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Energy of propagating electromagnetic waves can be fully absorbed in a thin lossy layer, but only in a narrow frequency band, as follows from the causality principle. On the other hand, it appears that there are no fundamental limitations on broadband matching of thin resonant absorbing layers. However, known thin absorbers produce significant reflections outside of the resonant absorption band. In this paper, we explore possibilities to realize a thin absorbing layer that produces no reflected waves in a very wide frequency range, while the transmission coefficient has a narrow peak of full absorption. Here we show, both theoretically and experimentally, that a thin resonant absorber, invisible in reflection in a very wide frequency range, can be realized if one and the same resonant mode of the absorbing array unit cells is utilized to create both electric and magnetic responses. We test this concept using chiral particles in each unit cell, arranged in a periodic planar racemic array, utilizing chirality coupling in each unit cell but compensating the field coupling at the macroscopic level. We prove that the concept and the proposed realization approach also can be used to create nonreflecting layers for full control of transmitted fields. Our results can have a broad range of potential applications over the entire electromagnetic spectrum including, for example, perfect ultracompact wave filters and selective multifrequency sensors.

In this article, we study spiral particles with optimized design parameters, which can make possible the realization of media with equal dielectric, magnetic, and chiral susceptibilities. Two different spiral structures are investigated: the canonical spiral (which consists of a split loop with two straight-wire sections, orthogonal to the loop plane and connected to the edges of the gap) and the true helix (which is obtained by bending a wire with a constant pitch angle). The transmission and reflection coefficients of arrays of spiral particles are obtained under plane wave excitation by numerical simulation. The properties of slabs formed by periodic chiral and racemic arrays of spirals are investigated. Good agreement is found between the presented results and the previously reported theoretical and experimental studies.

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The possibility of linearly polarized electromagnetic microwave conversion into a circularly polarized wave using single-turn helices with preliminary calculated optimal parameters is demonstrated. The helices are characterized by equal dielectric, magnetic, and chiral susceptibilities. Such optimal helices can be further used, for example, to develop reflectionless coatings and metamaterials with negative electromagnetic wave refraction. It is demonstrated that the examined helices activated by both electric and magnetic fields, that is, for any arbitrary orientation of the incident wave polarization plane, have optimal characteristics. This is the advantage of the optimal helices over other possible metamaterial elements, for example, rectilinear vibrators or circular resonators.