Effective Surface Impedance Research Articles

Atmospheric Sound Propagation over Rough Sea: Numerical Evaluation of Equivalent Acoustic Impedance of Varying Sea States

This work presents a numerical study on atmospheric sound propagation over rough water surfaces with the aim of improving predictions of sound propagation over long distances. A method for generating pseudorandom sea profiles consistent with sea wave spectra is presented. The proposed method is suited for capturing the logarithmic nature of the energy distribution of the waves. Sea profiles representing fully developed seas for sea states 2, 3, 4, and 5 are generated from the Elfouhaily et al. (ECKV) sea wave spectra. Excess attenuation caused by refraction and surface roughness is predicted with a parabolic equation (PE) solver. A novel method for estimating equivalent effective impedance based on PE predictions at different sea states is presented. Parametric expressions using acoustic frequency and significant wave height are developed for effective surface impedances. In this work, sea surface roughness is on a scale comparable with the acoustic wavelength. Under this condition, the acoustic scattering is primarily incoherent. This work shows the limitations of using an equivalent surface impedance in such incoherent scattering cases.

Effect of Molybdenum Coatings on the Accelerating Cavity Quality Factor

In this work, a detailed parametric study assessing the impact of low-conductivity coatings on the radio-frequency accelerating cavity quality factor and resonance frequency shift is presented. In particular, this study is aimed at proving the feasibility of molybdenum oxides deposited on copper to reduce the dark current in high-gradient applications due to its intrinsically high work function. In order to compute the effective surface impedance of the resulting layered structure, a transmission line-based approach is adopted. The present analysis demonstrates the potential effectiveness of molybdenum thin-films, which only slightly affects the accelerating cavity quality factor, with very low sensitivity to thickness and resistivity inhomogeneities.

Tensor surface impedance modelling of metasurfaces with printed elements

AbstractTwo methods for extraction of the effective tensor surface impedance representing metasurfaces with printed elements and arbitrary geometry are presented. The surface impedance can be modelled as a shunt admittance on a transmission line. The extracted surface impedance combined with transmission line theory enables to compute easily and efficiently the transmission coefficient through a multilayer metasurface structure. The metasurface modelling as a shunt admittance enables to conduct a multi‐parameter study on the transmission and reflection performance through the metasurface structure. The results of the proposed methods are compared, and very good agreement was obtained up to 30° incident angle. Beyond 30° it is suggested to recalibrate the model or upgrade it. For model validation purposes, a 2D flat lens antenna with patch elements loaded with capacitors was designed, fabricated, and measured. Simulation and experimental results of the lens near and far‐field are presented.

An Effective Surface Impedance Concept to Model Arbitrary Roughness Profiles on Printed Circuit Boards up to 110 GHz

This letter presents a method to determine the complex surface impedance of rough conductors utilizing the main idea of the gradient model. By introducing a frequency dependent, effective quadratic surface roughness parameter real conductivity profiles can be described without simplified approximations. This concept delivers highly accurate simulation models, which is proven for additive and subtractive manufactured microstrip lines (MS) up to 110 GHz.

A Comparative study of the effective surface impedance of an HTc superconducting thin film from visible to mid-IR region

A Comparative study of the effective surface impedance of an HTc superconducting thin film from visible to mid-IR region

Effect of net direct current on the properties of radio frequency sheaths: simulation and cross-code comparison

In order to understand, predict and control ion cyclotron range of frequency interactions with tokamak scrape-off layer plasmas, computational tools which can model radio frequency (RF) sheaths are needed. In particular, models for the effective surface impedance and DC rectified sheath potentials may be coupled with full wave RF simulation codes to predict self-consistent wave fields near surfaces and the resulting power dissipation and plasma–material interactions from ion sputtering. In this study, previous work assuming zero net DC current flow through the sheath is generalized to allow the surface to collect net positive or negative current, as is often observed in experiments. The waveforms, DC potential and RF admittance are investigated by means of analytical theory, nonlinear fluid and particle-in-cell codes. Cross-code comparisons provide detailed model verification and elucidate the roles of ion and electron kinetics. When the sheath draws negative (positive) DC current, the voltage rectification is reduced (increased) compared with the zero-current case, and the magnitude of both the real and imaginary parts of the admittance are increased (reduced). A previous four-input parametrization of the sheath rectification and admittance properties is generalized to include a fifth parameter describing the DC sheath current.

Tunable mantle cloaking utilizing graphene metasurface for terahertz sensing applications.

Design of sensors which are able to probe electromagnetic radiation with larger cross section and at the same time with having negligible perturbation in measurement has attracted significant attention. For this purpose, scattering-cancellation sensors or cloaking sensors are introduced. However, tunable cloaking sensors are very challenging. In this regards, here, a metasurface based on graphene strips is proposed to cloak a dielectric cylinder under illumination of TEz and TMz polarized incident waves in terahertz range. According to the in plane effective surface impedance tensor for the considered metasurface and the required surface impedance for achieving invisibility under TE and TM polarized impinging waves, the geometrical parameters of the covering structure and characteristics of graphene are obtained. Numerical simulations show radar cross section reduction for both TE and TM polarizations. Furthermore, the introduced metasurface is able to cloak the cylinder for incoming waves with circular polarization. In addition, it is shown that by properly adjusting the chemical potential of graphene, the required surface impedance to have cloaking for the two polarizations in other frequencies can also be achieved, which results in a tunable dual polarized cloaking. The proposed structure provides 2-11 dB reduction in scattering strength relative to the uncloaked configuration for 0.3eV variation of graphene chemical potential.

Homogenization of Fractal Metasurface Based on Extension of Babinet–Booker's Principle

In this letter, we develop a homogenization approach for a quasi-two-dimensional fractal-based metamaterial (fractal-based metasurface). The structure that we consider is a self-similar patterned metal screen based on the Hilbert curve. The screen can be placed above a metallic ground plane, be printed on a dielectric substrate, or it can be considered freestanding in air. In this study, we develop a theoretical model for the effective surface impedance of this structure by using the self-similarity of the fractal and the electromagnetic Babinet's principle. We numerically simulate the reflection of the plane waves incident on such a metasurface (with the substrate plane included). The properties of the Hilbert fractal metasurface are extracted from the data obtained in these simulations. Our aim is to find the effective parameters and construct an equivalent model for the fractal metasurface.

Analysis of Reflectarray Antenna Elements Under Arbitrary Incident Angles and Polarizations Using Generalized Boundary Conditions

A novel analysis method for reflectarray antenna elements is proposed in this letter, which is derived from generalized boundary conditions. First, the effective surface susceptibilities of the patch layer are extracted from two sets of simulations of given incident angles and polarizations. Then, the effective surface impedances of reflectarray antenna elements are analytically computed for arbitrary incident angles and polarizations, from which the reflection coefficients can be readily obtained. The detailed analysis procedure is presented, and its effectiveness is validated through several representative examples of reflectarray antenna elements.

Sideways adiabaticity: beyond ray optics for slowly varying metasurfaces.

Optical metasurfaces (subwavelength-patterned surfaces typically described by variable effective surface impedances) are typically modeled by an approximation akin to ray optics: the reflection or transmission of an incident wave at each point of the surface is computed as if the surface were "locally uniform," and then the total field is obtained by summing all of these local scattered fields via a Huygens principle. (Similar approximations are found in scalar diffraction theory and in ray optics for curved surfaces.) In this paper, we develop a precise theory of such approximations for variable-impedance surfaces. Not only do we obtain a type of adiabatic theorem showing that the "zeroth-order" locally uniform approximation converges in the limit as the surface varies more and more slowly, including a way to quantify the rate of convergence, but we also obtain an infinite series of higher-order corrections. These corrections, which can be computed to any desired order by performing integral operations on the surface fields, allow rapidly varying surfaces to be modeled with arbitrary accuracy, and also allow one to validate designs based on the zeroth-order approximation (which is often surprisingly accurate) without resorting to expensive brute-force Maxwell solvers. We show that our formulation works arbitrarily close to the surface, and can even compute coupling to guided modes, whereas in the far-field limit our zeroth-order result simplifies to an expression similar to what has been used by other authors.

A bandwidth-enhanced planar endfire antenna based on surface waves

ABSTRACTA surface-wave-based endfire antenna with an enhanced impedance bandwidth is presented in this paper. The antenna consists of a surface-wave guide (SWG) and five strips printed on the SWG. The endfire radiation benefits from the surface-wave diffraction of the SWG which is investigated with the Snell's law, effective surface impedance and eigenvalue analysis. A prototype is fabricated and measured, and the measured results indicate that the antenna has two resonances and its bandwidth is enhanced up to 16.1% (4.0–4.73 GHz). The proposed endfire antenna also exhibits high gain, good gain flatness, low cross-polarization level and low profile.

Microwave response of a cavity resonator with thin superconductor film depending on film temperature and orientation

We substantiate the earlier proposed hypothesis about the nature of an anomaly previously observed in the temperature dependence of microwave losses of a cavity resonator containing a thin superconducting film of FeSe1-xTex placed perpendicular to the magnetic field of the resonator. An experiment with a film oriented parallel to the field shows the absence of the anomaly. The appearance of the anomaly for T ≤ Tc in the case of the perpendicular orientation of the film is explained by a change in the field orientation and distribution in the film with changing temperature. Comparison of the experimental data with the theoretical model that does not take into account the field evolution in the film shows that the model is applicable to microwave impedance studies of films with a perpendicular orientation in the range from low temperatures to ∼2Tc/3. The expressions for the effective surface impedance of superconducting films in the N-state are also obtained, which are required for constructing the temperature dependence of the effective impedance in the temperature range including Tc.

High gain holographic antenna for terahertz applications

A high gain antenna with controlling electromagnetic surface propagation and radiation for terahertz application is proposed based on a holographic artificial impedance surface. The artificial impedance surface is composed of sub wavelength square metallic patches on a grounded dielectric slab. The effective surface impedance depends on the size of the patches and can be varied as a function of position and direction. Using the holographic technique enables the desired direction radiation and high gain to be achieved. The detailed design procedure of impedance modulation and the simulation results are presented.

Optimizing sound transmission through 3D pentamode materials in water

Pentamode materials (PMs) are engineered isotropic or anisotropic elastic structures that mimic the acoustic behavior of ideal fluids. Recently, many applications such as negative refraction lens, focal lens, and a cloak have been designed and demonstrated using 2D PMs. We consider metallic lattice structures in which only one wave mode propagates over a wide frequency band. Unlike the 2D PM which naturally has a boundary, the three dimensional PM needs to be sealed to isolate the interior structure from water. However, the material used for encapsulation alters the effective surface impedance of the material. In this talk, we present 3D PMs with acoustic properties matched to water and show that the transmission is sensitive to the thickness of the boundary plate. An analytic model is established to provide some insights into the boundary transmission. Iterations were performed in FEM simulations to achieve optimal transmission within the frequency range of interest (4 kHz–10 kHz). The model PM with partial boundary is made using 3D metal printing, and the remaining boundary is added post fabrication. Preliminary experimental results will be discussed. [Work supported by ONR/MURI.]

Effective Nonlinear Surface Impedance of Conductive Magnetic Slabs

A new formula is developed for the surface impedance of a nonlinear magnetic conductive semi-infinite slab. The impedance expression allows for proper incorporation of the nonlinear characteristic of the magnetic material, also using an analytical expression. Thus, the usual step representation of the $B$ – $H$ curve is superseded. Normal and tangential magnetic components may simultaneously exist, avoiding the classical premise that only the tangential component exists. This way, it is possible to represent highly magnetic permeable materials where the normal component of the magnetic field is the main one. Our analytical formulation and solution can be for example used for calculation of losses in electrical machines or along with FE software code to avoid meshing of magnetic conductive regions.

Infrared broadband metasurface absorber for reducing the thermal mass of a microbolometer

We demonstrate an infrared broadband metasurface absorber that is suitable for increasing the response speed of a microbolometer by reducing its thermal mass. A large fraction of holes are made in a periodic pattern on a thin lossy metal layer characterised with a non-dispersive effective surface impedance. This can be used as a non-resonant metasurface that can be integrated with a Salisbury screen absorber to construct an absorbing membrane for a microbolometer that can significantly reduce the thermal mass while maintaining high infrared broadband absorption in the long wavelength infrared (LWIR) band. The non-dispersive effective surface impedance can be matched to the free space by optimising the surface resistance of the thin lossy metal layer depending on the size of the patterned holes by using a dc approximation method. In experiments a high broadband absorption was maintained even when the fill factor of the absorbing area was reduced to 28% (hole area: 72%), and it was theoretically maintained even when the fill factor of the absorbing area was reduced to 19% (hole area: 81%). Therefore, a metasurface with a non-dispersive effective surface impedance is a promising solution for reducing the thermal mass of infrared microbolometer pixels.

Effective-medium description of a metasurface composed of a periodic array of nanoantennas coupled to a metallic film

We compute the reflectance properties of a metasurface that consists of a doubly periodic array of patch nanoantennas strongly coupled to a metallic film. Each plasmonic patch antenna can be accurately modeled as a polarizable, radiating, magnetic dipole. By accounting for interactions amongst the dipoles, an equivalent surface polarizability can be obtained, from which the effective surface impedance, reflectivity, and other homogenized quantities of interest can be obtained. When the metasurface is extremely close to the metal film, the interaction between constituent dipoles is dominated by surface plasmon mediation. We calculate analytically the dipole interaction constant by explicitly evaluating the infinite sum of fields from all the dipoles in the lattice. While a single film-coupled nanoparticle exhibits anomalous loss due to coupling to surface plasmons, we find that for the lattice of dipoles, the radiation reaction force due to the coupling to the surface plasmon modes is exactly canceled by the interaction constant; the lattice thereby conserves energy in the limit of zero Ohmic loss. When Ohmic losses are present, absorption to surface plasmons reemerges and can be compared with the losses to radiation and Ohmic absorption in the metasurface.

Novel Metamaterial Surfaces from Perfectly Conducting Subwavelength Corrugations

Motivated by the numerical experiments carried out in [S. C. Yurt, A. Elfrgani, M. I. Fuks, K. Ilyenko, and E. Schamiloglu, IEEE Trans. Plasma Sci., 44 (2016), pp. 1280--1286], we apply an asymptotic analysis to show that corrugated waveguides can be approximated by smooth cylindrical waveguides with an effective metamaterial surface impedance. We show that this approximation is in force when the period of the corrugations is subwavelength. Here the metamaterial delivers an effective anisotropic surface impedance and imparts novel dispersive effects on signals traveling inside the waveguide. These properties arise from the subwavelength resonances inside the corrugations. For sufficiently deep corrugations, the metamaterial waveguide predicts backward wave propagation. In this way we may understand backward wave propagation as a multiscale phenomenon resulting from local resonances inside subwavelength geometry. Our approach is well suited to numerical computation, and we provide a systematic investigatio...

A post-processing method to simulate the generalized RF sheath boundary condition

For applications of ICRF power in fusion devices, control of RF sheath interactions is of great importance. A sheath boundary condition (SBC) was previously developed to provide an effective surface impedance for the interaction of the RF sheath with the waves. The SBC enables the surface power flux and rectified potential energy available for sputtering to be calculated. For legacy codes which cannot easily implement the SBC, or to speed convergence in codes which do implement it, we consider here an approximate method to simulate SBCs by post-processing results obtained using other, e.g. conducting wall, boundary conditions. The basic approximation is that the modifications resulting from the generalized SBC are driven by a fixed incoming wave which could be either a fast wave or a slow wave. The method is illustrated in slab geometry and compared with exact numerical solutions; it is shown to work very well.

Wavelength-Selective Infrared Metasurface Absorber for Multispectral Thermal Detection

A wavelength-selective metasurface absorber suitable for use in multispectral microbolometer focal plane arrays in the long-wavelength infrared (LWIR) region is theoretically and experimentally investigated. We show that a thin metal metasurface, which is characterized by effective surface impedance, as an absorbing layer can be integrated with an asymmetric Fabry–Perot cavity to construct a wavelength-selective perfect absorber. The absorbed infrared energy is mainly dissipated in a thin metal metasurface, in contrast to a multilayer metamaterial absorber. The calculated and experimental spectral responses of the metasurface absorbers show excellent wavelength-selective narrowband absorption to allow multispectral imaging in LWIR.