Anisotropic Resonator Research Articles

Reflective mode conversions between extensional and flexural waves by ultrathin oblique anisotropic tri-component resonators

In this letter, we report a theoretical study for the conversions between extensional and flexural modes for Lamb waves. The incident extensional waves can be completely converted into reflected flexural waves, or vice versa. The used structure is oblique anisotropic tri-component resonators with locally dipolar resonators. Without special optimization designs, the thickness of conversion device is much smaller than the working wavelengths (1/150 and 1/15 for the extensional and flexural waves, respectively). Our proposed design is more feasible for low-frequency application scenarios.

Collective-Coupling Enhanced Ultrabroadband Linear Polarization Conversion on Zigzag-Split Metasurfaces

Polarization manipulation plays a pivotal role in integrated multifunctional devices. Various metasurface-based polarization converters successfully demonstrated high efficiency and broad bandwidth. However, new mechanisms to aggressively enhance performance are still in dire need. Herein, theoretical and experimental evidence corroborates an efficient ultra-wideband transmissive polarization converter based on a topological design method. The triple-layer meta-device consists of a layer of anisotropic zigzag-split resonator array amid two orthogonal wire-grating layers. By splitting double zigzag lines into compound 90°V-shaped resonators, the intra-unit cross-coupling extends to strong inter-unit cross-coupling, giving birth to the multi-resonances enhancement and significant bandwidth broadening. This polarization converter can efficiently convert linearly polarized incident waves into 90° cross-polarized transmitted waves, with a conversion efficiency above 80% over 3.98-22.71 GHz, reaching a fractional bandwidth of 140.4%. The proposed meta-device enables strong cross-coupling in mutual transition from intra-unit to inter-unit. From a physical viewpoint, the novel mechanism is elucidated by the surface current and electric field distributions upon constructive interferences stemming from V-shape-resonator combinations and the Fabry-Pérot-like cavity effect. With both spectrum and function extensions, the proposed polarization conversion strategy finds essential applications in polarization-related systematic scenarios, such as 6G communication, radar imaging, anti-interference, chiral sensing, etc.

High isolation circularly polarized in-band full-duplex anisotropic dielectric resonator antenna

A novel high-isolation, monostatic, circularly polarized (CP) simultaneous transmit and receive (STAR) anisotropic dielectric resonator antenna (DRA) is presented. The Proposed antenna is composed of two identical but orthogonally positioned annular sectoral anisotropic dielectric resonators. Each circularly polarized (CP) resonator consists of alternating stacked dielectric layers of relative permittivities of 2 and 15 and is excited by a coaxial probe from the two opposite ends to have left and right-hand CP. Proper element spacing and a square absorber are placed between the resonators to maximize Tx/Rx isolation. Such a structure provides an in-band full-duplex (IBFD) CP-DRA system. Measurement results exhibit high Tx/Rx isolation better than 50 dB over the desired operating bandwidth (5.87–5.97 GHz) with a peak gain of 5.49 and 5.08 dBic for Ports 1 and 2, respectively.

Trapped mode control in metasurfaces composed of particles with the form birefringence property.

Progress in developing advanced photonic devices relies on introducing new materials, discovered physical principles, and optimal designs when constructing their components. Optical systems operating on the principles of excitation of extremely high-quality factor trapped modes (also known as the bound states in the continuum, BICs) are of great interest since they allow the implementation of laser and sensor devices with outstanding characteristics. In this paper, we discuss how one can utilize the anisotropic properties of novel materials (transition metal dichalcogenides, TMDs), particularly, the bulk molybdenum disulfide (MoS2), to realize the excitation of trapped modes in dielectric metasurfaces. The bulk MoS2 is a thin-film structure in which the light wave behaves the same way as that in the uniaxial anisotropic material with the form birefringence property. Our metasurface is composed of an array of disk-shaped nanoparticles (resonators) made of the MoS2 material under the assumption that the anisotropy axis of MoS2 can be tilted to the rotation axis of the disks. We perform a detailed analysis of eigenwaves and scattering properties of such anisotropic resonators as well as the spectral features of the metasurface revealing dependence of the excitation conditions of the trapped mode on the anisotropy axis orientation of the MoS2 material used.

Mode conversions for elastic waves transmitted and reflected by ultrathin elastic metamaterial plates with anisotropic resonances

In this work, we study the conversions between longitudinal and transverse modes by one layer of oblique anisotropic dipolar resonators, whose size can be orders smaller than the wavelength. Mode conversions are found for both transmitted and reflected waves by such small resonators with a normal incidence. A mass-spring model is proposed to depict analytically the mode conversion, giving simplified analytical expressions for the conversion rates. The reflected conversion will gradually increase to the maximum while the solid material behind the resonators becomes soft, or vice versa. This work may provide theoretical guidance for practical applications.

Total conversion between the longitudinal and transverse waves by ultrathin elastic metamaterials with anisotropic resonances

In this work, total conversions between longitudinal and transverse modes are achieved within an elastic metamaterial plate with thickness of two orders smaller than the wavelength. The ultrathin metamaterial plate consists of an array of anisotropic dipolar resonators obliquely-oriented, which can transfer the longitudinal movement into the transverse one, or vice versa, accounting for this effect. A mass–spring model is developed to depict analytically the mode conversion with a quantitative agreement with the simulation. The conversion rate of the metamaterial plate remains above 95% in varying solids, showing good adaptability in practical applications.

A Novel Wideband and High-Efficiency Electronically Scanning Transmitarray Using Transmission Metasurface Polarizer

Inspired by the recent progress of metasurfaces, a novel wideband and high-efficiency electronically reconfigurable transmitarray (TA) is presented that can enable two-dimensional (2-D) beam steering and multiple beam mode switching in a digital manner. The proposed unit cell consists of a reconfigurable anisotropic metallic resonator, which locates between a pair of orthogonally oriented subwavelength metal gratings. One-bit phase quantization is obtained by controlling the ON or OFF state of two positive-intrinsic-negative (p-i-n) diodes integrated on the anisotropic metallic resonator layer. To choke the radio frequency (RF) signal effectively, a bias layer is connected by the metallic via hole to the center of the resonator, where the electric field is minimal. The simulated results show that the proposed unit cell can achieve a low insertion loss of 1 dB from 9.3 to 10.9 GHz for two phase states. Then, the radiation performance of a 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times\,\,10$ </tex-math></inline-formula> element array fed by a pyramidal horn antenna is theoretically calculated, simulated, and measured. The measured maximum gain is 19.1 dBi at 10.0 GHz with an aperture efficiency of 25.8% and a 3 dB gain bandwidth of 15.9%. The measured results demonstrate the 2-D beam scanning capability within the 50° range.

An ultrathin acoustic metasurface composed of an anisotropic three-component resonator

An ultrathin acoustic metasurface consisting of an anisotropic three-component resonator is proposed. The resonator can induce nondegenerate dipole resonances at the same resonant frequencies. A large phase delay can be obtained based on the resonance, which can be modulated by the direction of polarization. The anisotropic resonator can be regarded as an effective homogenous medium with an anisotropic mass density, and the phase change can also be attributed to the change of the effective material parameters. A good comparison between the results for the metasurface and its effective slab is obtained.

1Pb2-6 Stable modeling of free boundaries of an anisotropic plate resonator in the finite difference time domain method using staggered grid with collocated grid points of velocities

1Pb2-6 Stable modeling of free boundaries of an anisotropic plate resonator in the finite difference time domain method using staggered grid with collocated grid points of velocities

Compact Terahertz Dielectric Folded Metasurface

AbstractDespite modern terahertz (THz) systems benefit from the developments in metasurfaces with flexible wavefront manipulation capabilities, the large propagation space between the THz source/detector and metasurface component, as well as the inherent insertion loss of metasurfaces, are the two main bottlenecks that hinder metasurface‐based THz devices from becoming a practical technology. This article addresses these two problems by demonstrating a compact, high‐efficiency folded metasurface system in the THz band. The device comprises two parallel dielectric metasurfaces, between which the THz waves are confined to reduce the system volume. The bottom reflective metasurface consists of an array of anisotropic dielectric resonators, which can simultaneously manipulate THz waves’ phase and polarization properties. The upper metasurface is a dielectric Bragg polarizer that selects the desired linear polarization of THz waves to pass through. Low‐loss and nondispersive high‐resistivity silicon is used as the material for both metasurfaces in this design, which is essential to achieving high efficiency. The compact and high‐efficiency folded THz metasystem is further experimentally verified. The introduced folded device can be used in diverse applications such as high‐speed THz communications, non‐destructive detection, and imaging systems, significantly reducing their volume and increasing their radiation efficiency.

Controlling frequency distance between individual modes of dielectric resonator nanoantenna using uniaxial anisotropic materials

A technique to significantly shift down the resonant frequency of the higher order modes of a uniaxial anisotropic rectangular dielectric resonator nanoantenna (DRNA) without any considerable change in the resonant frequency of the fundamental mode is presented. The method aims to adjust the ratio of the resonant frequency of the higher order modes without changing the physical dimensions of DRA. This method can be used to adjust the frequency distance between the high-order and fundamental modes of the antenna. The performances of the method are numerically validated for different contrast values between the dielectric tensor entries.

Scattering by lossy anisotropic scatterers: A modal approach

Scattering from anisotropic geometries of arbitrary shape is relatively difficult to interpret physically, involving the intricate interplay between material and geometric effects. Insights into complex scattering mechanisms are often enabled by modal methods that decompose the response into the well-understood multipolar resonances. Here, we extend the generalized normal mode expansion to lossy and anisotropic scatterers. Unique to the method is that it decomposes the total response of any anisotropic resonator into the modes of the corresponding isotropic resonator. This disentangles the material and geometric contributions to the scattering of any anisotropic resonator. Furthermore, the method can identify absorption and scattering resonances with separate sets of modes. We illustrate our method by considering an infinitely long cylinder with concentric metallic/dielectric layers, targeting the complex case of an effective hyperbolic response. We show that by scanning the material composition of the hyperbolic medium, we can achieve any desired scattering effect, including backscattering cancellation.

A magnetoactive metamaterial based on a structured ferrite

Subject and Purpose. The use of spatially structured ferromagnets is promising for designing materials with unique predetermined electromagnetic properties welcome to the development of magnetically controlled microwave and optical devices. The paper addresses the electromagnetic properties of structured ferrite samples of a different shape (spatial geometry) and is devoted to their research by the method of electron spin resonance (ESR). Methods and methodology. The research into magnetic properties of structured ferrite samples was performed by the ESR method. The measurements of transmission coefficient spectra were carried out inside a rectangular waveguide with an external magnetic field applied. Results. We have experimentally shown that over a range of external magnetic field strengths, the frequency of the ferromagnetic resonance (FMR) of grooved ferrite samples (groove type spatial geometry) increases with the groove depth. The FMR frequency depends also on the groove orientation relative to the long side of the sample. We have shown that as the external static magnetic field approaches the saturation field of the ferrite, the FMR frequency dependence on the external static magnetic field demonstrates "jump-like" behavior. And as the magnetic field exceeds the ferrite saturation field, the FMR frequency dependence on the groove depth gets a monotonic character and rises with the further growth of the field strength. Conclusion. We have shown that the use of structured ferrites as microwave electronics components becomes reasonable at magnetic field strengths exceeding the saturation field of the ferrite. At these fields, such a ferrite offers a monotonically increasing dependence of the resonant frequency on the external magnetic field and on the depth of grooves on the ferrite surface. Structured ferrites are promising in the microwave range as components of controlled filters, polarizers, anisotropic ferrite resonators since they can provide predetermined effective permeability and anisotropy

A Numerical Approach to Investigate the Influence of Resonator Setting and Volume Fraction on Stop Bands in an Acoustic Metamaterial

A quest for physics-based understanding of engineered metamaterials inspired numerous researchers to extract intricate features such as guiding and filtering elastic waves, wave focusing, topological insulation, cloaking etc. A traditional metamaterial is composed of a heavy core along with some other materials of dissimilar mechanical properties (e.g. stiffness, density). It is well-established that the frequency band gap can be formed by introducing the desired inconsistency of material properties between stiffer resonators and adjacent embedding matrices. Frequency band gaps are fundamental requirements of many engineering applications such as vibration control, noise mitigation, and energy concentrations. Hence, advanced researches are being carried out continuously to understand the control parameters (e.g. bandwidth, starting and ending frequencies) of the frequency band gap. In this article, a mass-in-mass metamaterial using elliptical anisotropic resonators are considered to investigate the influence of resonators’ geometric factors and volume fraction on band gap parameters. While the elliptical resonator is splitted diagonally in one or both opposite ends to analyze the influence of volume fraction of the resonators on stop bands, a second set of half-circular resonators are also investigated to analyze the impact of resonator parameters on frequency bands.

Dual-channel sensing by combining geometric and dynamic phases with an ultrathin metasurface.

Ultrathin metasurfaces consisting of subwavelength anisotropic plasmonic resonators with spatially variant orientations are capable of generating local geometric phase profiles for circular polarizations (CP) and can be used for multiplexing of electromagnetic waves. As the geometric phase solely depends on the orientation of dipole antennas, the phase profiles cannot be changed dynamically with external environment once the structure is fabricated. Here, by incorporating geometric phase and resonance-induced dynamic phase in a monolayer of nano gold antennas, we show that phase profiles of different spin components can vary independently through modification of the external environment. Specifically, the intensities of the + 1 and -1 order diffracted waves vary asymmetrically with the refractive index of surrounding media, forming a dual-channel sensing system. Our dual-channel sensing method exhibits very high signal-to-noise ratio and stability for sensing of liquid, monomolecular layer and even nanoscale motion, which will have potential applications in various fields, including biosensing, precision manufacturing, monitoring of environment, and logic operations.

Homogenization of 3D metamaterial particle arrays for oblique propagation via a point-dipole analysis

A rigorous technique for the consistent calculation of the effective parameters of infinite three-dimensional metamaterial particle arrays in the realistic case of oblique wave propagation, is presented in this paper. The extracted polarizabilities of the consisting scatterer and the numerically retrieved wavenumber for the obliquely propagating electromagnetic waves are systematically inserted into a properly modified first-principles homogenization technique, thus leading to the characterization of the effective medium. Finally, the proposed methodology is applied to two popular, anisotropic metamaterial resonators.

Ultra‐wideband anomalous reflection realised by a gradient metasurface

In this work, an ultra-wideband gradient metasurface (GM) is designed by using Pancharatnam-Berry (PB) phase. In the design process, a circular polarisation (CP)-maintaining metasurface (CMM) is proposed at first, which can realise ultra-wideband CP-maintaining reflection in the frequency band of 7.7-19.9 GHz, moreover, PB phase will be generated in its co-polarised reflection coefficient under CP incidence by rotating the anisotropic resonators in its unit cells. Thus, based on the CMM, an ultra-wideband GM is designed by using PB phase. Both the simulation and experiment show that the designed GM can realise ultra-wideband anomalous reflection under arbitrary polarised incidence, the reflected wave under CP incidence will be deflected to an anomalous direction and even be converted to a surface wave; but under linearly polarised and elliptically polarised incidences, the reflected wave will be separated into two beams. In addition, to design more anomalous reflection metasurfaces, the generation principle of PB phase, together with the root cause of the CP-maintaining reflection of the proposed CMM, has been analysed in detail in the designing process.

A design of ultra-wideband linear cross-polarization conversion metasurface with high efficiency and ultra-thin thickness

In this work, a polarization conversion metasurface is proposed, which can convert linearly polarized electromagnetic (EM) waves into cross-polarized waves in ultra-wideband at high polarization conversion efficiency. The proposed sandwiched metasurface, composed of a top anisotropic resonator structure, an intermediate dielectric substrate, and a bottom metallic ground, has an extremely thin thickness (only 6.6% of the longest wavelength of the working frequency range). Both numerical simulation and experimental results illustrate that the metasurface can work in the ultra-wideband (ranging from 9.04 to 20.83 GHz) with a high polarization conversion ratio (&amp;gt;0.9). Polarization conversion in a wide bandwidth is attributed to multiple resonances in the operating frequency range. Additionally, the proposed metasurface also shows good performance for oblique incidence. It is expected that the designed metasurface will be more convenient for integration within ultra-thin EM devices.

The Antibody-Free Recognition of Cancer Cells Using Plasmonic Biosensor Platforms with the Anisotropic Resonant Metasurfaces.

It is vital and promising for portable and disposable biosensing devices to achieve on-site detection and analysis of cancer cells. Although traditional labeling techniques provide an accurate quantitative measurement, the complicated cell staining and high-cost measurements limit their further development. Here, we demonstrate a nonimmune biosensing technology. The plasmonic biosensors, which are based on anisotropic resonant split ring resonators in the terahertz range, successfully realize the antibody-free recognition of cancer cells. The dependences of Δf and the fitted phase slope on the cancer cell concentration at different polarizations give new perspective in hexagonal radar maps. The results indicate that the lung cancer cell A549 and liver cancer cell HepG2 can be distinguished and determined simply based on the enclosed shapes in the radar maps without any antibody introduction. The minimum concentration of identification reduces to as low as 1 × 104 cells/mL and such identification can be kept valid in a wide range of cell concentration, ranging from 104 to 105. The construction of two-dimensional extinction intensity cards of corresponding cancer cells based on the wavelet transform method also supplies corresponding information for the antibody-free recognition and determination of two cancer cells. Our plasmonic metasurface biosensors show a great potential in the determination and recognition of label-free cancer cells, being an alternative to nonimmune biosensing technology.

Continuously tunable acoustic metasurface with rotatable anisotropic three-component resonators

We propose a tunable acoustic metasurface consisting of identical units. Units are rotatable anisotropic three-component resonators, which can induce non-degenerate dipolar resonance, causing an evident phase change in low frequencies. Compared with the monopole resonance widely used in Helmholtz resonators, the polarization direction of the dipole resonance is a new degree of freedom for phase manipulation. The proposed metasurface is constructed by identical units that are made with real (not rigid) materials. The phase profile can continuously change by rotating the anisotropic resonators. We present a wide-angle and broad-band acoustic focusing by the metasurface under a water background.