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.

Control of electromagnetic waves using engineered materials is very important in a wide range of applications, therefore there is always a continuous need for new and more efficient solutions. Known natural and artificial materials and surfaces provide a particular functionality in the frequency range they operate but cast a "shadow" and produce reflections at other frequencies. Here, we introduce a concept of multifunctional engineered materials that possess different predetermined functionalities at different frequencies. Such response can be accomplished by cascading metasurfaces (thin composite layers) that are designed to perform a single operation at the desired frequency and are transparent elsewhere. Previously, out-of-band transparent metasurfaces for control over reflection and absorption were proposed. In this paper, to complete the full set of functionalities for wave control, we synthesize transmitarrays that tailor transmission in a desired way, being "invisible" beyond the operational band. The designed transmitarrays for wavefront shaping and anomalous refraction are tested numerically and experimentally. To demonstrate our concept of multifunctional engineered materials, we have designed a cascade of three metasurfaces that performs three different functions for waves at different frequencies. Remarkably, applied to volumetric metamaterials, our concept can enable a single composite possessing desired multifunctional response.Comment: 9 pages, 9 figures, journal pape

Cuesta, F. S.; Faniayeu, I. A.; Asadchy, V. S.; Tretyakov, S. A. Planar broadband Huygens' metasurfaces for wave manipulationsAbstract-Electrically thin and effectively two-dimensional material composites, metasurfaces, have been widely exploited for manipulation of electromagnetic waves. For many applications it is desired to transform incident waves of a specific frequency range keeping the metasurface invisible at other frequencies.Such frequency-selective response can be achieved based on subwavelength Huygens' inclusions. However, their fabrication requires sophisticated processes due to the three-dimensional geometry. Here, we propose a planar Huygens' meta-atom with the goal to open a way to realize broadband invisible metasurfaces with topologies suitable for the conventional printed circuit board fabrication technology. We synthesize and analyse, both numerically and experimentally, three different metasurfaces capable of polarization and amplitude transformations of incident waves.

Perfect electromagnetic metamaterial absorbers based on three-dimensional (3D) vertical split-ring resonators for an infrared spectral range were fabricated using a combination of high-resolution direct laser write lithography and a simple metalization by sputtering. In accordance with theoretical predictions, the fabricated samples exhibit perfect absorption resonances tunable in the wavelength range of 4.5–9.2 µm by changing the dimensions and spacing of the resonators. The structures exhibit polarization and incidence angle-invariant operation with absorbance in excess of 0.85 for incidence angles up to 30°. In the future, they may find applications as narrow-band thermal emitters and for signal enhancement in mid-IR photodetectors.

A twist polarizer metasurface for polarization rotation by an angle of 90° is proposed and realized at microwave frequencies. The metasurface consists of sub-wavelength metallic helices arranged periodically in a single layer and operates in transmission geometry with a nearly unity cross-polarization conversion coefficient at resonance. The structure exhibits low reflectivity R<0.06 within a decade-spanning frequency range of 0.1–5.5 GHz and is insensitive to the polarization orientation of the incident wave. Moreover, it can operate with high efficiency at oblique incidence angles of up to 35°. Such twist polarizer metasurfaces are potentially applicable as electromagnetic/optical isolators and frequency-selective polarization antennas.

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Nanophotonic chiral antennas exhibit orders of magnitude higher circular dichroism (CD) compared to molecular systems. Merging magnetism and structural chirality at the nanometric level allows for the efficient magnetic control of the dichroic response, bringing exciting new prospects to active nanophotonic devices and magnetochirality. Here we devise macroscale enantiomeric magnetophotonic metasurfaces of plasmon and ferromagnetic spiral antennas. Mixed 2D-and 3Dchiral nanoantennas induce large CD response, where we identify reciprocal and non-reciprocal contributions. The simultaneous chiroptical and magneto-optical response in a wide spectral range with these metasurfaces delivers an attractive platform for the study of magnetochirality at the nanoscale. Exploring further this type of magnetophotonic metasurfaces allows the realization of high-sensitivity chiral sensors and prompts the design of novel macroscopic optical devices operating with polarized light.

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