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Skin effect in electromagnetism describes the tendency of alternating current to predominantly flow near the surface rather than through the center of a conductor. Here, another skin effect of electromagnetic flux is reported to occur in non‐conductive materials, specifically within 3D anisotropic zero‐index metamaterials, as well as its potential application in subwavelength wave manipulation. It is found that when a medium of near‐zero longitudinal permittivity and permeability is embedded with inclusions whose transverse permittivity and/or permeability approach zero, the electromagnetic flux therein is directed and extremely compressed beneath the surfaces of inclusions within a skin depth less than λ0/150. This unusual effect empowers arbitrary control of electromagnetic flux at deep subwavelength scale. Customized subwavelength pathways for electromagnetic flux are demonstrated in full‐wave simulations via practical implementations. The study opens a new avenue for extreme confinement and flexible control of electromagnetic flux at deep‐subwavelength scale. [ABSTRACT FROM AUTHOR]

The realization of multifunction by simply combining acoustic devices together is always attractive, but it is never an easy task due to the complexity of most acoustic metamaterials. In this paper, a kind of unbalanced gain–loss acoustic metasurface is designed, which can work as an energy amplifier, suggesting that it may be applied to an acoustic signal detector or a sensor. In addition, an acoustic negative refraction system is established by simply putting two of these metasurfaces together, which can produce negative refraction for the acoustic waves from different directions and different incident angles, even focusing and imaging with precisely selected parameters. These functions are quite rare for materials without the double-negative-parameter. This work provides a new route to use the gain/loss materials and achieve multifunctions without complex design. [ABSTRACT FROM AUTHOR]

Manipulating radiation patterns is challenging, especially at low frequencies. In this paper, we demonstrate that acoustic metamaterials arranged as an array of quadrupoles remarkably improve the directionality of acoustic radiation at low frequencies, compared with previous metamaterials arranged as monopole and dipole structures. The directivity of the acoustic radiation can be adjusted by changing the characteristic parameter and the symmetry of the structure, which provides a flexible method of adjusting radiation directions. The directionality can be further improved by constructing a linear array. Our work establishes acoustic radiation control via quadrupolar metamaterials. [ABSTRACT FROM AUTHOR]

To design and optimize seismic metamaterials, the impacts of nonlinearity in different locations of locally resonant acoustic metamaterials on the dispersions and the variation of amplitude-dependent bandgaps are investigated in this paper. The research used theoretical calculations, namely, Lindstedt–Poincaré perturbation method and prediction method, and combined finite-element simulation. Summarizing from our research, the lower bandgap is sensitive when exposed to amplitude stimulation, when there arise nonlinear characteristics between matrices; while nonlinearity appears within the interior oscillator, amplitudes obtain a more intense influence on the bandgap, introducing an enormous magnitude of deviation between the upper bandgap and the lower bandgap. Based on the peculiar frequency-shift characteristics, an acoustic three-terminal controller is proposed as a conventional subsize acoustical device and nonlinear seismic metamaterials component. This controller enables the realization of modulating the value of output signals by adjusting the quantitative loading on the control port, without changing the input signals and the parameters of the apparatus validated with the finite-element simulation. The work may offer potential applications in low-frequency vibration reduction and external-controllable multi-functional acoustical devices. [ABSTRACT FROM AUTHOR]

Negative refraction plays an important role in acoustic wave manipulation and imaging. However, conventional systems based on acoustic metamaterials suffer from the limits induced by loss-related and resolution issues. In this work, a parity-time (PT)-symmetric system is introduced to realize loss-free bidirectional acoustic negative refraction. The system is composed of a pair of locally PT-symmetric multi-layer metasurfaces sandwiching a region of free space, which also forms a global PT symmetry. The property of bidirectional negative refraction, which is rare for general PT-symmetric structures, is related to the coexistence of amplification and absorption in the locally PT-symmetric metasurfaces at their PT-broken phases. Such metasurfaces can freely switch their states between coherent perfect absorber (CPA) and amplifier depending on the direction of incidence. Our results provide a physical mechanism for realizing bidirectional functions in acoustic PT-symmetric systems. [ABSTRACT FROM AUTHOR]

By investigating the transmission of electromagnetic waves through random media composed of a random cluster of inclusions embedded in a 'double-zero' medium with simultaneously near-zero permittivity and permeability, a percolation behavior of photons squeezing through the gaps between random inclusions with unity transmittance is observed. Interestingly, such a percolation exhibits a threshold induced by the long-range connectivity of the 'nonconducting' component in the transverse direction instead of the 'conducting' component in the propagation direction, which is distinctly different from those in normal percolations. This unusual phenomenon, obtained by full wave simulations, is explained analytically through the introduction of a geometric concept hereby denoted as 'free surfaces'. This work reveals a unique type of percolation threshold for electromagnetic waves with potential applications in energy harvesting, sensors and switches. [ABSTRACT FROM AUTHOR]

As the typical building blocks of metamaterials, the cut wire and the split ring resonator have been extensively studied in recent years. Besides them, the space-filling curve-based metamaterials are receiving great attentions because of their intrinsic subwavelength and multi-bands characteristics. In this work, we have investigated experimentally and numerically the electromagnetic characteristics of such Hilbert curve metamaterial in the microwave frequency regime and found a deeply subwavelength magnetic resonance supported by the fractal pattern and featuring the wavelength-to-size ratio more than 20. The subwavelength electromagnetic properties of the Hilbert curve will be beneficial to realize high-performance metamaterials. [ABSTRACT FROM AUTHOR]

We investigate the wave transmissions through an index-near-zero (INZ) or epsilon-near-zero (ENZ) metamaterial containing various kinds of coated cylindrical defects. We find that thin coatings of the defects can dramatically change the transmission behaviors. For example, perfect magnetic conductor (PMC) defects embedded in an INZ or ENZ metamaterial yield total reflections for transverse magnetic polarized waves (Hao et al., Appl Phys Lett 96:101109, ). However, if the PMC defects are coated with dielectric shells, total transmissions could be achieved by tuning their permittivity values or geometric sizes. The permittivity differences of dielectric shells for total reflections and transmissions in the INZ or ENZ metamaterial could be very small, implying potential applications, such as ultrasensitive sensors and switches. [ABSTRACT FROM AUTHOR]

Metamaterials can exhibit electromagnetic and elastic characteristics beyond those found in nature. In this work, we present a design of elastic metamaterial that exhibits multiple resonances in its building blocks. Band structure calculations show two negative dispersion bands, of which one supports only compressional waves and thereby blurs the distinction between a fluid and a solid over a finite frequency regime, whereas the other displays 'super anisotropy' in which compressional waves and shear waves can propagate only along different directions. Such unusual characteristics, well explained by the effective medium theory, have no comparable analogue in conventional solids and may lead to novel applications. [ABSTRACT FROM AUTHOR]

A zero-refractive-index metamaterial is one in which waves do not experience any spatial phase change, and such a peculiar material has many interesting wave-manipulating properties. These materials can in principle be realized using man-made composites comprising metallic resonators or chiral inclusions, but metallic components have losses that compromise functionality at high frequencies. It would be highly desirable if we could achieve a zero refractive index using dielectrics alone. Here, we show that by employing accidental degeneracy, dielectric photonic crystals can be designed and fabricated that exhibit Dirac cone dispersion at the centre of the Brillouin zone at a finite frequency. In addition to many interesting properties intrinsic to a Dirac cone dispersion, we can use effective medium theory to relate the photonic crystal to a material with effectively zero permittivity and permeability. We then numerically and experimentally demonstrate in the microwave regime that such dielectric photonic crystals with reasonable dielectric constants manipulate waves as if they had near-zero refractive indices at and near the Dirac point frequency. [ABSTRACT FROM AUTHOR]

We derive and numerically demonstrate that perfect absorption of elastic waves can be achieved in two types of ultra-thin elastic meta-films: one requires a large value of almost pure imaginary effective mass density and a free space boundary, while the other requires a small value of almost pure imaginary effective modulus and a hard wall boundary. When the pure imaginary density or modulus exhibits certain frequency dispersions, the perfect absorption effect becomes broadband, even in the low frequency regime. Through a model analysis, we find that such almost pure imaginary effective mass density with required dispersion for perfect absorption can be achieved by elastic metamaterials with large damping. Our work provides a feasible approach to realize broadband perfect absorption of elastic waves in ultra-thin films. [ABSTRACT FROM AUTHOR]

We report an experiment on an inside-out Eaton lens fabricated using H-fractal metamaterials, copper printed on printed circuit boards in H-fractal patterns. The lens can perform good imaging functionality with both sources and images inside the vacuum core. The H-fractal metamaterials also provide design technique to achieve refractive index ranging in [0,1] with little loss in microwave spectrum. Excellent agreements between numerical and experimental results have been demonstrated. [ABSTRACT FROM AUTHOR]

We study metamaterials with an anisotropic permittivity tensor in which one component is near zero. We find that such an anisotropic metamaterial can be used to control wave propagation and construct almost perfect bending waveguides with a high transmission rate (>95%). This interesting effect originates in the power flow redistribution by the surface waves on the input and output interfaces, which smoothly matches with the propagating modes inside the metamaterial waveguide. We also find that waves in such anisotropic epsilon-near-zero materials can be reflected by small-sized perfect magnetic conductor defects. Numerical calculations have been performed to confirm the above effects. [ABSTRACT FROM AUTHOR]