Broadband Reflectionless Metasheets: Frequency-Selective Transmission and Perfect Absorption

Abstract

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.

Stored and absorbed energy of fields in lossy chiral single-component metamaterials

“…As shown by numerical simulations [18], under the condition of half-wave resonance, the resonant frequency depends mainly on the length of the wire, if its thickness is small. In this case, the shape of the wire, that is, the shape of the metamaterial element, has little effect on the resonance frequency.…”

“…In this case, the shape of the wire, that is, the shape of the metamaterial element, has little effect on the resonance frequency. For example, in paper [18] the change in the number of turns for optimal helices was investigated. In this case, the shape of the helix, that is, its pitch angle, also changed.…”

“…Optimal helices, introduced and studied in Refs. [8][9][10][12][13][14][15][16][17][18], interact only with waves of one of the two orthogonal circular polarizations. The geometrical parameters of such helices are known.…”

“…In that paper, "maximum chirality" corresponds to the optimal helices, introduced and studied in Refs. [12][13][14][15][16][17][18], that is, to helices which interact only with waves of one of the two orthogonal polarizations. However, the present results show that extreme chirality can be possibly defined based on other criteria: maximally strong interaction with one of the CP modes or maximum difference of reactive energies of an object in the field of right-or left-hand polarized waves.…”

The equilibrium state of bifilar helix as element of metamaterials

“…The metal helices are widely used as elements of metamaterials because at transmission of electrical current in them the electric dipole moments and magnetic moments are simultaneously generated. Consequently metamaterial based on helical elements displays as dielectric and magnetic properties, which enhances its use [1][2][3][4][5][6][7][8][9][10][11] (Fig.1). A special place among the helices take bifilar helices consisting of two helical conductors.…”