alloy components with minimal geometric constraints. The SLM process is multi-physics in nature and its study requires development of complex simulation tools. The purpose of this paper is to study – for the first time, to the best of the authors’ knowledge – the electromagnetic wave interactions and thermal processes in SLM based dense powder beds under the full-wave formalism and identify prospective metal powder bed particle distributions that can substantially improve the absorption rate, SLM volumetric deposition rate and thereby the overall build time.Design/methodology/approachWe present a self-consistent thermo-optical model of the laser-matter interactions pertaining to SLM. The complex electromagnetic interactions and thermal effects in the dense metal powder beds are investigated by means of full-wave finite difference simulations. The model allows for accurate simulations of the excitation of gap, bulk and surface electromagnetic resonance modes, the energy transport across the particles, time dependent local permittivity variations under the incident laser intensity, and the thermal effects (joule heating) due to electromagnetic energy dissipation.FindingsLocalized gap and surface plasmon polariton resonance effects are identified as possible mechanisms toward improved absorption in small and medium size titanium powder beds. Furthermore, the observed near homogeneous temperature distributions across the metal powders indicates fast thermalization processes and allows for development of simple analytical models to describe the dynamics of the SLM process.Originality/valueTo the best of the authors’ knowledge, for the first time the electromagnetic interactions and thermal processes with dense powder beds pertaining to SLM processes are investigated under full-wave formalism. Explicit description is provided for important SLM process parameters such as critical laser power density, saturation temperature and time to melt. Specific guidelines are presented for improved energy efficiency and optimization of the SLM process deposition rates.

Broadband High-Efficiency Chiral Splitters and Holograms from Dielectric Nanoarc Metasurfaces.

Simultaneous broadband and high efficiency merits of designer metasurfaces are currently attracting widespread attention in the field of nanophotonics. However, contemporary metasurfaces rarely achieve both advantages simultaneously. For the category of transmissive metadevices, plasmonic or conventional dielectric metasurfaces are viable for either broadband operation with relatively low efficiency or high efficiency at only a selection of wavelengths. To overcome this limitation, dielectric nanoarcs are proposed as a means to accomplish two advantages. Continuous nanoarcs support different electromagnetic resonant modes at localized areas for generating phase retardation. Meanwhile, the geometric nature of nanoarc curvature endows the nanoarcs with full phase coverage of 0-2π due to the Pancharatnam-Berry phase principle. Experimentally incorporated with the chiral-detour phase principle, a few compelling functionalities are demonstrated, such as chiral beamsplitting, broadband holography, and helicity-selective holography. The continuous nanoarc metasurfaces prevail over plasmonic or dielectric discretized building block strategies and the findings lead to novel designs of spin-controllable metadevices.

Towards x-ray waveguide formation upon ion irradiation of a Co thin film on Si(111)

We have obtained the essential features of x-ray waveguide formation in an ion-irradiated cobalt thin film on Si substrate. Ion irradiation was carried out with 1 MeV Si+ ions at a fluence of 5 × 1016 ions cm−2. X-ray reflectivity (XRR) experiment shows characteristic dips in the total external reflection region indicating formation of resonant electromagnetic modes—an essential requirement for x-ray waveguides. XRR analysis further reveals formation of a low electron-density layer sandwiched between higher electron-density layers. These are two key requirements for an x-ray waveguide. Computation of x-ray intensity within the irradiated sample shows about 15 times flux enhancement of the electric field within the low electron-density guiding layer.

Electromagnetic resonance in the asymmetric terahertz metamaterials with triangle microstructure

We investigate terahertz transmission properties and electromagnetic resonance modes in the asymmetric triangle structures with the change of asymmetric distance and the direction of electric field. When the THz electric field is perpendicular to the split gap of triangle, the electric field can better excite the THz absorption in the triangle structures. Importantly, electromagnetically induced transparency (EIT) characteristics are observed in the triangle structures due to the destructive interference of the different excited modes. The distributions of electric field and surface current density simulated by finite difference time domain indicate that the bright mode is excited by the side of triangle structures and dark mode is excited by the gap-side of triangle. The present study is helpful to understand the electromagnetic resonance in the asymmetric triangular metamaterials.

The application of a high-order discontinuous Galerkin time-domain method for the computation of electromagnetic resonant modes

• A high-order discontinuous time-domain method is presented for computing resonant frequencies and modes. • The method incorporates the exact boundary representation of the computational domain given by a CAD model. • The results demonstrates that the rate of convergence can be affected by an inaccurate geometric description. • The method is able to provide a broad band of resonant frequencies for two and three dimensional cavities. This work presents a highly accurate and efficient methodology for the computation of electromagnetic resonant frequencies and their associated modes in cavities. The proposed technique consists of a high–order discontinuous Galerkin time-domain solver combined with a signal processing algorithm for extracting the frequency content. The methodology is capable of incorporating the CAD boundary representation of the domain. The numerical results demonstrate that incorporating the exact boundary representation results in a improved convergence rate, a phenomenon that has not been previously reported. Several numerical examples in two and three dimensions show the potential of the proposed technique for cavities filled with non-dispersive or dispersive media.

Toroidal eigenmodes in all-dielectric metamolecules

We present a thorough investigation of the electromagnetic resonant modes supported by systems of polaritonic rods placed at the vertices of canonical polygons. The study is conducted with rigorous finite-element eigenvalue simulations. To provide physical insight, the simulations are complemented with coupled mode theory (the analog of LCAO in molecular and solid state physics) and a lumped wire model capturing the coupling-caused reorganizations of the currents in each rod. The systems of rods, which form all-dielectric cyclic metamolecules, are found to support the unconventional toroidal dipole mode, consisting of the magnetic dipole mode in each rod. Besides the toroidal modes, the spectrally adjacent collective modes are identified. The evolution of all resonant frequencies with rod separation is examined. They are found to oscillate about the single-rod magnetic dipole resonance, a feature attributed to the leaky nature of the constituent modes. Importantly, we observe that ensembles of an odd number of rods produce larger frequency separation between the toroidal mode and its neighbor than the ones with even number of rods. This increased spectral isolation, along with the low quality factor exhibited by the toroidal mode, favors the coupling of the commonly silent toroidal dipole to the outside world, rendering the proposed structure a prime candidate for controlling the observation of toroidal excitations and their interaction with the usually present electric dipole

Metasurfaces with Fano resonances for directionally selective thermal emission

ABSTRACTThermal emission impacts a wide variety of applications, including thermophotovoltaics, photovoltaics, photon-enhanced thermionic emission, selective solar absorption, incandescent lighting, and spectroscopy. Ordinary structures generally emit a broad range of wavelengths, angles, and polarizations. However, highly selective thermal emission has potential to greatly improve performance in many of these applications. While prior work has explored a wide range of structures to provide some degree of control of one or more of these attributes, there is an ongoing challenge in combining readily-fabricated, simple structures made of appropriate (e.g., heat-resistant) materials with the desired functionality. Here, we will focus on using metasurfaces in conjunction with refractory materials as a platform for achieving selective control of emission. These structures are built from sub-wavelength elements that support localization of surface plasmon polaritons or electromagnetic resonant modes with appropriate attributes. Modeling is performed using rigorous coupled wave analysis (RCWA), plus Kirchhoff’s law of thermal radiation, which is further validated using finite-difference time domain (FDTD) simulations and coupled-mode analysis. Such structures can be considered arbitrarily directional sources that can be carefully patterned in lateral directions to yield a thermal lens with a designed focal length and/or concentration ratio; the benefit of this approach is that it can enhance the view factor between thermal emitters and receivers, without restricting the area ratio or separation distance. This design and modeling platform is then applied to exclude thermal radiation over a certain range of angles. In this work, we study the effect of controlling the angular width and direction on the view factor, and we explore angular dependence of these angular selective structures.

Electromagnetic resonance modes on a two-dimensional tandem grating and its application for broadband absorption in the visible spectrum.

In this work, we numerically investigate the electromagnetic resonances on two-dimensional tandem grating structures. The base of a tandem grating consists of an opaque Au substrate, a SiO(2) spacer, and a Au grating (concave type); that is, a well-known fishnet structure forming Au/SiO(2)/Au stack. A convex-type Au grating (i.e., topmost grating) is then attached on top of the base fishnet structure with or without additional SiO(2) spacer, resulting in two types of tandem grating structures. In order to calculate the spectral reflectance and local magnetic field distribution, the finite-difference time-domain method is employed. When the topmost Au grating is directly added onto the base fishnet structure, the surface plasmon and magnetic polariton in the base structure are branched out due to the geometric asymmetry with respect to the SiO(2) spacer. If additional SiO(2) spacer is added between the topmost Au grating and the base fishnet structure, new magnetic resonance modes appear due to coupling between two vertically aligned Au/SiO(2)/Au stacks. With the understanding of multiple electromagnetic resonance modes on the proposed tandem grating structures, we successfully design a broadband absorber made of Au and SiO(2) in the visible spectrum.

Long bunch trains measured using a prototype cavity beam position monitor for the Compact Linear Collider

The Compact Linear Collider (CLIC) requires beam position monitors (BPMs) with 50 nm spatial resolution for alignment of the beam line elements in the main linac and beam delivery system. Furthermore, the BPMs must be able to make multiple independent measurements within a single 156 ns long bunch train. A prototype cavity BPM for CLIC has been manufactured and tested on the probe beam line at the 3rd CLIC Test Facility (CTF3) at CERN. The transverse beam position is determined from the electromagnetic resonant modes excited by the beam in the two cavities of the pickup, the position cavity and the reference cavity. The mode that is measured in each cavity resonates at 15 GHz and has a loaded quality factor that is below 200. Analytical expressions for the amplitude, phase and total energy of signals from long trains of bunches have been derived and the main conclusions are discussed. The results of the beam tests are presented. The variable gain of the receiver electronics has been characterized using beam excited signals and the form of the signals for different beam pulse lengths with the $2/3\text{ }\text{ }\mathrm{ns}$ bunch spacing has been observed. The sensitivity of the reference cavity signal to charge and the horizontal position signal to beam offset have been measured and are compared with theoretical predictions based on laboratory measurements of the BPM pickup and the form of the resonant cavity modes as determined by numerical simulation. Finally, the BPM was calibrated so that the beam position jitter at the BPM location could be measured. It is expected that the beam jitter scales linearly with the beam size and so the results are compared to predicted values for the latter.

Planckian Power Spectral Densities from Human Calves during Posture Maintenance and Controlled Isometric Contractions.

BackgroundThe relationship between muscle anatomy and physiology and its corresponding electromyography activity (EMGA) is complex and not well understood. EMGA models may be broadly divided in stochastic and motor-unit-based models. For example, these models have successfully described many muscle physiological variables such as the value of the muscle fiber velocity and the linear relationship between median frequency and muscle fiber velocity. However they cannot explain the behavior of many of these variables with changes in intramuscular temperature, or muscle PH acidity, for instance. Here, we propose that the motor unit action potential can be treated as an electromagnetic resonant mode confined at thermal equilibrium inside the muscle. The motor units comprising the muscle form a system of standing waves or modes, where the energy of each mode is proportional to its frequency. Therefore, the power spectral density of the EMGA is well described and fit by Planck’s law and from its distribution we developed theoretical relationships that explain the behavior of known physiological variables with changes in intramuscular temperature or muscle PH acidity, for instance.MethodsEMGA of the calf muscle was recorded during posture maintenance in seven participants and during controlled isometric contractions in two participants. The power spectral density of the EMGA was then fit with the Planckian distribution. Then, we inferred nine theoretical relationships from the distribution and compared the theoretically derived values with experimentally obtained values.ResultsThe power spectral density of EMGA was fit by Planckian distributions and all the theoretical relationships were validated by experimental results.ConclusionsOnly by considering the motor unit action potentials as electromagnetic resonant modes confined at thermal equilibrium inside the muscle suffices to predict known or new theoretical relationships for muscle physiological variables that other models have failed to do.

Design and optimization of a SiC thermal emitter/absorber composed of periodic microstructures based on a non-linear method**Project supported by the National Natural Science Foundation of China (Grant No. 51076002), the National Basis Research Program of China (Grant No. 2013CA328900), and the Key Project of Complicated Electromagnetic Environment Laboratory of CAEP, China (Grant No. 2015E0-01-1).

Spectral and directional control of thermal emission based on excitation of confined electromagnetic resonant modes paves a viable way for the design and construction of microscale thermal emitters/absorbers. In this paper, we present numerical simulation results of the thermal radiative properties of a silicon carbide (SiC) thermal emitter/absorber composed of periodic microstructures. We illustrate different electromagnetic resonant modes which can be excited with the structure, such as surface phonon polaritons, magnetic polaritons and photonic crystal modes, and the process of radiation spectrum optimization based on a non-linear optimization algorithm. We show that the spectral and directional control of thermal emission/absorption can be efficiently achieved by adjusting the geometrical parameters of the structure. Moreover, the optimized spectrum is insensitive to 3% dimension modification.

Electromagnetic resonant modes of dielectric sphere bilayers

Sphere bilayers have been proposed as promising structures for electromagnetic management in photonic crystal devices. These arrangements are made of two intertwined subsets of spheres of different size and refractive index, one subset filling the interstitial sites of the other. We present a systematic study of the electromagnetic resonant modes of the bilayers, in comparison with those of the constituent subsets of spheres. Three samples were built with glass and Teflon spheres and their transmission spectra measured in the microwave range (10–25 GHz). Simulations with finite integration time-domain method are in good agreement with experiments. Results show that the bilayer presents the same resonances as one of the subsets but modified by the presence of the other in its resonant frequencies and in the electric field distributions. As this distortion is not very large, the number of resonances in a selected spectral region is determined by the dominant subset. The degree of freedom that offers the bilayer could be useful to fine tune the resonances of the structure for different applications. A map of modes useful to guide this design is also presented. Scale invariance of Maxwell equations allows the translation of these results in the microwave range to the visible region; hence, some possible applications are discussed in this framework.

Control of thermal radiative properties using two-dimensional complex gratings

The present work theoretically investigates the electromagnetic resonance modes on two-dimensional (2-D) complex gratings consisting of a metallic grating, a dielectric film, and a metal substrate. We treat a complex grating as geometrical combination of two square grating elements with different sizes in a single unit cell. In order to identify various resonance modes, such as surface plasmon and magnetic resonance (or magnetic polariton) existing in gratings, the finite-difference time-domain method is employed in solving Maxwell’s equation. Differently from 2-D simple grating structures, we observe the diverse magnetic polariton modes associated with each grating elements in a single unit cell. Furthermore, if two different grating elements are merged together, magnetic polariton will be branched out into multiple modes. These characteristics of 2-D complex grating will be desirable for achieving broad-band absorption, which is critical for energy harvesting applications.