Narrow Results

Based on the variational differential quadrature (VDQ) method, the bending and buckling characteristics of circular plates made of functionally graded graphene origami-enabled auxetic metamaterials (FG-GOEAMs) are numerically studied in this paper. It is considered that the plate is composed of multiple GOEAM layers with graphene origami (GOri) content that changes in layer-wise patterns. The results from genetic programming-assisted micromechanical models are also employed in order to estimate the material properties. The plate is modeled according to the first-order shear deformation plate theory whose governing equations are obtained using an energy approach in the context of VDQ technique. The governing equations are given in a new vector-matrix form which can be easily utilized in coding process of numerical methods. By means of VDQ matrix differential and integral operators, the governing equations are discretized and solved to calculate the lateral deflection and critical buckling load of plates under various boundary conditions. Selected numerical results are presented to investigate the influences of boundary conditions, GOri content, folding degree and distribution pattern on the buckling and bending behaviors of FG-GOEAM plates.

In this letter, the influence of an applied electric field on a periodic structure composed of alternating layers of DPS (Double Positive index of refraction) and DNG (Double Negative index of refraction) has been studied. We present theoretical and numerical results of our investigation of the reflection spectra of the visible waves on this structure in the presence of an electric field which will cause redistribution of charges and will modify the index of refraction of materials and enable us to control the phase and/or intensity of propagating electromagnetic waves.

Nonreciprocal devices such as isolators are well-known in light wave communication systems. The integrated optical isolator is presented where the magnetic field modes propagate perpendicular to the in-plane magnetization of a planar magnetooptical waveguide. The cladding and film of optical isolator are made of magnetic materials with magnetization adjusted to be parallel to their plan. The substrate is filled with new artificial metamaterials (MTMs). The dispersion equation of the transverse magnetic (TM) fields is derived. The difference Δβ between the phase constant for forward and backward propagation is calculated numerically for different values of MTMs permittivity (ε_s) and permeability (μ_s). Results show that the cutoff thickness is different for forward and backward propagation and varies with the MTMs parameters. The cutoff thickness of the isolator is selected such that the coupling coefficients of the isolator with optical fiber are obviously different for forward and backward propagation.

The condition for the formation of Dirac cones at arbitrary points in the Brillouin zone by the accidental degeneracy of two photonic bands was examined by the degenerate perturbation theory and group theory based on the spatial symmetry of two modes. The analysis was applied to a two-dimensional square-lattice photonic crystal with C_4v symmetry and the dispersion relation in the vicinity of the M point was examined. Exact agreement between the analytical and numerical calculation was obtained.

Superconductor-ferromagnetic (FN) metamaterial with effective magnetic shielding and transmittal properties that allow the cloaking and transferring of static magnetic fields has been introduced. Most metamaterials consist of different arrangements of superconducting and ferromagnetic materials whose performance and feasibility mainly depend on the involved materials, their geometrical distribution and the permeability of each. In this paper, combining the method of transformation optics with the design of metamaterials, we experimentally demonstrated a superconductor-FM metamaterial system, composed of two coaxial cylinders of different lengths, to investigate the influence of the length and the properties of superconducting material on the magnetic transferring properties of the magnetic field produced by the permanent magnets. By comparing the transmittal magnetic field of different cases, the optimal structure has been ultimately achieved in terms of calculating the transmitted magnetic field ratios. The insights attained by the present study are aimed to provide useful implications for the design of wireless energy transmission and increasing the efficiency of magnetic transmittal devices.

In this paper, we demonstrate that an electromagnetically induced transparency (EIT)-like behavior is induced by the coupling of two magnetic resonances in a planar terahertz (THz) metamaterial structure, which consists of a U-shaped split ring (USR) as one bright resonator and a cut wire pair (CWP) as one dark resonator. It is found that the EIT-like spectral response is insensitive to the lateral distance between the two magnetic resonators, whereas an on-to-off modulation of the amplitude of the transparency window can be achieved by moving the USR vertically along the CWP. The underlying physical mechanism is mainly attributed to magneto-inductive excitation of the CWP. This investigation may provide an insight of the role of magnetic coupling in achieving EIT-like effect, inspiring interest in the developments of magnetically tunable transparency metamaterial for a wide range of optical devices with switching and modulation capabilities.

The asymmetric pentamode metamaterial structure which is built by connecting double-cones with different cross-section shapes (regular triangle, square, pentagon and hexagon) to form diamond lattice is proposed in this paper. Then its phonon band structure is calculated by finite-element method (FEM), and its pentamodal behaviors and acoustic bandgaps are studied in detail. Results show that in the process of adjusting geometrical parameters, the asymmetric case performs similar pentamodal behaviors [ratio of bulk modulus to shear modulus $B/G$ and single-mode bandgap (SBG)] with the symmetric cases. And the asymmetric case not only remains the intrinsic complete bandgap (CBG) of mode 12-13 like symmetric cases, but also opens new and wide CBG of mode 10-11 and mode 14-15 for appropriate parameters. Therefore, introducing structural asymmetry should be an effective way to open CBG in pentamode elastic metamaterials.

Thermal cloak has potential applications in thermal protection and sensing. Based on the theories of spatial transformation and effective medium, layered structure of elliptic cylindrical thermal cloak was designed. According to theoretical analysis and numerical simulation, the layered structure has typical characteristics of perfect thermal cloak. The external temperature field remains unchanged, while the internal temperature gradient decreases obviously. Meanwhile, the cloaking effect is stable in any direction. The cloaking effect can be improved by increasing the number of discretization layers or reducing the cloak thickness. The elliptic cylindrical cloak can be considered as cylindrical cloak when the focal distance is close to zero. This study has provided an effective way for realizing thermal cloak with more complex shapes.

Emerging nanotechnologies have revealed carbon nanotubes (CNTs) as one of the best materials with immense potential. Considering the outstanding physical, mechanical, electrochemical, thermal, and optoelectronic properties of CNTs, extensive studies have been reported assessing their applications in several disciplines. This paper presents a broad review of the studies in the literature that address the contribution of CNTs in terms of their applications as different parts of solar cells such as photoelectrode, photoconductor, top and back electrode, replacement of indium tin oxide (ITO) as transparent conducting electrode (TCE) in a variety of photovoltaics such as silicon, organic, polymer and dye-sensitized with their subsequent efficiencies.

In this paper, the impact of coupling between unit cells on the performance of linear-to-circular polarization conversion metamaterial with half transmission and half reflection is analyzed by changing the distance between the unit cells. An equivalent electrical circuit model is then built to explain it based on the analysis. The simulated results show that, when the distance between the unit cells is 23 mm, this metamaterial converts half of the incident linearly-polarized wave into reflected left-hand circularly-polarized wave and converts the other half of it into transmitted left-hand circularly-polarized wave at 4.4 GHz; when the distance is 28 mm, this metamaterial reflects all of the incident linearly-polarized wave at 4.4 GHz; and when the distance is 32 mm, this metamaterial converts half of the incident linearly-polarized wave into reflected right-hand circularly-polarized wave and converts the other half of it into transmitted right-hand circularly-polarized wave at 4.4 GHz. The tunability is realized successfully. The analysis shows that the changes of coupling between unit cells lead to the changes of performance of this metamaterial. The coupling between the unit cells is then considered when building the equivalent electrical circuit model. The built equivalent electrical circuit model can be used to perfectly explain the simulated results, which confirms the validity of it. It can also give help to the design of tunable polarization conversion metamaterials.

A multiband tunable electromagnetic induced transparency (EIT) effect in metamaterial at microwave frequency range is investigated. The sandwich structure contains silicon dioxide and gold layers. The metamaterial structure has multiband EIT phenomenon due to coupling with U-Shaped split-ring resonators (SRRs) and cut wire (CW). Two different modes can be obtained in CW and a single band EIT effects in SRRs. Results show that the different resonances in the structure lead to multiband EIT. By adding the finding of the graphene layer on top of the structures, EIT window can be changed obviously. It is shown that the graphene can adjust EIT phenomenon. The group index is calculated to exhibit the slow light effect. The demonstrated phenomenon can provide valuable variety of important applications, including microwave communication technology, microwave devices, slow light and switch devices.

In this paper, a sensor structure operating in the microwave frequency band is used and it is aimed to determine whether fuel samples are branded or not. In the literature, the studies on the sensor applications of metamaterials (MTMs) have increased considerably in recent years. However, this study, unlike other studies, focuses on determining the changes in electromagnetic parameters of different fuel samples at X band frequencies. For this purpose, the electromagnetic properties of the branded and unbranded fuel samples are measured experimentally. Then, simulation studies are carried out using the experimental results and the dimensions of the resonators are optimized to determine the most suitable structure. Fabrication of MTM sensor structure is then performed using optimized dimensions. Finally, the experimental study is carried out using the sensor structure.

We design and build an acoustic invisible medium (AIM) based on first principles. Isotropic, inhomogeneous mass density and bulk modulus profiles of the AIM are derived from scattering-free conditions. Acoustic invisibility is illustrated by two types of examples: the AIM under plane-wave incidence and point sound sources. It is found that an AIM of arbitrary shape is invisible to plane waves of any incident angle. The invisibility of the AIM holds for any sound source sitting anywhere except for in the AIM region. Acoustic field distributions show that the acoustic wave enters the AIM from one side and recovers its initial shape at the other side. The designed AIMs are numerically verified by the finite element method, and are highly consistent with theoretical results. This work offers an alternative strategy to achieve a scattering-free AIM and could be extendable to the three-dimensional cases.

In this paper, a new method is proposed to manipulate seismic Rayleigh waves using phase-gradient metasurfaces. This highly compact artificial structure enables the anomalous refraction of Rayleigh waves according to the generalized Snell’s law (GSL). The soil-embedded metasurface is composed of only one column of commercial rubber blocks, which can provide an accurate phase shift to the Rayleigh wave. To verify the flexibility of this method, several metasurfaces are designed. Numerical results demonstrate that the Rayleigh waves can be focused, split, or converted into evanescent waves by using specific phase gradient configurations. The investigation also suggests the strong potential of metasurface as a smart device for shielding of seismic surface waves.

Bound states in the continuum (BICs) are a special kind of resonant states that remain localized even though they coexist with a continuous spectrum of radiating waves. They have already emerged as an important design principle for creating systems with high-quality ($Q)$ factor states to enhance light–matter interaction. Many approaches have been proposed to achieve BICs, but it is still of great challenge to design multiple BICs simultaneously, and especially working at terahertz (THz) frequencies. In this paper, we propose an all-dielectric metamaterial, consisting of four hollow cylinders in each unit cell. We show dual BICs exist in such a simple structure, and as breaking the symmetry via varying the inner radius of two diagonal cylinders, they will turn to quasi-BICs with high yet finite Q-factor. These quasi-BICs are manifested themselves as two new dips in the transmission spectrum, of which the resonance shapes can be well described by the Fano formula with a few fitting parameters. We find the evolution of their Q-factors still follows the simple linear relationship with respect to the inverse square of the asymmetry parameter. Based on the multipole decomposition method, two BICs are further investigated to show different multipole components for them. Interestingly, the higher frequency BIC is closely related to the excitation of toroidal dipole (TD), which will split into two TDs as breaking the symmetry. The proposed metamaterial provides an alternative platform to merge the physics of BICs, Fano, and TD, and to pave the way for potential device applications (since the states of extremely high-Q factor).

In this paper, a polarization-insensitive magnetic tunable perfect metamaterial absorber is analyzed which shows its functionality as a refractive index (RI) sensor. The unit cell structure of reported absorber consists of a cross-shaped Indium Arsenide (InAs) array over aluminum ground plane and has dimensions less than ${\lambda }_c/4$ with thickness of 7.5$\mu \mathrm{\text{m}}$ and periodicity of 9$\mu \mathrm{\text{m}}$ making it an ultra-thin absorber. The proposed absorber provides an average absorption of 99.99% on the implementation of magnetic field (B) at 0.4T. The resonance frequency and corresponding absorption rates can be controlled by varying the magnetic field from $B=0.1$–0.4T which provides high absorption as well as shift in the frequency spectrum by 0.3THz/T. Additionally, parametric analysis has also been done to confirm the selection of the optimum value of the designed parameters of the structure. Furthermore, simulation results reveal that the proposed structure is very sensitive to the change of RI values in the surroundings thus, the proposed structure can be employed as a RI sensor high sensitivity of 1.083THz/RIU over the range of 1–1.6RI. In addition, the result shows that the sensor can detect different chemical compounds such as ethanol, gasoline, paraffin, sodium chloride, carbon disulfide with an average sensitivity of 0.925THz/RIU. Thus, the proposed absorber with obtained sensitivity can be utilized as a biochemical sensor, making it one of the best contenders for chemical industry uses in diagnosis of different hazardous chemicals as well as finding its applicability in biomedical uses.

We derive an analytic expression for the Goos–Hänchen shift experienced by an acoustic wave that is totally reflected at the interface between a negative-refractive-index acoustic medium and a positive-index acoustic medium. Upon obtaining the condition necessary for phonon-trapping in a three-layer heterostructure, we introduce a design for an "acoustic clepsydra" by using a suitable acoustic heterostructure.

A novel microwave sensor based on waveguide filled with a metamaterial particle, which is composed of meander line and split-ring-resonator (SRR), is presented and modeled using a full wave electromagnetic commercial simulator CST. Simulating results show that evanescent mode are enhanced and transmitted through the structured particle within the frequency band that exhibits negative permittivity and permeability. Evanescent mode is sensitive to permittivity of dielectric material in the detecting zone. Comparing with conventional microwave resonant sensor, the microwave sensor using evanescent mode allows for much higher detection sensitivity.

An electromagnetic cloak is a device which makes an object "invisible" for electromagnetic irradiation in a certain frequency range. Material parameters for the complementary medium-assisted external cylindrical cloak with N-sided regular polygonal cross-section are derived based on combining the concepts of complementary media and transformation optics. It can make the object with an arbitrary shape outside the cloaking domain invisible, as long as an "anti-object" is embedded in the complementary layer. The external cloaking effect has been verified by full-wave simulation.

We developed a new method of an earthquake-resistant design to support conventional aseismic system using acoustic metamaterials. The device is an attenuator of a seismic wave that reduces the amplitude of the wave exponentially. Constructing a cylindrical shell-type waveguide composed of many Helmholtz resonators that creates a stop-band for the seismic frequency range, we convert the seismic wave into an attenuated one without touching the building that we want to protect. It is a mechanical way to convert the seismic energy into sound and heat.