Cross Resonator Research Articles

Electromagnetically induced transparent bimodal to unimodal conversion realized by the nematic liquid crystal

In this paper, under the action of a gigahertz (GHz) band circularly polarized (CP) wave, a kind of CP metastructure (MS) is proposed. Recently, the nematic liquid crystal (NLC) has received great attention, but it is still difficult to realize the electromagnetically induced transparency (EIT) bimodal to unimodal under the control of NLC. Through the direct mutual coupling of the metal cross resonator, metal ring resonator, and dielectric cross resonator, modes coupling can be formed and generate transparent windows. Two different EIT states (unimodal and bimodal EIT) accompanied by slow light effects can be observed by regulating the NLC with the applied bias voltage (V bias). When the V bias is 0 V, two transparent windows can be achieved in the 6.595–8.443 GHz and 8.443–10.161 GHz bands. When V bias is 20 V, only one transparent window can be achieved in the 6.891–8.682 GHz band. Due to the high symmetry, the MS has the polarization insensitivity to CP waves. These extremely excellent properties make this MS have a wide application prospect. Noteworthy, the theoretical calculation result of EIT is verified by the two-oscillator theory model and circuit model, the results obtained are basically consistent with the simulation results.

Multi-band tunable electromagnetically induced transparencies based on active metasurface with polarization-independent property

The analogue of an electromagnetically induced transparency (EIT) spectrum is achieved using a metasurface that consists of an array of cross and L-shaped resonators. The mechanism of the EIT-like phenomenon are analyzed, and the influence of geometrical parameters on the effect is discussed. We achieve a multi-band EIT-like through structure configuration. In particular, polarization-independent EIT is realized. A potential application in sensing is suggested and the sensitivity of 65 GHz RIU−1 is obtained. We also investigate the impact of analyte and substrate on the transmission spectrum. We find that when the analyte thickness is higher than 40 μm, the EIT spectral shift is less affected by the analyte thickness. The thickness insensitive characteristics are beneficial for reducing errors caused by analyte dose, thereby improving the accuracy of sensing. By introducing a phase change material, vanadium dioxide, we obtain active control of the EIT effect. Our results provide valuable insights into the development of multi-band tunable compact devices based on metasurfaces.

Compact dual‐band balanced bandpass filter with controllable passbands using coplanar waveguide spiral cross resonators

AbstractIn this article, a novel coplanar waveguide (CPW) spiral cross resonators (SCRs) is introduced for creating a dual‐band balanced bandpass filter (BPF) with adjustable passbands. The resonant properties of the SCRs in both the differential mode (DM) and common mode (CM) are scrutinized. Furthermore, DM bisection offers two distinct parameters for internal coupling adjustment, enabling independent passband width control. A novel grounding wire feeding arrangement matches the external Q factor of the dual DM passbands. An additional stepped‐impedance short‐circuited conducting stub between resonators prevents excessive coupling while obtaining moderate bandwidths. Inherent CM suppression can be attained by distinguishing CM resonant frequencies distinct from the DM frequencies. To validate the design, a dual‐band balanced BPF designed and fabricated to operate at 2.42 and 5.2 GHz exhibits closely matching measured results compared to the simulated ones.

Design and analysis of X-band metamaterial absorber

This study introduces a novel metamaterial absorber operating in X-band frequency range, characterized by two resonating peaks. The proposed structure consist of four circular sectors connected by two cross resonators enclosed by a circular strip with the dielectric between them. FR4 is used as a substrate backed by a copper ground plane. Simulation results reveal that the reflection co-efficient remains consistently below -10dB across the entire X-band, ensuring effective absorption of incident electromagnetic waves. Moreover, the absorber achieves 100% absorptivity with the gain of -25dB under normal incidence. The validity of absorber performance is confirmed through impedance plot and Voltage Stand Waving Ration (VSWR) analysis, verifying the accuracy of S11 parameter. Furthermore, the proposed structure has a compact size of 0.1666 λ. The absorber’s robust performance validated through numerical analysis, positions it as a compelling candidate for various X-band applications, including radar systems, communication devices, and stealth technology. This research establishes the proposed metamaterial absorber as a promising solution for X-band electromagnetic wave attenuation needs, offering both superior performance and compact design.

Electromagnetically induced transparency in a strongly coupled orthogonal polarization-insensitive planar terahertz metamaterial

We demonstrate experimentally and numerically the electromagnetically induced transparency (EIT) effect in a strongly coupled planar terahertz (THz) metamaterial. The circular-shaped four-arc geometry combined with cross resonators ensures the polarization-insensitive response of the EIT for the orthogonal polarization incident THz. The EIT response can be varied by changing the gap between the arc and cross-shaped resonators. The field profiles indicate a strong coupling between the resonators leading to the EIT effect. In order to understand the underlying physical mechanism, we employed a coupled harmonic oscillator model, which suggests an increase in coupling when the distance between resonators is reduced. THz time-domain spectroscopy of the fabricated samples with the same shape and size of the simulated structures was used to verify the numerical findings. Our study uses a symmetric and easy-to-fabricate planar metasurface that can pave the way for the design and construction of THz photonic components, such as optical switches and slow light devices.

Switchable optical vortex beam generator based on an all-dielectric metasurface governed by merging bound states in the continuum.

The polarization topology around the bound states in continuum (BIC) affects the optical vortex (OV) beam generation. We propose a cross-cross-shaped resonator based on a THz metasurface to realize an OV beam generator in real space by exploiting the inherent winding topology around the BIC. The BIC merging at the point Γ is achieved by tuning the width of the cross resonator, which significantly improves the Q factor and enhances the field localization. Furthermore, the switching between the high-order OV beam generator governed by the merged BIC and the low-order OV beam generator is realized. This extends the application of BIC in modulating orbital angular momentum.

Bandwidth Improvement of a Cone-Inverted Cylindrical and Cross Hybrids Dielectric Resonator Antennas

This work employs the coaxial probe method to stack and excite two cone-inverted cylindrical and cross-hybrid dielectric resonator antennas at a standard operating resonant frequency of 5.438 GHz. A drawback of these standard Dielectric Resonator Antennas (DRAs) is their narrow bandwidth. For good antenna performance, a stacked DR geometry and a thick dielectric substrate with a low dielectric constant are desired since this provides large bandwidth, better radiation power, reduces conductor loss and reduces the nonappearance of surface waves. Many approaches, such as changing the shape of the dielectric resonator, have been used to enhance bandwidth. Using DRA, which has the lowest dielectric constant, increases the bandwidth and the electromagnetic energy. In the current work, bandwidth improvement was significantly achieved by the proposed geometry by varying the antenna size. A novel hybrid DRA configuration is used to increase the bandwidth of the antenna to 89.27% and 149.23% due to cone-inverted cylindrical and cross-hybrid dielectric resonator antennas, respectively. The DRA is designed numerically via the Finite Difference Time Domain (FDTD) method. Several parameters like return loss, input impedance (verified at ) and radiation pattern are calculated. Furthermore, the stacked-hybrid technique is used to enhance the antenna's performance, which is useful for broadband communication and the demand for wireless.

A new method with integration of matching and decoupling for MIMO antenna applications

A novel design concept of multiple-input and multiple-output (MIMO) antenna with aiming to improve matching and port isolation at the same time is proposed. As the only additional component, a cross resonator is connected between the two patch antennas. The equivalent circuit model and calculation formula of resonant frequency of cross resonator in differential mode (DM) and common mode (CM) state are given, respectively. The same resonant frequency in the two states is the goal to want, and which is proved to be achievable by the relevant expression. Then, the two port antenna system can obtain good matching and high isolation. The coincidence between simulation and measured results brings the possibility of practical implementation.

Simultaneous matching, decoupling, and miniaturization in MIMO antenna by loading a folded cross resonator

A novel concept of simultaneous matching, decoupling and miniaturization in multiple input multiple output (MIMO) antenna is proposed. Firstly, the integration of matching and decoupling can be achieved by making the resonant frequency of common mode (CM) and differential mode (DM) consistent. Then, the same resonant frequency of CM and DM will move to lower frequency through changing parameters value of the folded cross resonator. The MIMO antenna will operate normally at lower frequency with low mutual coupling without increase the volume. The operating frequency of antenna is reduced from 3 to 2.5 GHz of achieving 16.7% miniaturization, and the isolation is increased from 20 to 45 dB at 2.5 GHz at the same time. The measurement and simulation results are in good agreement, which verifies the correctness of the design. The goal of matching, decoupling and miniaturization for MIMO antenna will be completed without too much software optimization and other complex structures.

Fabrication of a Multilayer X-Band Band-Pass Metasurface Using Liquid Metal

Fabrication of a physically flexible multi-layer metasurface composed of gallium-based liquid metal encapsulated in polydimethylsiloxane (PDMS) is reported. The second-order bandpass frequency selective surface (FSS) operating in the X-band is comprised of two modified Jerusalem cross resonator layers separated by an aperture layer. The multi-layer spatial filter has a total thickness of 4.81 mm ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda _{0}$ </tex-math></inline-formula> /7.5). An inverse image mold of the FSS layer patterns is fabricated in silicon, and FSS patterned PDMS is replicated from the silicon mold. The FSS patterned PDMS is spray coated with liquid metal to fabricate a massive array of discrete conductor patterns in PDMS and subsequent encapsulation by an additional layer of PDMS. Multiple versions of the m <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> n metasurfaces have been fabricated and have been repeatedly tested for their bendability, stretchability, twistability, and foldability. The fabricated metasurface in a waveguide environment demonstrated a band-pass center frequency at 8.33 GHz which agreed well with the simulated frequency at 8.31 GHz.

Perfect Solar Absorber with Extremely Low Infrared Emissivity

We propose a multi-materials solar absorber consisting of regularly arranged cross resonators made of Ni, and nanocylinders made of Ti on their four corners. The finite difference time domain (FDTD) method is used to study the absorption performance of solar energy, the inherent absorption mechanisms and influencing factors of the absorber submerged in water. The proposed absorber achieves a nearly perfect absorption in the wavelength range of 300–2400 nm with an average absorptance higher than 98%, indicating that most of solar energy will be harvested. In addition, the high absorptance in the solar spectrum originates from the plasmon resonances of cross resonators and the magnetic polariton from the interaction between cross nanostructures and multi-material nanocylinders. Moreover, our absorbers are insensitive to the polarization of light due to the symmetry of the structures, but sensitive to the incident angle of the light. Most significantly, our absorbers can effectively prevent heat loss via radiation heat transfer by lowering their emissivity in the infrared region. The present study demonstrates a new design strategy for perfect solar absorbers consist of simple nanostructures made from multi-materials, which promise potential applications of solar energy harvesting and solar steam generation for desalination and wastewater treatment.

Three port photonic and plasmonic demultiplexers based on Cross and U-shaped stub structures: Application for filtering and sensing

We propose the design of three port photonic and plasmonic demultiplexers where filtering toward the two outputs is based on the phenomena of Fano resonances and electromagnetically induced transparency (EIT). We use a Cross-shape resonator in one output and a U-shape resonator composed of two stubs in the other output. We give a theoretical demonstration of the geometrical parameters of both resonators in order to filter one wavelength in one output while leaving the other output unperturbed. These results are confirmed by experimental validation in the radio frequency domain and a numerical simulation in the infrared (IR) domain using plasmonic metal–insulator–metal waveguides. The Cross resonator in the first output can give rise to an EIT resonance, whereas the U-shaped resonator in the second output may exhibit both EIT and Fano resonances depending on the lengths chosen for the stubs. Therefore, different demultiplexing schemes can be proposed such as achieving a Fano resonance in one output and an EIT in the other, or EIT resonances in both outputs. The Fano resonance is obtained by bringing resonance close to transmission zero, whereas the EIT results from the squeezing of resonance between two transmission zeros. When the widths of the resonances tend to zero, they transform to trapped or bound states in the continuum with an infinite lifetime. We show that the crosstalk between the two channels can be reduced to −82 dB and the sensitivity can reach 2390.8 nm/RIU, RIU is the refractive index units. Finally, we highlight the performance of our design as a high sensitive filter and sensor in the IR domain. In this work, the analytical calculations and demonstrations are performed by using Green’s function approach, the experimental verifications are realized by means of coaxial cables operating in the radio frequency range and the numerical simulations are obtained using the finite element method via Comsol Multiphysics software.

High absorption and a tunable broadband absorption based on the fractal Technology of Infrared Metamaterial Broadband Absorber

This paper proposes a dual-band tunable broadband metamaterials absorber, achieved absorption in 3-5 μm mid-wavelength infrared (MWIR) and 8-12 μm long-wavelength infrared (LWIR). The absorber is composed of periodic units, and each unit is composed of a bottom ideal electric conductor-dielectric layer-graphene layer-metal fractal cross resonator in which a graphene layer-metal fractal cross is embedded. The metal structure uses the multi-scale self-similar characteristics of fractal geometry to create multiple resonances for the same element structure. Numerical simulations were carried out using the finite element method (FEM). The optimization results show that the designed single-layer 3-Level graphene-metal fractal cross absorber produces broadband absorption of 2.86 μm (absorption >90%) in the LWIR region and an average absorption rate of 92.1%. By adding the bottom layer 3 × 3 graphene-metal fractal cross array structure, a resonant absorption peak is generated in the MWIR region, and the broadband absorption of 0.38 μm (absorption >80%), and it achieves broadband absorption of 2.48 μm (absorption >90%) in the LWIR region. For the double-layer graphene-metal structure, the resonance bandwidth and absorptivity of the absorber can be tuned by adjusting the Fermi energy of each layer of graphene. Due to its broadband, polarization insensitivity, high absorption, flexible tunable, and other characteristics, the proposed metamaterials absorber is beneficial for fabricating nano-optoelectronic devices and can be applied in thermal imaging and infrared detection.

Switchable and tunable triple-channel bandpass filter

A triple-channel bandpass filter with switchable and tunable functions is proposed, which is based on a triple-mode cross resonator. The varactors and switching diodes are loaded at the end of the resonator. Because of the use of switches, resonators have four working states: conventional single-mode, two dual-modes, and one triple-mode. The varactor makes the channel independently adjustable. Finally, a triple-channel bandpass filter with switchable and tunable functions is designed by using two identical triple-mode resonator coupling structures. To solve the problem of whether each channel in the multi-channel filter is independently adjustable, this paper gives a simple and rigorous judgment method, namely rank criterion, which is a necessary and sufficient condition for each channel to be independently adjustable. The method of designing an element variable coupling matrix (EVCM) is adopted, which can not only obtain the desired frequency response through adjustable elements but also help to select resonators and coupling modes in the actual circuit design. The final circuit size of the designed filter is 0.29λg × 0.26λg. The measured results are in good agreement with the simulation results.

Complex Dielectric Constant Extraction of Substrate Materials Using Cross-Resonator Method

This article presents a novel resonance method by using a cross-resonator (CR) for the complex permittivity measurement of planar dielectric substrate materials with its full theoretical and analytical transmission line (TL) model. Due to its symmetrical structure, it offers two main advantages over its conventional counterpart. First, the unwanted asymmetricity mode is eliminated due to the centric magnetic wall. Second, the electric field at the junction of the open-ended stub is uniform. The propagation constant of the structure in this method is directly extracted from the <inline-formula> <tex-math notation="LaTeX">$S$ </tex-math></inline-formula>-parameters. The total losses without approximation are computed by using a robust theoretical and analytical TL modeling. The proposed resonator is implemented on RO4003C and FR4 substrate materials, and <inline-formula> <tex-math notation="LaTeX">$S$ </tex-math></inline-formula>-parameters are measured in the frequency range of 1 MHz to 8 GHz. Then the complex dielectric constant of the substrate is extracted at each resonant frequency. A full-wave simulation also confirms the accuracy of the proposed resonator. The comparison between the simulated and the measured <inline-formula> <tex-math notation="LaTeX">$S$ </tex-math></inline-formula>-parameters demonstrate a relative error less than 1&#x0025; over the frequency range.

A compact metamaterial broadband THz absorber consists of graphene crosses with different sizes

A compact tunable graphene-based broadband metamaterial absorber in the terahertz (THz) frequency band was presented in this paper. The proposed absorber is the classic sandwich structure, which is composed of graphene cross resonators of different sizes placed on the top of a dielectric spacer backed with a gold ground at the bottom. It is shown that the absorber can achieve a broad absorption band from 2.81 THz to 4.67 THz with absorption up to 97 % under the normal incident. Meanwhile, the absorption property can be kept to a moderate level when the incident angle is up to 45° for both TE and TM incident light, and the absorption is polarization insensitive. By increasing the chemical potential of graphene from 0.40 eV to 0.60 eV, the broad absorption band can be tuned to shift blue with slightly changing maximum absorption rate. What's more, the absorption bandwidth can be further extended to over 2.34 THz with absorption larger than 97 % by adding another graphene cross layer. Based on the optimal and novel characteristics, we hope that the designed absorber may have potential applications in THz absorbing, sensing, switching, modulating, stealthy and so on.

Design and analysis of an ultra-thin dual-band wide-angle polarization-insensitive metamaterial absorber for C-band application

This paper presents an ultra-thin metamaterial absorber (MMA) which can execute two absorption peaks in the C-band. It is polarization-insensitive to the incident wave and has the satisfactory absorption under the wide incident angle. The proposed dual-band MMA is a three-layer structure, the top layer is consisted of a central cross resonator and an external circle ring resonator, the bottom layers is made by copper, and the middle layer is filled in the dielectric FR-4. The calculation results show two diverse absorption bands at the resonance frequency of 4.44 GHz and 7.85 GHz, whose absorbance can reach 99.93 % and 99.99 %, respectively. The structure of the proposed MMA is simple and its thickness is only 1.47 % of the wave length at minimal resonance frequency. In addition, the calculation demonstrates the proposed MMA is insensitive to the various angles of polarization, and it also shows the proposed MMA can attain high absorptivity under the wide angles of incidence in TE and TM modes. Furthermore, the absorption mechanism of the proposed MMA is studied, and the specific effect of the geometric parameters of top resonator and the dielectric thickness on absorptivity are also analyzed. At last, the absorption performance of the proposed dual-band MMA is verified by experiments, which is basically consistent with the simulation effect. The proposed MMA can be applied in C-band radar antenna, satellite communication or other relative fields.

Broadband long-wave infrared metamaterial absorber based on single-sized cut-wire resonators.

Broadband absorption is critical for the applications of metamaterial absorbers. In this work, a broadband long-wave infrared (LWIR) absorber with classical metal-dielectric-metal configuration is numerically demonstrated. The absorber consists of single-sized cut-wire arrays that show broadband and high extinction ratio, attributed to polarization-selective simultaneous excitation of propagated and localized surface plasmon resonances. The average absorption rate of the TM wave reaches 91.7% and 90% of the incident light is absorbed by the resonator in the wavelength range of 7.5-13.25µm so that the average extinction ratio in the resonator layer reaches 125. The polarization insensitive broadband absorption can be obtained by a cross resonator which can be treated as a pair of cut-wires perpendicular to each other. Our metamaterial absorber with single-sized resonators shows spatially concentrated broadband absorption and may have promising applications for hot-electron devices, infrared imaging, and thermal detection.

Polarization independent triple band ultrathin conformal metamaterial absorber for C- and X-frequency bands

A triple-band polarization-independent conformal absorber for C- and X-frequency bands have been presented. The designed absorber comprises a Jerusalem cross resonator at the center, surrounded by a quarter part of a square rings resonator with different sizes at each corner on a metal grounded dielectric substrate. The unit cell size is 0.23λo × 0.23λo and thickness of λo/153 at frequency 6.16 GHz. The absorption peaks were obtained at frequencies 6.16 GHz, 9.22 GHz, and 11.6 GHz with absorptivity 95%, 96%, and 97%, respectively. It exhibited full wave at half maximum (FWHM) bandwidths of 160 MHz bands (6.04 GHz–6.20 GHz, 240 MHz (9.09 GHz–9.33 GHz), and 260 MHz (11.57 GHz−11.83 GHz), respectively. The designed absorber is shown four-fold symmetry, which makes it polarization independent. Normalized impedance and surface current distributions for all three absorption frequencies are analyzed for understanding the absorption phenomenon. The designed absorber has been tested for the curved surfaces with different central angles to show its conformal behavior, and it is perceived that measured results and simulated results have good agreement between them.

Jerusalem cross geometry magnetic substrate absorbers for low-frequency broadband applications

In this work, an ultra-thin microwave absorber with a Jerusalem cross (JC) resonator exhibiting a broad low-frequency absorption bandwidth was successfully designed and fabricated. The system is composed of a perfect electric conductor-magnetic-periodic metal three-layer structure. The top metal layer consists of a periodic JC, which can be used to obtain dual-bands and low-frequencies in L-band, S-band, and C-band applications. The simulations show that the magnetic absorber with a thickness of 2 mm exhibits two absorption peaks at 1.4 and 4.7 GHz, with a reflectivity of −10 and −8 dB, respectively. The L-band RL (reflection loss) of −4 dB bandwidth has 91% (1–1.91 GHz), whereas the S-band and the C-band of −4 dB bandwidth have 47% (2.87–5.68 GHz). The total thickness of the absorber measures only 1/100λ at 1.5 GHz. When a magnetic harmonic TM is polarized at an incidence angle of 75°, an average RL of −10 dB in the 1–8 GHz range is obtained. The equivalent L–C circuit and the power loss density clarify the mechanism behind the absorption. Such a low-frequency absorber can be used in various applications to quench microwave energy.