Lossy Periodic Structures Research Articles

Generalized normal mode expansion method for open and lossy periodic structures

We describe and demonstrate the extension of permittivity mode expansion, which is also know as generalized normal mode expansion (GENOME), to open and lossy periodic structures. The resulting expansion gives a complete spatial characterization of any open periodic structure, via the quasi-periodic Green’s tensor, by a complete, discrete set of modes rather than a continuum. The method has been validated by comparing our expansion of an open waveguide array with a direct scattering calculation. Good agreement was obtained regardless of the source location or detuning from resonance.

Resonant field enhancement in lossy periodic structures supporting complex bound states in the continuum

Resonant modes in a lossy periodic structure sandwiched between two lossless homogeneous media form bands that depend on the Bloch wave vector continuously and have a complex frequency due to radiation and absorption losses. A complex bound state in the continuum (cBIC) is a special state with a zero radiation loss in such a band. Plane waves incident upon the periodic structure can be resonantly enhanced. An existing scaling law states that the field enhancement η , defined as the ratio between the amplitudes of the local field and the incident wave, is proportional to Q r a d Q d i s / ( Q r a d + Q d i s ) , where Q r a d and Q d i s are the quality factors associated with radiation and absorption losses, respectively. In this paper, we show that this scaling law is not always valid when there are multiple radiation channels. Using a perturbation method, we derive a rigorous formula for field enhancement, and show the scaling law is valid for specific incident fields, but not valid for optimal incident fields that maximize the field enhancement.

Bound states with complex frequencies near the continuum on lossy periodic structures

On a lossless periodic dielectric structure sandwiched between two homogeneous media, bound states in the continuum (BICs) with real frequencies and real Bloch wavevectors may exist, and they decay exponentially in the surrounding homogeneous media and do not couple with propagating plane waves with the same frequencies and wavevectors. The BICs are of significant current interest, because they give rise to high-$Q$ resonances when the structure or the Bloch wavevector is slightly perturbed. In this paper, the effect of a small material loss on the BICs is analyzed by a perturbation method and illustrated by numerical results. It is shown that bound states with complex frequencies near the continuum appear, but they behave differently depending on whether the BIC is symmetry-protected or not. The Bloch wavevector of a bound state with a complex frequency can be real if the original BIC is symmetry-protected, and it is usually complex if the original BIC is not symmetry-protected. Our study improves the theoretical understanding on BICs and provides useful insight for their practical applications.

Earth-Effect Emulation Using Periodic Structures in Scaled-Model Characterization of HF Antennas

We present a technique for emulating the impact of the presence of earth on the responses of high-frequency (HF) antennas characterized using scaled-model measurements. Scaled-model measurements are routinely used to characterize the responses of HF antennas, particularly those mounted on physically large platforms. The presence of the ground in the vicinity of such antennas and its electrical parameters can significantly impact the antenna response. However, this impact is not usually taken into account in scaled-model measurements. In this letter, we demonstrate that the earth impact in such scaled-model measurements can be emulated by using a lossy periodic structure, with subwavelength unit-cell dimensions, designed to emulate the electrical properties of the ground at the scaled frequency of operation. The efficacy of this approach is demonstrated using a design example for a platform-based HF antenna operating in the vicinity of wet earth. A scaled-model prototype of the antenna and the ground-emulating periodic structure are fabricated and experimentally characterized. Measurement and simulation results demonstrate a good agreement and verify the capability of using this approach to perform more accurate, scaled-model measurement of HF antennas.

Modal transmission-line modeling of propagation of plane radiowaves through multilayer periodic building structures

The modal transmission-line (MTL) method is used to analyze the shielding effectiveness of buildings, which is an important parameter for the accurate planning of microcellular communication networks. For that, the buildings are modeled as multilayer lossy periodic structures. After a description of the extended MTL-theory, the newly developed MTL model is applied to brick walls, reinforced-concrete walls and concrete block walls. From a comparison of the MTL-results with previously published theoretical or experimental results it is concluded that the MTL method is very well suited, because it is accurate as well as computational time-efficient.

Multimode Network Analysis of Absorption Characteristics of Lossy Dielectric Periodic Structures

In this paper, the absorption characteristics of lossy periodic structure are investigated using a method which combines the multimode network theory with the rigorous mode-matching method. The dependence of the absorption characteristics on the incident wave and structure parameters is quantitatively analyzed; the guidelines to the design and optimization of absorbers in anechoic chamber and other applications are thus given.

Shielding and reflection properties of periodic fiber-matrix composite structures

The plane-wave shielding and reflection properties of a single-layer composite material are analyzed, based on a filament-current phase-correction model that treats the composite material as a lossy periodic structure of fibers situated in a dielectric matrix. The scattering effect of the fibers is dealt with by calculating the scattered fields from the fiber currents, which are assumed to be concentrated along the centers of the fibers. To include the effect of the dielectric matrix, multiple reflection and transmission at the air-matrix interfaces, as well as the phase correction across the fiber grating, are incorporated in the analysis. In this study, only numerical results for the TM case are presented for graphite/epoxy composite materials. In particular, the parameters that influence the shielding and reflection characteristics, such as the angle of incidence and the frequency of the incident wave, the conductivity and radius of the fibers, and the dielectric constant of the matrix, are investigated in detail. Also included are results to illustrate the frequency-selective phenomena associated with composite structures.