Asymmetric Split Ring Resonators Research Articles

Tunable asymmetric square split ring resonator based triple band metamaterial absorber for wireless communication system

This article presents a triple band square split ring with double circular split ring resonator incident angle-insensitive perfect metamaterial absorber. The MMA unit cell was manufactured using a FR4 substrate that had dimensions of 8 mm by 8 mm. With the flexibility of on-design adjusting of the highest absorption frequencies, the suggested MA displays peak absorption of 99.67 %, 99.85 %, and 99.97 % at 2.52 GHz, 6.99 GHz, and 9.28 GHz, correspondingly. The tuning metallic two circular split rings and square split ring resonator provide the possibility of frequency modification throughout three distinct frequency ranges: the first frequency range extends from 2.45 GHz to 2.53 GHz, the second frequency range extends from 6.91 GHz to 7 GHz, and the third frequency range extends from 8.85 GHz to 10.32 GHz. Furthermore, the suggested MA displays near-zero absorption at an oblique incidence angle of up to 45° for both the TE as well as TM modes. The unit cell demonstrates single negative properties, such as an effective medium ratio (EMR) of 14.88 and a quality factor (Q factor) that is more than 30 for both the TM mode as well as the TE mode. An equivalent circuit successfully demonstrated its performance capability, which indicated that it would create an MMA of good quality when using ADS software. A comparison between the measured results of the suggested MMA and the simulated one reveals that there is a significant amount of a similarity between both of them. Because of its easy design within small dimension, wide angular stability, strong absorption, good EMR, and flexibility to frequency tuning is suitable for a wide range of wireless communication system, especially satellite applications like electromagnetic interference reduction, radar systems, imaging, and stealth technology.

Fano resonance and sensing application based on terahertz asymmetric split-ring metasurfaces

Recently, Fano resonance metamaterials have attracted considerable attention in the field of terahertz modulator and sensitive detection due to its high quality factor, sensitivity to the environment, and simple structure. Here, a Fano resonance and sensing application based on terahertz asymmetric split-ring resonators metasurfaces been investigated. The Fano resonance exhibits a strong dependence on both the symmetry of the meta-atoms and the rotation angle of the metasurface. Exploiting the high sensitivity of Fano resonance to conductivity, we employed indium oxide film as the analyte to validate the sensitive detection of Fano resonance to conductivity. Furthermore, we enhanced the sensitivity by introducing rotation, enabling the detection of even smaller variations in the conductivity of perovskite film. This research provides an important experimental and theoretical foundation for further investigating the mechanisms of Fano resonance and exploring its potential in sensor applications.

Resonator-Loaded Waveguide Notch Filters with Broad Tuning Range and Additive-Manufacturing-Based Operating Frequency Adjustment Procedure

This article presents a new class of ring-resonator-loaded waveguide notch filters with a broad tuning range, low cost, and improved performance. The proposed approach employs a complementary asymmetric split-ring resonator coupled to a microstrip transmission line and excited in a rectangular waveguide. An equivalent circuit model is proposed to explain the working principle of the proposed notch filter. The adjustment of the operating frequency is based on the additive manufacturing of a metallic copper patch allocated on the microstrip transmission line, which enables extensive tuning capabilities and consistent performance with minimum variations across the tuning window. For demonstration purposes, a filter employing a WR-28 waveguide and photolithography-manufactured resonator is prototyped and experimentally validated. The measured results indicate a broad 8 GHz tuning range with a consistent insertion loss, ranging from 23.4 to 31.4 GHz. An inverse regression model is constructed using measurement data obtained for tuning patches of various sizes, which allows for determining the relationships between the operating frequency and the copper patch size. The resulting calibration curve enables rapid filter tuning to the required frequency by inserting metallic patches of the model-predicted size.

Electromagnetic characteristics of antisymmetric toroidal dipole array of plasmonic metasurfaces.

An antisymmetric toroidal dipole array of plasmonic metasurfaces, whose unit cell consisted of a pair of physically connected asymmetric split-ring resonators, is presented in this study. Moreover, a new paradigm was established to control toroidal electric dipole properties. Toroidal electric dipoles and electric and magnetic hybrid pseudo-anapole states are excited owing to imperfect and perfect destructive interference, respectively, which leads to the spatial separation of the electric and magnetic fields and a distinct asymmetric Fano line shape in the transmission spectrum. The imperfect destructive interference was further modified by adjusting the relative position between the even and odd layers of the metasurfaces. The scattered power of the toroidal electric dipole is tuned continuously and linearly, which enables the tailoring of the electromagnetic response. The displacement sensitivity is approximately 0.13 GHz/mm over the range 0-8 mm. The modulation depth of the scattered power of the toroidal electric dipole can reach 740%, realising a toroidal electric-dipole-to-electric-dipole transition. The proposed plasmonic metasurfaces provide a platform to efficiently control near-field enhancement, far-field radiation, and electric-magnetic separation and find potential applications in frequency-selective surfaces, sensors, filters, spectroscopic tests, and many other areas.

Resonant toroidal metasurface as a platform for thin-film and biomaterial sensing.

Toroidal resonances with weak free-space coupling have recently garnered significant research attraction toward the realization of advanced photonic devices. As a natural consequence of weak free-space coupling, toroidal resonances generally possess a high quality factor with low radiative losses. Because of these backgrounds, we have experimentally studied thin-film sensing utilizing toroidal resonance in a subwavelength planar metasurface, whose unit cell consists of near-field coupled asymmetric dual gap split-ring resonators (ASRRs). These ASRRs are placed in a mirrored configuration within the unit cell. The near-field coupled ASRRs support circulating surface currents in both resonators with opposite phases, resulting in excitation of the toroidal mode. In such a way, excited toroidal resonance can support strong light-matter interactions with external materials (analytes to be detected) placed on top of the metasurface. Further, our study reveals a sensitivity of 30GHz/RIU while sensing AZ4533 photoresist film utilizing the toroidal mode. Such detection of thin films can be highly beneficial for the development of sensing devices for various biomolecules and dielectric materials that can be spin coated or drop casted on metasurfaces. Hence, the toroidal mode is further theoretically explored towards the detection of avian influenza virus subtypes, namely, H5N2 and H9N2. Our study reveals 6 and 9GHz of frequency redshifts for H5N2 and H9N2, respectively, in comparison to the bare sample. Therefore, this work shows that toroidal metasurfaces can be a useful platform to sense thin films of various materials including biomaterials.

Transflective spatial terahertz wave modulator

Spatial light modulators can digitally manipulate the amplitude, phase, and polarization of light. Their counterparts in the terahertz band are highly pursued to meet the requirements of numerous applications such as wireless communications and biomedical detection. Here, we propose a spatial terahertz wave modulator based on a liquid-crystal-integrated metadevice. The modulator consists of 8 × 8 pixels. The liquid crystal layer is sandwiched between an asymmetric split ring resonator array and pixelated interdigital electrodes. Fano resonance occurs for the transmitted wave, while the reflected wave is perfectly absorbed. By separately driving the liquid crystal with pixelated interdigital electrodes, both the Fano resonance and absorption peak can be continuously tuned due to the variation in the environmental refractive index. This work provides a transflective spatial terahertz wave modulator that can dynamically reconfigure a terahertz wavefront.

Substrate-integrated microfluidics for sensitive biosensing with complementary THz metamaterials in water

Metamaterials can be utilized for a variety of applications and have emerged as a valuable tool in THz technologies. Used as THz biosensors, metasurfaces can significantly improve the sensitivity in the detection of biomolecules, but the high THz absorption of water represents a major challenge for the realization of a sensor for measurements in liquids. In this article, we propose an approach where the resonance feature of complementary asymmetric split ring resonators (CASRR) is maintained even for measurements in water, allowing highly sensitive detection of biomolecules in strongly absorbing liquids. This is enabled by the introduction of substrate-integrated microfluidics, which are shown to have a minimal effect on the transmission properties of the metamaterial. Due to this approach, the metamaterial structure design is independent from the microfluidic channels. Our simulations also show that the sensitivity of CASRR changes only marginally for lossless and highly absorbing materials. At the same time, the presented concept is easy to fabricate by standard lithography methods and can be applied to other metamaterial structures as well.

Research on a high-sensitivity asymmetric metamaterial structure and its application as microwave sensor

In this paper, an Asymmetric Electric Split-Ring Resonator (AESRR) metamaterial structure is proposed to explore the interaction between metamaterials and electromagnetic waves with the influence of Fano resonance on electromagnetic properties. With the symmetry of basic electric Split-Ring Resonator (eSRR) being broken, a new Fano resonant peak appears at around 11.575 GHz and this peak is very sensitive to the dielectric environment. Based on the proposed high sensitivity of AESRR, a microwave sensor based on a 13 × 13 arrays of AESRR was designed and verified using printed circuit board (PCB) technology. T-shape channel was integrated to the sensor by grooving in the FR-4 substrate which improved the integration and provided the feasibility of liquids detection. Seven organic liquids and four dielectric substrates are measured by this sensor. The measured results show the transmission frequency shifts from 11.575 to 11.150 GHz as the liquid samples permittivity changes from 1 to 7 and the transmission frequency shifts from 11.575 to 8.260 GHz as the solid substrates permittivity changes from 1 to 9. The results have proven the improved sensitivity and the larger frequency shift ∆f on material under test (MUTs) compared with the conventional reported sensor. The relative permittivity of liquid samples and solid samples can be obtained by establishing approximate models in CST, respectively. Two transcendental equations derived from measured results are proposed to predict the relative permittivity of liquid samples and solids samples. The accuracy and reliability of measured results and predicted results are numerically verified by comparing them with literature values. Thus, the proposed sensor has many advantages, such as low-cost, high-sensitivity, high-robustness, and extensive detecting range, which provided a great potential to be implemented in a lab-on-a-chip sensor system in the future.

Hybridization of dark resonant states in terahertz metasurfaces

Dark resonating states render an important role in curtailing the dominant radiative losses in various photonic structures like metamaterials (MMs), photonic crystals, waveguides, etc. Currently, radiative losses are the major degrading factors toward the strong confinement of electromagnetic radiation for MMs’ applications in sensing, imaging, filters, modulators, nonlinear studies, etc. Generally, the dark states cannot be excited through the direct interaction of electromagnetic radiations with the metastructures. However, indirect excitation at suitable structural asymmetries can trigger the evolution of dark resonating states or trapped modes. In this work, we have experimentally demonstrated mode hybridization of such dark (sub-radiant) resonating states without the direct involvement of interacting bright (super-radiant) states in a metasurface consisting of asymmetric split-ring resonators. Our study further reveals that the lattice mode plays a crucial role in determining the line shapes of the split (hybridized) modes. Such tuning of dark resonance modes can find potential applications in metasurfaces-based sensing, non-linear devices besides other potential applications desiring multi-channel strong field confinement.

Actively Controlled Frequency-Agile Fano-Resonant Metasurface for Broadband and Unity Modulation

The active control to the local resonant mode of metasurface is a promising route for improving the operation bandwidth limitation of metasurface. Here, we propose and experimentally demonstrated the active tunabilities in a frequency-agile Fano-resonant metasurface. The metasurface with a pair of asymmetric split ring resonators is integrated with double varactor diodes for active control of the sharp Fano resonance. It is found that the sharp Fano-type spectrum appears due to the near-field interferences between the collective electric and magnetic dipole modes. The physical insight is revealed through local field analysis, multipole decomposition and temporal coupled-mode theory. It is also found that the metasurface can be employed as a broadband and unity modulator. Hopefully, our results could inspire sophisticated electrically controlled photonic devices with novel functions.

Balanced wideband bandpass filter design based on asymmetric split ring resonators (ASRRs) with wide stopband and multi-transmission zeros

In this paper, a balanced wideband bandpass filter (BPF) using the asymmetric split ring resonators (ASRRs) is proposed. The analysis of resonant characteristics and high freedom of design of the multi-physical parameters ASRRs are performed and utilized to design the balanced wideband BPF. Based on the subwavelength size of ASRR, the miniaturization of BPF is realized. To obtain a wide stopband and enable the suppression of spurious resonance in differential mode, the efficient generation of the desired multiple transmission zeros (TZs) is implemented with the employment of the quarter/half-wavelength uniform impedance resonators. By maintaining the performance of filtering function, the different generation mechanisms of TZs based on the multi-path transmission of signal and terminated short-circuited coupled lines are theoretically explored and discussed. The designated balanced BPF is centered at 3.5 GHz with a fractional bandwidth of 32.8%, an extended stopband bandwidth and a miniaturized circuit size of 0.10λ g × 0.28λ g. Two prototypes are fabricated and measured to validate the design methods and the significant filtering performance of wideband BPFs constructed by ASRRs. The simulated and measured results are in good agreement to verify the design.

3D Bended Metasurfaces: Asymmetrical Chirality in 3D Bended Metasurface (Adv. Funct. Mater. 31/2021)

In article number 2100689, Changzhi Gu, Junjie Li, and co-workers successfully fabricate a 3D bended metasurface with arrays of asymmetric split ring resonators using focused ion beam-defined origami. Both giant circular dichroism (CD) and asymmetric chirality properties are realized simultaneously in the same metasurface, and the CD for the forward and backward incidence at mid-infrared range are −0.29 and 0.71, respectively.

Asymmetrical Chirality in 3D Bended Metasurface

AbstractChiral metasurfaces offer crucial opportunities to expand the application potential for chiral photonics, but its desired freely structural control and giant circular dichroism (CD) allow for more and greater challenges. Here, different from previous metasurfaces with single structural regulation, a kind of bifunctional chiral metasurface based on 3D bended asymmetric construction is proposed, which shows giant CD combined with asymmetric chirality at 5.2 µm. The bended metasurface with abundant spatial freedom, consisting of an array of asymmetric bended split ring resonators (SRRs), can be built up from the tensile stress induced by focused ion beam (FIB)–matter interaction. The results show that the CD increases with the bending angle of the metasurface and reaches a maximum at a bending angle of 60°. Particularly, it is found that the CD is 0.71/0.85 (experiment/simulation) for the backward incidence but −0.29/−0.29 for the forward incidence, indicating that the giant CD and asymmetric chirality properties are realized simultaneously in the same metasurface. The calculated results of near‐field distribution and the electric/magnetic dipoles show that the giant CD and the asymmetric chirality come from and , respectively. The findings inspire a high efficiency approach to design multifunctional chiral optical devices.

Switchable Fano Resonance Based on Cut-Induced Asymmetric Split-Ring Resonators with Dirac Semimetal Film

We present cut-induced asymmetric split-ring resonator (CASRR) structure that can achieve switchable Fano resonance based on Dirac semimetals at the terahertz region. By changing the asymmetric geometric parameter from 3 to 13 µm, the transmission spectrum can be switched from single to dual channel and the Fano resonance can be gradually enhanced, which is consistent with the two coupled oscillators model. Meanwhile, the Fano effect can also be effectively switched by adjusting the Fermi energy of Dirac semimetal film (DSF) and it is suitable for practical applications. In the view of sensing application, the sensitivity based on Fano resonance is up to 69.75 GHz/RIU and 134.25 GHz/RIU with the thickness of sample 1 μm and 20 μm, respectively. Such manipulation of Fano resonance by using DSF may have potential applications in terahertz switch, sensor, and multichannel communication.

Multi-Band Electromagnetically-Induced-Transparency Metamaterial Based on the Near-Field Coupling of Asymmetric Split-Ring and Cut-Wire Resonators in the GHz Regime

A metamaterial (MM), mimicking electromagnetically-induced transparency (EIT) in the GHz regime, was demonstrated numerically and experimentally by exploiting the near-field coupling of asymmetric split-ring and cut-wire resonators. By moving the resonators towards each other, the original resonance dip was transformed to a multi-band EIT. The phenomenon was explained clearly through the excitation of bright and dark modes. The dispersion characteristic of the proposed MM was also investigated, which showed a strongly-dispersive behavior, leading to a high group index and a time delay of the MM. Our work is expected to contribute a simple way to develop the potential devices based on the multi-band EIT effect.

Asymmetric complementary split ring resonator for identification of materials

In this paper, a new microwave sensor for identification of materials is presented. The sensor is composed of an asymmetric complementary split ring resonator etched on the ground side of the microstrip line. The sensor is designed using low cost FR4 substrate. The S21 simulation result shows the unloaded resonator operates at 6.45 GHz. The overall size of the sensor is 100mm×50mm×0.8mm. When the sensor is loaded with sample material, the resonant frequency shift is noticed. This change in frequency shift is used to identify the material type and also the thickness of the material under test. The sensor is tested for different test samples placed on the ground and is correctly able to identify the material under test and its thickness by its resonant frequency variations.

Dual-band reflection polarization converter for circularly polarized waves based on a zigzag asymmetric split ring resonator

Polarization converters based on metasurfaces are one of the recently developed metadevices that can change the polarization state with designated modes, utilizing the sub-wavelength unit construction. In this paper, a kind of planar zigzag asymmetric split ring resonator (Z-ASRR) metasurface with dual bands is proposed to achieve nearly perfect polarization conversion for circularly polarized waves. Compared with the original prototype asymmetric resonant ring (ASRR), both magnitude and bandwidth have been remarkably improved for achieving a higher resonance, with the introduction of zigzag metallic wires. The reflection polarization conversion ratio possesses two peak values with 0.94 and 0.99 at 5.39 GHz and 9.65 GHz, respectively. It is also demonstrated that the introduction of extra gaps, which are closely linked with the multi-node standing wave characteristic, can control the number of resonant modes or modulate the relative bandwidth. Besides, an equivalent circuit model, the degree of zigzag bending, and the oblique incidence are further analyzed in detail. The experimental results agree well with the simulations, and this chiral metadevice could be applied for on-chip integration in an optical detection/laser, a chiral biosensor, and molecular spectroscopy.

Reconfigurable linear polarization conversion based on spatial-order kirigami metasurfaces

With the development of intelligent technology, it is essential to develop polarization-conversion devices with adaptable electromagnetic (EM) performance for practical applications. Up to now, most of attempts have relied on PIN diodes and varactor diodes for electrical tuning, typically featuring simplicity and timelineness. However, the shortcomings are also notable, such as less degrees of freedom (DoFs), more complex circuits and more expensive. In view of this, here we propose a kind of spatial-order metasurface for reconfigurable polarization conversion based on kirigami concept. By adjusting the folding angle <i>β</i>, the interaction between neighboring dipoles can be progressively changed and thus the operation frequency of polarization conversion can be shifted. Such a mechanical reconfigurable strategy brings about more DoFs for tuning and is cheaper and extraordinary convenient in practice. To verify the feasibility of our concept, a proof-of-concept spatial-order kirigami metasurface is proposed for the dual-band reconfigurable linear polarization conversion based on asymmetric chiral split ring resonators (SRRs). Experimental results show that the linear polarization operates at 5 and 5.8 GHz when folding angle is <i>β</i> = 10°, these frequencies are shifted to 5.8 and 7.2 GHz when <i>β</i> = 45°: a tuning range is expanded by 18.5%. In addition, the Poisson’s ratio and relative density of proposed kirigami metasurface as a function of <i>β</i> are also theoretically analyzed. The results show that the Poisson’s ratio increases with the value of <i>β</i> increasing. The relative density can be reduced to 1.5% of its unfolded planar counterpart. Our spatial-order kirigami metasurface strategy paves the way for implementing the reconfigurable linear polarization conversion and multifunctional devices.

Incidence angle-dependent broadband chiral metamaterial for near-infrared light absorption

The ability to spin-selectively absorb circularly polarized light plays a critical role in various photonic devices. Here we propose and investigate a broadband chiral metamaterial composed of asymmetric split-ring resonators, showing a wide spin-selective absorption band from 950 to 1200 nm with pronounced circular dichroism up to 20°. We demonstrate that the broadband absorption spectra originate from induced dual chiral resonance modes. Meanwhile, the two different resonances can be adjusted independently, suggesting great flexibility of the designed chiral absorption band for different purposes. Also, the chiral-selective absorption performance is highly dependent on the oblique incident angle due to the extrinsic chirality. The chiral resonance modes can be either enhanced or destroyed under oblique incidence. Such angle-dependent broadband chiral metamaterials may find potential applications for spin-orbit communications, chiral detection, polarimetric imaging, and biosensors.

Quad band metamaterial absorber based on asymmetric circular split ring resonator for multiband microwave applications

This article presents a metamaterial based on a modified asymmetric circular split ring resonator for absorber applications in the microwave regime. This metamaterial absorber (MMA) exhibits four absorption peaks to cover C, X, and Ku band applications. The MMA unit cell is formed on a low-cost FR4 substrate with electrical dimensions of 0.106 λ0 × 0.106 λ0, for which wavelength, λ0 is calculated at 4.1 GHz. The resonator of the unit cell is structured with three concentric intercoupled circular split rings with dimensions modified to obtain maximum absorption peaks of 97.9%, 99.1%, 99.5%, and 99.95% at 4.1, 6.86, 11.3, and 13.45 GHz, respectively. Metamaterial and absorber properties are investigated in the analysis of the surface current, electric, and magnetic fields. The unit cell exhibits single negative metamaterial properties with an effective medium ratio (EMR) of 9.15 and quality factor (Q factor) greater than 20. Beyond that, the equivalent circuit of the MMA unit cell is modeled and validated by comparing S11 obtained from Advanced Design System (ADS) and CST software. The MMA array’s absorption properties are also examined in the simulations. Measured results of the unit cell and array correspond well with the result of simulation in terms of absorption at intended frequencies. Given its good EMR, superior quality factor, and high maximum absorption, the MMA can be a good candidate for a multiband absorber in microwave applications like sensing, detecting, notch filtering and to decrease the indirect reverberation and reflections caused by the metallic part of the radar and satellite antennas.