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.
3 More- Received 23 February 2015
DOI:https://doi.org/10.1103/PhysRevX.5.031005
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Published by the American Physical Society
Synopsis
The Helical Factor
Published 14 July 2015
An array of helical elements absorbs radiation of a certain frequency while casting no shadow in light over a range of other frequencies.
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Popular Summary
Electromagnetic waves play a pivotal role in our everyday lives; they are what we see and how we communicate. The manipulation of waves has led to a myriad of applications and has revolutionized society through communications, medicine, and entertainment. New electromagnetic and optical devices with properties that exceed those achieved by conventional lenses and mirrors are currently being developed, and planar films of advanced artificial engineered materials have opened up new degrees of freedom to guide and bend electromagnetic waves. Such films, so-called metasurfaces, are capable of modifying the intensity, direction, and even the shape of wave beams passed through them. However, metasurfaces are all characterized by the same major drawback: They efficiently manipulate transmitted radiation of a desired frequency, but they do not fully transmit electromagnetic waves of other frequencies. This limitation creates a perceptible shadow, while the overwhelming majority of applications require wave-controlling devices that do not create a shadow and are transparent to radiation at nonoperational frequencies. In this work, we demonstrate the first realization of such devices that are free of shadows.
We employ a single-layer metasheet that contains inclusions that can be polarized both electrically and magnetically. These inclusions, whose sizes are on the order of the impinging microwave radiation, are smooth helices with one or two turns. We design and experimentally test metasurfaces that totally absorb incident radiation at the operational frequency and are fully transparent elsewhere in the spectrum. We prove that such unparalleled operation can be achieved in wave-manipulating devices using only structural inclusions whose electric and magnetic properties are strongly coupled.
We expect that our results will motivate future work to extend the functionality of the proposed structures and design new devices to fully control electromagnetic waves.