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Design of two invisibility cloaks using transmissive and reflective metamaterial-based multilayer frame microstructures
Abstract
Ultrathin metamaterials provide new possibilities for the realization of cloaking devices because of their ability to control electromagnetic waves. However, applications of metamaterials in cloaking devices have been limited primarily to reflection-type carpet cloaks. Hence, a transmissive free-space cloak was developed using a multilayer frame structure, wherein highly transparent metamaterials were used to guide incident waves into propagating around an object. The cloaking effect was quantitatively verified using near-field and far-field distributions. Metamaterials allow for the cloaking shells of transmissive cloaks to be developed without spatially varying extreme parameters. Moreover, a transmissive invisible cloak with metamaterial-based mirrors was designed. The design principle of this cloak with a frame structure consists of four metamaterial-based mirrors and two metal mirrors. After covered with the designed metamaterials-based mirrors cloak, the outgoing electromagnetic wave is restored greatly as if the wave passes directly through the obstacle without distortion. This cloak used the metamaterials mirrors to adjust the reflected angle, so that the outgoing electromagnetic wave does not change direction, thereby achieving the cloaking effect.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The realization of invisibility has always been a significant goal and has inspired considerable research. The concept of the invisibility cloak has attracted significant attention from the scientific community in the past decade [1]. Metamaterials provide routes to manipulating electromagnetic (EM) waves [2–7] as a method for achieving cloaking and invisibility [8–16]. Early cloaks were designed mainly based on transformation optics (TO) [17], with which EM waves can be guided around a hidden object via control over the complex distribution of spatial permittivity and permeability. Such cloaks indeed cause the target object to appear invisible. However, this method generally requires bulk metamaterials to satisfy the requirements for some anisotropic and inhomogeneous material parameters. Based on this background, some simplified methods, such as quasi-conformal mapping [18] and homogeneous coordinate transformation [19], have been proposed. However, the resulting cloaks are bulky and suffer from loss, which makes them difficult to use in a laboratory. To overcome bulkiness, plasmonic cloaks based on scattering cancellation have been developed [20]. These devices can produce cloaking effects while having small thicknesses, but usually perform poorly under high-sensitivity detection.
Other methods, such as the use of near-zero refractive index metamaterials [21] and microstrip patch antennas [22], which can also direct EM wave energy around hidden objects, have also been used to create cloaking devices.
Recently, metamaterials, which are artificially ultrathin layers, have attracted significant attention from researchers. These materials have extraordinary control over the phases, amplitudes, and polarizations of incident waves. Metamaterials can break the thickness limits of conventional cloaks that have been developed based on metamaterials [23]. As a result, metamaterials provide possibilities for the design of an effective invisibility cloak. Several metamaterial-based carpet cloaks that can control the phases of reflected waves to restore the near-field wavefront have been proposed.
Metamaterial-based carpet cloaks have many advantages, such as low loss, high shapeability, ease of fabrication, and high flexibility. However, metamaterial-based carpet cloaks based on local phase compensation operate predominantly in the reflection mode. Although several designs for transmissive metamaterial-based cloaks have already been explored [24–27], the transmissive cloak developed in this study may substantially widen the practical applicability of metamaterial-based cloaks.
In this paper, the design of a transmissive invisibility cloak based on a multilayer frame structure of metamaterials is proposed. Such a metamaterial cloak increases the capability of traditional cloaks to achieve the transmissive mode. The developed cloak can considerably simplify the design and experimental process of TO-based cloaks, which currently require complex material parameters and a complex spatial distribution. The proposed invisibility cloak is composed of three metamaterials, the functions of which are beam splitting, steering, and collection of incident waves. With this cloak, the near-field wavefront of the transmitted EM wave can be effectively restored, and scattering can be minimized. Numerical simulations not only verify the cloaking effect but also create possibilities for the design of a transmissive cloak while avoiding the use of complex material parameters.
2. Cloaking theory
Currently, metamaterials are widely used in ultrathin carpet cloaks because of their extraordinary control over the phases and amplitudes of waves. Such metamaterials are composed of many unit cells, which permit local phase compensation to be accomplished through variations in the size parameters of the unit cells. In this study, a transmissive metamaterial-based cloak was developed based on a novel three-layer frame structure composed of metamaterials. Figure 1(a) shows a schematic diagram of the new invisibility cloak based on this metamaterial frame structure, where a y-polarization plane wave with normal incidence through the invisibility cloak can bypass the obstacle and emerge as a plane wave.
Fig. 1. (a) 3D scheme of transmissive invisibility cloak. (b) Schematic diagram of transmissive metamaterial cloak. I, II, III represent three transparent metamaterials with their respective phase gradients.
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5. Conclusions
A transmissive metamaterial cloak based on a frame structure composed of metamaterial layers was presented in this paper. The designed cloak is different from a TO-based cloak, which requires complex material parameters and spatial distribution. This cloak can guide EM waves into propagating around an object, thus creating an ideal hidden free space. The proposed cloak has a simple spatial structure, which simplifies its manufacturing for practical experiments. We also designed a transmitted invisible cloak with the metamaterials-based mirrors. Four metamaterials mirrors were used to control the reflective electromagnetic wave, and two metal mirrors on both sides of rhombus frame were also used to reflect the incoming wave from the metasurface-based mirrors. The application of the cloak can also be easily extended to other frequency domains, and the cloak can conceal large obstacles when carefully designed.
For the full paper:
https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-28-24-35528&id=442493
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