Reflection phase dispersion editing generates wideband invisible acoustic Huygens's metasurface.

Acoustic metasurfaces show non-traditional abilities in wave manipulation and provide alternate mechanisms for information communication and invisibility technology. However, most of the mechanisms remain narrow band (relative bandwidth ∼5%), and a wideband trait is essential for engineering applications. For example, controllable effective material properties-reflection or transmission phase-has barely been realized in wideband because the intrinsic dispersion relation is not always editable. In this paper, wideband reflection phase editing is realized, and wideband invisibility of a phase preserved Huygens's metasurface on a flat background is achieved with anomalous reflection. This metasurface is built with proposed unsymmetrical twin Helmholtz resonators which reach a predefined dispersion relation target value. The total instantaneous acoustic fields show nearly identical carpeting effects in a consecutive band with relative bandwidth 52.1% (from 5400 to 9200 Hz) in simulation and experiment.

Flexible thin broadband microwave absorber based on a pyramidal periodic structure of lossy composite.

Microwave absorber with broadband absorption and thin thickness is one of the main research interests in this field. A flexible ultrathin and broadband microwave absorber comprising multiwall carbon nanotubes, spherical carbonyl iron, and silicone rubber is fabricated in a newly proposed pyramidal spatial periodic structure (SPS). The SPS with equivalent thickness of 3.73 mm covers the -10 dB and -15 dB absorption bandwidth in the frequency range 2-40 GHz and 10-40 GHz, respectively. The excellent absorption performance is achieved by concentration and dissipation of the electromagnetic field inside different parts of the magnetic-dielectric lossy protrusions in different frequency ranges.

Symbiotic Electromagnetic Shadow for Regional Invisibility and Camouflage.

Low detectability and camouflage skills in the electromagnetic wave and light frequency range provide survival advantages for natural creatures and are essential for understanding the operational principles of the biosphere. Taking inspiration from natural mutualistic symbiosis, this paper proposes a symbiotic electromagnetic shadow camouflage mechanism based on a superdispersive surface, aiming to investigate its impact on the observability of specific objects. The design and experimental results indicate that the symbiotic shadow dihedral can significantly reduce overall scattering quantity, which reaches at least 10 dB shrink in the 12-18 GHz frequency range compared to the contrast object. Unlike known camouflage methods, the electromagnetic shadow technology shrinks the overall scattering without any coating and shield metal target while probably offering extensive functional design freedom for the concealed object, creature, or equipment. This also provides a hint to explore symbiosis-related camouflage phenomena in nature.

Evidence of negative-index refraction in nonlinear chemical waves.

The negative index of refraction of nonlinear chemical waves has become a recent focus in nonlinear dynamics researches. Theoretical analysis and computer simulations have predicted that the negative index of refraction can occur on the interface between antiwaves and normal waves in a reaction-diffusion (RD) system. However, no experimental evidence has been found so far. In this Letter, we report our experimental design in searching for such a phenomenon in a chlorite-iodide-malonic acid (CIMA) reaction. Our experimental results demonstrate that competition between waves and antiwaves at their interface determines the fate of the wave interaction. The negative index of refraction was only observed when the oscillation frequency of a normal wave is significantly smaller than that of the antiwave. All experimental results were supported by simulations using the Lengyel-Epstein RD model which describes the CIMA reaction-diffusion system.

Ultrabroadband Three-Dimensional Printed Radial Perfectly Symmetric Gradient Honeycomb All-Dielectric Dual-Directional Lightweight Planar Luneburg Lens.

An ultrabroadband all-dielectric planar Luneburg lens has been designed and fabricated in this study, which is in the form of a radial gradient lightweight honeycomb column. Because of the novel design of a radial symmetric honeycomb-like microstructure in the subwavelength dimension and the radial gradient configuration according to the refractive index distribution of Luneburg lens, the present lens can focus incident plane waves on the opposite side with high convergence, and its operating frequency range is rather broadband, spanning from 6 to 16 GHz. Besides, the all-dielectric honeycomb-like lens is lightweight with a mass density of 0.23 g/cm3, and its broadband transmittance is higher than the reported cases consisting of metallic metamaterial or gradient photonic crystal structures. A prototype of the lens is fabricated by using 3D printing techniques, on which the electric near-field distribution and far-field radiation pattern measurements have been carried out, and the aforementioned performances were demonstrated experimentally. It was also observed that for two point sources placed at the edge of the lens whose intersection angle with the center of the lens is 90°, the far-field radiation pattern was still kept highly directional, which means that the lens can generate two highly directional beams simultaneously, and is an efficient double input-double output device.

Ultrathin Flexible Carbon Fiber Reinforced Hierarchical Metastructure for Broadband Microwave Absorption with Nano Lossy Composite and Multiscale Optimization.

The implementation of thin structure for broadband microwave absorption is challenging due to the requirement of impedance match across several frequency bands and poor mechanical properties. Herein, we demonstrate a carbon fiber (CF) reinforced flexible thin hierarchical metastructure (HM) composed of lossy materials including carbonyl iron (CI), multiwall carbon nanotube (MWCNT), and silicone rubber (SR) with thickness of 5 mm and optimal concentration selected from 12 formulas. Optimization for the periodical unit size is applied, and impacts of structural sizes on absorption performance are also investigated. An effective process combining the vacuum bag method and the hand lay-up technique is used to fabricate the HM. Experimental reflectivity of the absorber achieves broadband absorption below -10 dB in 2-4 GHz and 8-40 GHz. The full band in 2-40 GHz is covered below -8 dB. Yielding stress of the HM is increased to 24 MPa with attachment of CF, while the fracture strain of the composite reaches 550%. The soft HM is suitable to adhere to the curved surface of objects needed to be protected from microwave radiation detection and electromagnetic interference. Enhanced mechanical properties make it possible for further practical applications under harsh service environments such as the ocean and machines with constant vibration.

Graphene-Based Sandwich Structures for Frequency Selectable Electromagnetic Shielding.

Due to substantial development of electronics and telecommunication techniques, materials with electromagnetic interference (EMI) shielding performance are significant in alleviating the interference impacts induced from a remarkable variety of devices. In the work, we propose novel sandwich structures for manipulating the EM wave transport, which holds unique EMI shielding features of frequency selectivity. By employing electrical and magnetic loss spacers, the resultant sandwich structures are endowed with tunable EMI shielding performance, showing substantial improvements in overall shielding effectiveness along with pronounced shielding peak shift. The mechanisms suggest that the multiple interfaces, electromagnetic loss media, and changes of representative EM wavelength could be critical roles in tailoring the EMI shielding performance. The results provide a versatile strategy that could be extended in other frequency ranges and various types of sandwich structures, promising great opportunities for designing and fabricating advanced electromagnetic attenuation materials and devices.

Disordered plane waves in the transition between target and antitarget patterns.

Since the experimental observation of antiwaves in reaction-diffusion (RD) systems, the discrepancy between the theoretical prediction and the experimental observation on the transition from inwardly rotating chemical waves to normal waves remains an unsolved problem. Theoretical predictions using both RD model and complex Ginzberg-Landau equation indicate that there exists a trend in which wave vector approaches to zero in the transition process, while disordered plane waves near the onset were observed in experiment. This discrepancy motivated us to conduct a systematic research to investigate the transition. Using chlorite-iodide-malonic acid reaction as model system and with a thorough parameter scanning, we found clear trend that the wave vector decreased near the transition point, where wavelength diverged. This observation is consistent with the theoretical predictions. However, disordered plane waves appeared in the region near the onset. Comparing the experimental results with the results from numerical simulation, we found that spatial inhomogeneity of the diffusion coefficient was the main cause of the disordered plane waves.