Planar Metamaterials Research Articles

Multifold coupling enabled high quality factor toroidal resonances in metasurfaces

The phenomena of toroidal resonances can lead to extraordinary strong confinement of electromagnetic energy inside a physical system. Very recently, toroidal resonances were observed in metamaterial systems too. Because of strong electromagnetic field confinements, associated radiative losses in toroidal modes can be tailored to extremely low values leading to simultaneous high quality (Q) factor and high figure of merit (FoM) resonances. Such metamaterial systems can be attractive for sensors, lasing spasers, nonlinear devices, narrow band filters, etc. However, demonstrating toroidal modes with high Q factors is a challenging task in a typical metamaterial system. In this work, we have experimentally demonstrated a strategy for achieving toroidal resonances with high Q factor and FoM in single resonator based planar metamaterials. At resonance, magnetic dipoles akin to toroidal topology are excited in these structures. The anti-aligned circulating surface currents are observed along the circumference of metal split rings which resulted in a head to tail arrangement of the magnetic dipoles across the unit cells. We attribute the observation of high-quality factor toroidal modes to the strong inter-unit cell near field coupling of the induced magnetic field lines. Such multifold toroidal mode leads to resonances with exceptionally high quality factors because of significant reduction in radiation losses. Our experiments demonstrate Q factor and FoM of 183 and 76, respectively, making these metasurfaces a strong candidate for practical applications where high Q resonances are crucial.

Planar Mechanical Metamaterials with Embedded Permanent Magnets.

The design space of mechanical metamaterials can be drastically enriched by the employment of non-mechanical interactions between unit cells. Here, the mechanical behavior of planar metamaterials consisting of rotating squares is controlled through the periodic embedment of modified elementary cells with attractive and repulsive configurations of the magnets. The proposed design of mechanical metamaterials produced by three-dimensional printing enables the efficient and quick reprogramming of their mechanical properties through the insertion of the magnets into various locations within the metamaterial. Experimental and numerical studies reveal that under equibiaxial compression various mechanical characteristics, such as buckling strain and post-buckling stiffness, can be finely tuned through the rational placement of the magnets. Moreover, this strategy is shown to be efficient in introducing bistability into the metamaterial with an initially single equilibrium state.

Asymmetric chiroptical effect from chiral medium filled golden slit grating on substrate.

In this Letter, we report a giant and robust asymmetric chiroptical effect (ACOE) in the chiral medium filled golden slit grating on glass substrate (CMGSG-GS). This ACOE comes from the influence of interface asymmetry on the electromagnetic cross-coupling in the CMGSG-GS, and it is inherently different than that reported in the Faraday medium and the planar anisotropic chiral metamaterials. Both the polarization eigenstate and the transmission matrix are highly dependent on the metal structure used in the CMGSG-GS. The polarization eigenstates of the CMGSG-GS are two co-rotating elliptical states with ellipticity of nearly 0, and they remain mostly unchanged for opposite directions. The transmission matrices of opposite directions are normal matrices, which do not show any symmetric law although the geometry of the CMGSG-GS owns a high rotational symmetry. The reported ACOE gives a measurable physical parameter to reveal the events happening at interface.

Integrated Metasurfaces with Microprints and Helicity‐Multiplexed Holograms for Real‐Time Optical Encryption

AbstractOptical encryption with multichannel, high complexity, and artistry characteristics has become one of the most significant approaches for modern information security. Recently emerged metasurface‐based optics consisting of planar subwavelength metamaterials has been engineered as an ideal platform for optical encryption because of its capability of manipulating various optical parameters and enhancing information storage capacity. However, limited encrypted channels and insufficient real‐time encryption abilities hinder its practical applications. A novel integrated metasurface is proposed, which can regulate the amplitude, phase, and polarization of light wave simultaneously to realize the combination of microprint and helicity‐multiplexed meta‐holography with negligible crosstalk. By encoding an online editable quick response code into the microprint as a keystore, an optical encryption device is demonstrated with 26‐1 storage capacity endowed by wavelength‐ and helicity‐dependent hologram images that can be employed in real time. The integration scheme enables multidimensional information storage and real‐time encryption and decryption capability, which provides an approachable way for the practical application of metasurface devices in the field of optical encryption.

Metamaterial Cell-Based Superstrate towards Bandwidth and Gain Enhancement of Quad-Band CPW-Fed Antenna for Wireless Applications.

A multiband coplanar waveguide (CPW)-fed antenna loaded with metamaterial unit cell for GSM900, WLAN, LTE-A, and 5G Wi-Fi applications is presented in this paper. The proposed metamaterial structure is a combination of various symmetric split-ring resonators (SSRR) and its characteristics were investigated for two major axes directions at (x and y-axis) wave propagation through the material. For x-axis wave propagation, it indicates a wide range of negative refractive index in the frequency span of 2–8.5 GHz. For y-axis wave propagation, it shows more than 2 GHz bandwidth of near-zero refractive index (NZRI) property. Two categories of the proposed metamaterial plane were applied to enhance the bandwidth and gain. The measured reflection coefficient (S11) demonstrated significant bandwidths increase at the upper bands by 4.92–6.49 GHz and 3.251–4.324 GHz, considered as a rise of 71.4% and 168%, respectively, against the proposed antenna without using metamaterial. Besides being high bandwidth achieving, the proposed antenna radiates bi-directionally with 95% as the maximum radiation efficiency. Moreover, the maximum measured gain reaches 6.74 dBi by a 92.57% improvement compared with the antenna without using metamaterial. The simulation and measurement results of the proposed antenna show good agreement.

Lattice‐Enhanced Fano Resonances from Bound States in the Continuum Metasurfaces

AbstractFano resonances in metamaterials are known for their high quality (Q) factor and high sensitivity to external perturbations, which makes them attractive for sensors, lasers, and nonlinear and slow light devices. However, Fano resonances with higher Q factors obtained through structural optimization of individual resonators are accompanied by lower resonance intensity, thereby limiting the overall figure of merit (FoM) of the resonance. Here, a strategy for simultaneously enhancing the Q factor and FoM of Fano resonances in terahertz metamaterials is reported. Coupling of the Fano resonance, which arises from a symmetry‐protected bound state in continuum, to the first‐order lattice mode of the metamaterial array leads to stronger field confinement and substantial enhancement of both Q factor and FoM. As such enhancement occurs in planar metamaterials independently of the resonator geometry, the proposed approach can be utilized for a wide range of high‐Q and high‐sensitivity terahertz metadevices.

Tunable GRIN lensing based on graphene-dielectric multilayer metamaterials

In this paper, we have proposed effective gradient refractive index structures at terahertz frequencies, which function as a planar lens, focusing reflector, and a generalized Luneburg lens (GLL). We base the structures on planar and cylindrical graphene-dielectric multilayer metamaterials. The structures are designed by tailoring the surface conductivity of the graphene layers to provide a given phase profile along with the structures. Therefore, the customizable focus area of the focusing devices, and the tunable terahertz jet effect that is produced by the GLL result. The proposed structures have potential applications in terahertz imaging, sensing, detecting, and communication areas.

Microwave toroidal dipolar response in an asymmetric planar metamaterial

Toroidal multipoles are the third type of multipoles which are fundamentally different from conventional electric multipoles and magnetic multipoles. The implementation of toroidal multipoles is hard since its energy is too low to be discovered. This paper proposes a simple planar metamaterial to achieve an often ignored toroidal dipolar response in the microwave band. The metamaterial is designed with a special asymmetric arrangement of asymmetric U-ring resonators to attain a toroidal resonance with high quality factor at 13.5 GHz. Its toroidal nature can be verified by calculated scattered power by multipoles, magnetic field and the surface current. The presented planar metamaterial owns toroidal resonance with high quality factor and it can be applied in the field of antennas, photonics, sensing and energy.

Cross-Entropy Method for Design and Optimization of Pixelated Metasurfaces

Electromagnetic metasurfaces are planar two-dimensional metamaterials, typically of subwavelength thickness. Unit cell elements of different shapes have been widely explored, including electric and magnetic dipoles, patches, arbitrary geometries and pixelated surfaces. Although pixelated metasurfaces have a great advantage of geometric versatility, their design and analysis requires algorithmic approach. One of the techniques for their design is via evolutionary simulation-driven optimization. Since full-wave electromagnetic simulations are time-consuming, optimization methods with fast convergence properties are preferable. In this article, we demonstrate the application of the cross-entropy optimization method to design of artificial magnetic conductors (AMCs) and thin printed phase shifters. Single-frequency AMCs at 10 GHz (X band) and dual-frequency AMCs at 8 and 12 GHz (X and Ku band) were produced that are more manufacturing-friendly, and thus cost effective, than previously reported AMCs. We also show that phase-shifting unit cells with transmission magnitudes over 0.9 (linear) can be designed using the proposed optimization technique. Other potential applications of these unit cells are in phase-correcting and beam-steering metasurfaces.

Wideband square spiral metamaterial absorbers based on flaky carbonyl iron/epoxy composites

A broadband and thin microwave absorber composed of metal-base planar metamaterials (MMs) sandwiched between two layers of flaked-shaped carbonyl iron (FCI) composites is presented. FCI powder was prepared by ball milling spherical carbonyl iron powder and then added to epoxy resin to prepare FCI composite (FCIC) samples with different weight fractions. With these electromagnetic characteristics, a broadband, polarization-insensitive and thin metamaterial absorber comprising of arrays of square spirals embedded between two different layers of FCIC is optimally designed through a genetic algorithm. The reflection loss of the measured square spiral MM absorber with a thickness of 3.4 mm can reach –26 dB at matching frequency 4.4 GHz and the absorbing bandwidth below –9.3 dB is covered from 3.6 to 18.0 GHz which almost 4 fourfold bandwidth compared to that of the FCIC without the incorporation of MM layers.

Optical Activity Effect on Planar Chiral Metamaterials

Metamaterial is an artificial material which has a negative refractive index. Both of permitivitty and permeability have a negative value to possess negative refraction. The breakthrough of metamaterial development was present as the invention of planar chiral metamaterials. This research was focussed on the development of a new design of planar chiral metamaterial with different depth. The developed metamaterial design was analyzed by chirality analysis in relation to the optical activity characteristics.The planar chiral metamaterial development was following a design, a structure examination, and a fabrication process using a focused ion beam system. Chirality characterization were measured using a femtosecond Ti:Sapphire pumped terahertz emission spectroscopy system. THz signal was occurred due to the optical rectification process as a result of an intense femtosecond laser pulses radiation on planar chiral metamaterial sample. The samples were capable to generate a THz emission over a frequency range of 0 − 2.24 THz, while the THz signal was measured as ∼5 ps. The effect of optical activity was able to rotate the polarization plane on a frequency range of 0 – 1.4 THz for both of the clockwise and the counter-clockwise samples. The maximum polarization plane rotation were +13° and −18° at a frequency 1.35 THz. It can be concluded that the new design planar chiral metamaterial with different depth has been successfully obtained with its elliptic polarized THz emission characteristic.

A technique for non-contact identification of liquids in closed containers using microwave planar metamaterial

At present, the fast non-contact identification of liquids is a real-life task for security systems and quality testing of various drinks. The possibility of identifying liquids in containers using a technique based on two oppositely connected planar photonic crystals (PPCs) with microwaves was experimentally demonstrated. The influence of the thickness of the container's walls, the container's diameter, and the inclination angle of the container axis relative to the plane of the planar photonic crystal on the reliability of the non-contact identification of liquids was experimentally investigated. The minimum change in the concentration of aqueous solutions of ethyl alcohol, sugar, and food salt that could be reliably distinguished in solutions was defined. For liquids containing water in thin plastic containers, the optimum operating frequency band (about 2 GH to 4 GHz) of the experimental setup for their identification was found. The maximum thickness of the container wall at which the required reliability of the identification of liquids was achieved was evaluated for several selected operating frequencies. The possibility of non-contact analysis of not only aqueous solutions of ethyl and methyl alcohol, but also of their mixes, was experimentally shown. The developed technique for fast non-contact identification of liquids can be applied for food quality control, security systems, and chemical technologies.

Design and study on the tailorable directional thermal expansion of dual-material planar metamaterial

Current studies on tailoring the coefficient of thermal expansion of metamaterials focused on either complex bending-dominated lattice or the stretching-dominated lattice which transforms the spaces of triangle and tetrahedron. This paper proposes a kind of dual-material rectangular cell of tailorable thermal expansion, which reduces the complexities of design, calculation, and manufacture of lattice materials. The theoretical derivation using the matrix displacement method is adopted to study the thermal expansion properties of rectangular cell in the direction of height, the analytical expressions of coefficient of thermal expansion and optimization model are used to design the sizes of rectangular cell, and experimental verification is carried out. It is found that the middle cell of lattice had the same thermal expansion law as that of the unit cell. The rectangular cells of negative coefficient of thermal expansion −7 ppm/℃, zero coefficient of thermal expansion, and large positive coefficient of thermal expansion 36.2 ppm/℃ in the direction of height were realized, respectively. The consistency of theory, simulation, and experiment verifies that rectangular lattice material made of two kinds of common materials with a different coefficient of thermal expansions can achieve the design of coefficient of thermal expansion in the direction of height by choosing different material distribution and geometric parameters.

Wavefront shaping in underwater with soft gradient-index metasurfaces at ultrasonic frequencies

Metasurfaces are planar metamaterials with a subwavelength thickness that allows wavefront shaping by introducing in-plane variations, namely, gradients, in the spatial wave response of these flat structures. Most of the recently reported acoustic metasurfaces were designed to control air-borne waves. Here, we report a new class of acoustic gradient-index (GRIN) metasurfaces engineered from soft graded-porous silicone rubber with a high and tunable acoustic index for broadband ultrasonic 3-D wavefront shaping in aqueous media. The functionalities of these soft flat lenses are illustrated through various experiments, which demonstrate beam steering and beam focusing, as well as vortex beam generation in free water. These new GRIN metasurfaces may have important applications in various domains using designed ultrasonic fields (biomedical imaging, industrial non-destructive testing, contactless particle manipulation), since their fabrication is very straightforward with common polymer science engineering. Y. Jin, R. Kumar, O. Poncelet, O. Mondain-Monval, and T. Brunet, Nat. Commun. 10, 143 (2019).

Discriminating between Coherent and Incoherent Light with Planar Metamaterials

Planar metamaterials represent a powerful paradigm of optical engineering, which enables one to control the flow of light across structured material interfaces in the absence of high-order diffraction modes. We report on a discovery that planar metamaterials of a certain type, formed by nanopatterned metal films, respond differently to spatially coherent and incoherent light, enabling robust speckle-free discrimination between different degrees of light coherence. The effect has no direct analogue in natural optical materials and may find applications in nanoscale metadevices enhancing imaging, vision, detection, communication, and metrology.

Planar metamaterial analogue of electromagnetically induced transparency for a miniature refractive index sensor

A planar metamaterial of analogue of Electromagnetically Induced Transparency (EIT-like) is theoretically and experimentally investigated. The EIT-like metamaterial unit cell, whose electrical size is only 0.2λ0, consists of a split ring resonator (SRR) and a folded-line pair resonator (FLPR). For the TM wave incidence, a sharp transmitted peak with high quality factor can be observed. The EIT-like metamaterial is fabricated and measured. Good agreement is obtained between the simulation and measurement results. A miniature refractive index sensor based on the proposed metamaterial is simulated and exhibits high sensitivity and Figure of Merit (FOM).

Demonstration of group delay above 40 ps at terahertz plasmon-induced transparency windows.

Herein, we demonstrate one of the highest terahertz group delay of 42.4 ps achieved experimentally at 0.23 THz, on a flexible planar metamaterial. The unit cell of metasurface is made up of a textured closed cavity and another experimentally concentric metallic arc. By tuning the central angle of the metallic arc, its intrinsic dipolar mode is in destructive interference with the spoof localized surface plasmon (SLSP) on textured closed cavity, which results in a plasmon-induced transparency phenomenon. The measured transmittances of as-fabricated samples using terahertz-time domain spectroscopy validate numerical results using extended coupled Lorentz oscillator model. It is found that the coupling coefficient and damping ratio of SLSP relies on the radius of the ring structure of textured closed cavity. As a consequence, the slow light maximum values become manoeuverable in strength at certain frequencies of induced transparency windows. To the best of our knowledge, our experimental result is currently the highest value demonstrated so far within metasurface at terahertz band.

Localized surface magnetic modes transfer along bend and T-splitter chain arrays of subwavelength metamaterial resonators

We present a coupled resonator optical waveguide (CROW) based on planar plasmonic metamaterials, which consists of metallic spiral structures arrays. The waveguide can confine the energy in subwavelength scale and transport spoof magnetic LSPs modes. Attributing to these features, the bend waveguide and T-splitter are designed. To verify the functionality and performance of the proposed waveguides, we implemented microwave simulations and experiments. The tested waveguide performances have clearly validated the functionality of our designs. It is believed to be applicable for future plasmonic circuit in microwave and THz ranges.

A zero refraction metamaterial and its application in electromagnetic stealth cloak

In this paper, a "+−" type metamaterial planar zero refraction metamaterial is proposed, which is composed of "+" and "−" shapes printed on both sides of the FR4 substrate, with a zero refraction bandwidth of 5.86GHz. By simulating the electric field distribution of the triangular prism and measuring the sample with the free space method, the zero refraction property of the sample is verified. A rectangular stealth cloak is constructed using the designed zero refraction metamaterials, which can make electromagnetic stealth of the shielding object in 2.2–4.3 GHz, overcome the strong distortion of the exposed object in the electromagnetic field, achieve fine wave front reconstruction, and effectively reduce the shadow behind the object. The basic structure of zero refraction metamaterials and the design method of stealth cloaks proposed in this paper have great potential to expand the application space, which will provide important reference for the application of zero refraction metamaterials in the field of electromagnetic stealth.

Symmetry Breaking and Resonances Hybridization in Vertical Split Ring Resonator Metamaterials and the Excellent Sensing Potential

The resonances with narrow linewidths generated in metamaterials are valuable for numerous applications ranging from bio-sensing to narrow-band filter. And breaking structural symmetry of metamaterials is one of the common and efficient approaches to achieve such sharp resonances. In this paper, we present symmetry breaking introduced in vertical split ring resonators (VSRR) metamaterials to excite narrow resonance. Different from the previously reported planar asymmetric metamaterials, the three-dimensional metamaterials are dominantly excited by the magnetic field component of the terahertz illumination together with the electric component. In this case, sharp resonances with characteristic magnetic field distributions are excited. And the sharpest resonance shows up with an ultra-narrow line-width (full width at half maximum, FWHM is 5.90 GHz) and high Q of 327 at 1.93 THz. Through systematic theoretical analysis and numerical mode implementation, we reveal the sharp resonance dips are produced by resonance hybridization between a resonance mode induced by the destruction of the structural symmetry and another special resonance inherent in the symmetric VSRR metamaterials. In addition, the fabrication robustness and sensing potential of the asymmetric VSRR metamaterials are explored in this paper. The results show that the AVSRR design is insensitive to the errors in the fabrication process, like rough metal surface and smooth metal corner, but highly sensitive to the approaching substance, which prove the excellent sensing performance.