The classic anisotropic spherical cloak can be mimicked by many alternating thin layers of isotropic metamaterials [Qiu et al. Phys. Rev. E 79 (2009) 047602]. We propose an improved method of designing permittivity and permeability in each isotropic layer, which eliminates the jumping of the refractive index at the interface. Multilayered spherical cloaks designed by the present method perform much better than those by Qiu et al., especially for forward scattering. It is found that the ratio of layer thickness to the operating wavelength plays an important role in achieving invisibility. The presented cloak should be discretized to at least 40 layers to meet the thickness threshold corresponding to 10% scattering.
The origami structure has caused a great interest in the field of engineering, and it has fantastic applications in the deployable and reconfigurable structures. Owing to the unique multi-stable states, here a typical hexagon-twist origami structure is fabricated via multi-material printing technology. The printed structure has multi-stable features and the stiffness of the deformable structure is dramatically reduced under thermal triggering. Such behavior causes an increase in the structural degree of freedom, allowing for self-deployment via releasing the prestored energy in the elastic crease. The response time and reaction time of the self-deployment process are also studied and illustrate the higher energy barrier of the folded state, the longer self-deployment time. Utilizing such unique features and design principles, a prototype of frequency reconfigurable origami antenna of nine diverse operating modes is subsequently designed and assembled with the hexagon-twist origami structure as the dielectric substrate. The antenna implements the cross-band from two different frequency bands, enabling to realize frequency reconfigurable under thermal condition.
The achromatic subdiffraction lens with large numerical aperture (NA) is of significant importance in optical imaging, photolithography, spectroscopy, and nanophotonics. However, most of the previous research on subdiffraction lenses has been restricted by limited bandwidth and efficiency as well as severe chromatic aberrations. In this paper, a semicircular gradient index lens (sGRIN) with a modified refractive index profile originated from a Maxwell fish-eye lens is put forward to achieve highly efficient (above 81%) achromatic (4–20 GHz) subdiffraction focusing at the focusing line (around 0.28λ ) with large NA of 1.3 and broadband diffraction-limited far-field radiation (4–16 GHz) theoretically, which overcomes the drawbacks of previous works. The presented lens is designed by gradient dielectric metamaterials. Evanescent waves ignited at the lens/air interface and transformation of electromagnetic (EM) waves with high spatial frequency in sGRIN to EM waves with low spatial frequency in air are responsible for subdiffraction focusing and diffraction-limited far-field radiation, respectively. Experimental results demonstrate the excellent performance of achromatic subdiffraction focusing and diffraction-limited far-field radiation. The presented lens has great potential to be applied in subdiffraction imaging systems.
Abstract Smart structures with manipulatable properties are highly demanded in many fields. However, there is a critical challenge in the pursuit of transparent windows that allow optical waves (wavelength of µm–nm) for transmitting while blocking microwave (wavelength of cm) in terms of absorbing electromagnetic energy, specifically for meeting the frequency requirement for the 5th generation (5G) mobile networks. For fundamentally establishing novel manipulatable microwave absorbing structures, here, new polymeric aqueous gels as both optically transparent materials and microwave absorbing materials are demonstrated, in which polar networks play significant roles in attenuating electromagnetic energy. By manipulating the hydrogen bonding networks, the resulting optically transparent solid‐state gels are able to offer the capabilities for absorbing microwaves. Interestingly, such gels can be switched into an optically opaque state via converting the amorphous state into a polycrystal state when the temperature is decreased. Such ionic conductive gels can endow the assembled sandwich windows with effective microwave absorbing capability in the range of 15–40 GHz, covering a branch of 5G frequency bands. The results highlight a new strategy for using ionic conductive gels to design and fabricate manipulatable microwave stealth structures for various applications.
Abstract A 3D-printed ultra-broadband planar Luneburg lens composed of periodic gradient structures is proposed in this paper. Compared to the previous studies, the proposed lens has a pretty large operating frequency range, spanning from 5 to 22 GHz. The measured results of the near-field and far-field illustrate the feasibility and validity of this lens antenna. High efficiency achromatic sub-diffraction focusing with a full-width at half-maximum of 0.3 λ and a highly directive far-field radiation pattern with side lobes below −10 dB are achieved. Triple input and triple output function is also realized experimentally.
Abstract Based on the perspective of a wide scanning range and ultra-broad bandwidth, Luneburg lenses are highly anticipated to be an outstanding option for multibeam radiation. However, owing to the lack of low-loss continuously varying permittivity materials, the practical application of Luneburg lenses is far below the expected level. In this paper, an ultra-wideband planar Luneburg lens (PLL) is proposed. Due to the novel design of an all dielectric lightweight radially symmetric periodic gradient metamaterial, the presented lens is able to yield highly directional emission with side lobes all below −8 dB and achromatic sub-diffraction focusing with full width at half maximum about 0.4 λ from 4 GHz to 22 GHz. The prototype of the lens is manufactured by computer-numerical controlled machining. The measured data of the near field and far field agree well with that of the simulated data, verifying the effectiveness of the proposed design methodology. The superiority of the presented approach to design a Luneburg lens is demonstrated. Therefore, the PLL has the advantages of being lightweight, with a compact structure, low profile, ultrabroadband function, high resolution, and convenient fabrication, giving it great potential to be practically deployed.
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