Asymmetric Split Ring Research Articles

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.

Wideband circularly polarized low profile dielectric resonator antenna with meta superstrate for high gain

A low profile circularly polarized high directive Dielectric Resonator Antenna (DRA) is presented for X - band wireless applications. DRA is excited by microstrip aperture slot coupling which is employed on the bottom side of the substrate. Two asymmetric rectangular split rings are created adjacent to the feed line on the substrate to enhance the 3-dB axial ratio bandwidth and impedance bandwidth. Corrugated circular ring shaped single layer double sided meta superstrate is loaded on the DRA to enhance the peak gain to 11.9 dBi. The extracted lumped element model of the Superstrate unitis found to be inconcurrence with Electromagnetic (EM) simulations. The proposed geometry offers a 1.1 GHz axial ratio bandwidth with 2.6 GHz impedance bandwidth. A prototype is fabricated and experimentally verified.

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.

Silicon‐Based Terahertz Meta‐Devices for Electrical Modulation of Fano Resonance and Transmission Amplitude

AbstractSeeking active and effective control over electromagnetic waves has always been an important focus in optics. Fano resonances occur in planar terahertz (THz) metamaterials by introducing a weak asymmetry in a two‐gap split ring resonator. Without extra layers of photoactive materials and microelectromechanical structures, a novel and economical scheme based on silicon‐integrated THz asymmetric metallic split ring metamaterial is proposed to control Fano resonance and transmission amplitude via electrical excitation. The results show that Fano resonance and transmission amplitude abate drastically with the increase of current bias, due to the loading of electrically formed silicon carrier layer. As the current bias is increased, both the thickness and conductivity of silicon carrier layer are modulated simultaneously. The depth range of modulated silicon carrier layer could reach 250 µm. Besides, a THz transmission amplitude modulator with a modulation depth of 93% is also demonstrated. This work significantly expands the function of silicon‐based metamaterials and opens up opportunities for the realization of switchable sensors, filters, and nonlinear devices.

Coherently controllable terahertz plasmon-induced transparency using a coupled Fano-Lorentzian metasurface.

Metamaterials have been engineered to achieve electromagnetically induced transparency (EIT)-like behavior, analogous to those in quantum optical systems. These meta-devices are opening new paradigms in terahertz communication, ultra-sensitive sensing and EIT-like anti-reflection. The controlled coupling between a sub-radiant and a super-radiant particle in the unit cells of these metamaterial can enable multiple narrow plasmon induced transparency (PIT) windows over a broad band, with considerable group delay of electromagnetic field (slow light effect). Phase coherence between these PIT windows is highly desired for next-generation multichannel communication network. Herein, we numerically and experimentally validate a controllable frequency hopping mechanism between "slow light" windows in the terahertz (THz) regime. The effective media are composed of plasmonic "molecules" in which an asymmetric split-ring resonator (ASRR) or Fano resonator is displaced on the side of a cut-wire (Lorentz oscillator). Two metasurfaces where ASRR is on opposite side of the cut-wire are investigated. In these two cases, the proximity of the cut-wire to the gap on the ASRR having asymmetry is different. On one side, when the gap is nearer to the cut wire, displacing the ASRR along the cut-wire, produces only one narrow transparency window at 0.8 THz, corresponding to 20 ps group delay. When the ASRR is positioned on the opposite side, such that the gap is further, two transparency windows are observed when the ASRR is displaced along the cut-wire. That is, the transparency window hops from 0.8 THz to 1.2 THz. This corresponds to an increase from 20 to 30 ps in slow light effect. Numerical simulations suggest these single or multiple PIT windows occur if the couplings between the plasmonic modes in the different arrangements are either in-phase or out-of-phase, respectively.

High-sensitivity biosensor for identification of protein based on terahertz Fano resonance metasurfaces

Terahertz metasurface biosensors have gained significant attention in biological field due to its label-free, real-time and in-situ detection properties. In this paper, a planar array of Fano asymmetric split ring resonators (ASRs) fabricated on 25μm flexible polyimide substrate was proposed for new terahertz metasurface biosensors. It is shown that 160 GHz/RIU and 240 GHz/RIU refractive index sensitivity can be severally obtained near 0.81 and 1.13 THz transmission dips of the proposed system. Correspondingly, for biosensor samples that protein A/G, A/G + IgG were spotted on ASRs surface in our experiment, shifts of 8.9 GHz and 11.7 GHz, 17.6 GHz and 52.9 GHz toward low frequency were observed, respectively. These results illustrate high-sensitivity and label-free detection of proteins, implying a new feasible way of sensing in terahertz domain.

Electromagnetically induced transparency metamaterial with strong toroidal dipole response

This paper designs a planar electromagnetically induced transparency (EIT) metamaterial, which comprises an asymmetric ellipse split resonance ring (AESRR) and cut wire (CW). The proposed EIT metamaterial works in the wide range of incident angles and has polarization-sensitive at two transmission dips. The frequency of transparency peak is 10.67 GHz and maintains a high quality-factor (180.84). By calculating the multipole’s radiated power, it can be found that the toroidal dipole response is enhanced, while the electric dipole response is suppressed at the transparency peak. Interestingly, this paper firstly uses the radiated power of electric dipole to elucidate the polarization sensitivity in two minimal transmissions. Meanwhile, the coupling mechanism of the EIT metamaterial is analyzed by the two-oscillator model and equivalent circuit.

Polarization independent double-band electromagnetically induced transparency effect in terahertz metamaterials

In this article, we experimentally and numerically investigated a metamaterial (MM) geometry capable of exhibiting polarization independent double-band electromagnetic induced transparency (EIT) effect. The meta-molecule unit of the proposed MM configuration composed of a strip and two asymmetric split ring resonators (SRRs). For polarization independent double-band EIT effect, the existing meta-molecule unit is converted into a cross-like structure adorned with four SRRs. Terahertz transmission response is analyzed for two orthogonal polarization directions of the incident light to confirm the polarization independent response. In order to understand and explain our numerical findings more elaborately we have employed four-level tripod atomic system based analytical model. The transmission response is also analyzed for different angle of incidence of the two orthogonal polarizations. In order to demonstrate the practical applicability of our study, we have studied the effect of transmission with the change of refractive index of analyte of thickness 10 μm coated on the top of the MM resonators. The calculated sensitivities for the 1st, 2nd and 3rd dips are 121 GHz/RIU, 138 GHz/RIU and 135 GHz/RIU (RIU, refractive index unit) respectively. Our study can also play an important role in the advancement of slow light devices, modulators and filters.

Dynamically tunable Fano resonance with high Q factor based on asymmetric Dirac semimetal split-ring structure

We propose an asymmetric split-ring (ASR) structure based on Dirac semimetal, which has Fano resonance with a high quality (Q) factor in the terahertz (THz) band, the Q factor can reach a maximum value of 20.19. Amplitude modulation can be achieved by increasing the degree of asymmetry Δθ of the asymmetric split ring. As a result, in this study, an amplitude modulation of 27.19% has been achieved by increasing the asymmetry from 10° to 40°. Furthermore, our full-wave electromagnetic simulations show that the frequency sensitivity values of Fano and quadrupole resonance are as high as 0.6 THz/refractive index unit (RIU) and 0.933 THz/RIU, respectively. In addition, the sensing range can be adjusted by changing the Fermi levels of Dirac semimetal. Our study can guide the practical application of ultrasensitive THz sensors.

A novel miniaturized asymmetric CPW split ring resonator with extended field distribution pattern for sensing applications

Novel modified split-ring resonator (SRR) for expanded field distribution and miniaturized dimensions for sensing with double sided detection capability is proposed. The proposed design is based on an asymmetric coplanar waveguide type SRR and demonstrate a smaller radius of 0.035 × λ with a resonance of 1.57 GHz at its first harmonic. The proposed structure has the potential of higher sensitivity, requires smaller sample volume and is miniaturized compared to the conventional ring resonators. The long but narrow overlap gaps evident in the proposed miniaturized SRR create capacitive and inductive contributions for better sensing performance regime and optimized for superior efficiency and miniaturization capability. For sensitivity evaluation, a coplanar SRR prototype operating at the same resonance is compared with the proposed miniaturized coplanar SRR to underscore its superior performance profile. The resulting design is prototyped on RO3003 microwave laminate with dielectric permittivity (ℇr) of 3 and measured. Sensitivity analysis is conducted by loading the both resonator on both planes with Teflon cladding sample of permittivity of 2.33, thickness of 3 mm and the results are compared. The final design is validated by loading the proposed resonator with a 0.05λ × 0.05λ sq.m and a strip of 0.045λ × 0.015λ sq.m moisture-responsive commercially available 3 M transparent water-soluble wave solder tape 5414 Poly-vinyl alcohol (PVA) film having a thickness of 20 μm.

Ultrasensitive THz biosensor for PCR-free cDNA detection based on frequency selective surfaces.

THz technologies are a powerful tool for label-free detection of biomolecules. However, significant reduction of the lower detection limit is required to apply THz-sensors in biomedical diagnosis. This paper reports an ultrasensitive THz-biosensor based on asymmetric double split ring resonators (aDSRR) for the direct label- and PCR-free detection of DNA at physiologically relevant concentrations. We introduce selective functionalization and localized electric field concentration to enhance aDSRR sensitivity and specificity. The sensor characteristics are demonstrated using the human tumor marker MIA in cDNA samples produced from total RNA without PCR-amplification. Measurements of DNA samples with concentrations as low as 1.55 × 10-12 mol/l are presented.

Wideband circularly polarized parasitic patches loaded coplanar waveguide-fed square slot antenna with grounded strips and slots for wireless communication systems

In this paper, the design, implementation, and experimental validation of a wideband circularly polarized square slot antenna (CPSSA) is presented. The proposed antenna comprises of an irregular square patch, asymmetrical grounded-L strips, an inverted-L grounded slot, a parasitic asymmetrical rectangular split ring resonator (RSRR) and inverted-L strip patches with the overall size of 0.305λL × 0.305λL × 0.007λL. By appropriately embedding strips, slots and parasitic patches on the antenna structure, the measured wideband 3-dB axial ratio bandwidth (ARBW) of 83.2% (2.33–5.65 GHz) is obtained, while the measured impedance bandwidth (IBW) of 86.72% (2.165–5.48 GHz) is achieved. Overall, CP bandwidth is 80.66%. A parametric study of different design parameters is investigated, and the overall performance of the antenna is presented.

Ultra-thin subwavelength film sensing through the excitation of dark modes in THz metasurfaces

We demonstrate ultra-subwavelength thin film sensing through experimental and numerical studies of the transmission characteristics of several symmetric and asymmetric planar split ring resonator (SRR) based terahertz metasurfaces coated with thin film of analyte layer(s). A sharp narrow-band dark mode resonance excited in the asymmetric metasurfaces is seen to be particularly very interesting to be exploited in thin film sensing, which is otherwise absent in the symmetric case. For this mode, our experiments clearly have demonstrated sensing of extreme subwavelength (close to thousandth part of the wavelength) thickness film. With the increasing thickness of the analyte layer a gradual red-shift of the resonance mode is observed. The sensitivities and figure of merit (FoM) of all the metasurfaces are derived and compared. The resonant dark modes in asymmetric metasurfaces are found to display substantially better FoM as compared to the typical dipolar modes.

The Realization of Chipless RFID Resonator for Multiple Physical Parameter Sensing

We present the design of asymmetric circular split ring resonator as a chipless radio frequency identification (RFID) sensor tag design that can be deployed to measure multiple physical parameters wirelessly in ultrawideband frequencies. Secondarily, the proposed tag design can extend to be used as a chipless RFID tag with actual data encoded for product identification purposes. The proposed resonator design consists of four complementary split circular rings. Each gap between the rings acts as a capacitive sensor which is optimized to have a resonance with a high-quality factor. Each resonance can be considered as a sensor to deploy smart materials that are sensitive to different environmental physical parameters, which can be operated in wireless conditions. The fabricated chipless RFID sensor provides a higher ${Q}$ factor, aligned with the optimized simulation model. Chipless RFID sensors are comparatively low-cost, but they provide enormous design flexibility in diversified applications. The proposed design holds a great promise for sensing multiple physical parameters in a remote setting.

All-dielectric metamaterial analogue of electromagnetically induced transparency and its sensing application in terahertz range.

A novel electromagnetically induced transparency (EIT) all-dielectric metamaterial is proposed, fabricated, and characterized. The unit cell of the proposed metamaterial comprises of two asymmetric split ring resonators (a-SRRs) positioned with a mirror symmetry. The asymmetric nature of a-SRRs results from the length difference of two arcs. Optical properties of the fabricated metamaterial are investigated numerically using finite difference method, as well as experimentally using a terahertz time-domain spectroscopy. The results confirm that the proposed metamaterial exhibits an EIT transparent window in the frequency range around 0.78THz with a Q-factor of ~75.7 and a time-delay up to ~28.9ps. Theoretical investigations show that EIT effects in our metamaterial are achieved by hybridizing two bright modes in the same unit cell, which are aroused by the excitation of magnetic moments. We also confirm that the proposed metamaterial has great potential for sensing applications with high sensitivity and high figure of merit (FOM), which guarantees potential applications in in situ chemical and biological sensing.

Liquid crystal enabled dynamic cloaking of terahertz Fano resonators

Terahertz (THz) metadevices featured by high-Q Fano resonance are applicable for ultrasensitive biodetection. The active tuning of Fano resonance further extends their applications to switching and filtering. Here, we propose a dynamic Fano cloaking in a liquid crystal (LC) integrated THz metasurface. The metasurface is composed of two-gap asymmetric split rings. Its Fano resonance is intensively dependent on the incident polarization. The Fano resonance occurs when illuminated by THz waves with polarization perpendicular to the gaps, while for parallel polarization, the Fano resonance vanishes, namely, the cloaking of Fano resonators. A 250-μm-thick LC layer functions as an integrated tunable polarization converter. Thus, the device can be electrically switched between the sharp Fano state and the high-transmission state. The modulation depth reaches over 50% in a broad frequency range of 660 GHz. This work may inspire various advanced active THz apparatuses for biosensing, switching, and filtering.

Dual Notch Microwave Sensors Based on Complementary Metamaterial Resonators

In this paper, three dual notch microwave sensors are presented based on a microstrip transmission line and complementary metamaterial resonators. The main aim of this paper is to compare the constitutive parameters and sensitivity of all three dual notch sensors which are based on complementary symmetric split ring resonator (CS-SRR), complementary asymmetric split ring resonator (CAS-SRR) and complementary bisymmetric split ring resonator (CBS-SRR). The main motivation beyond the presented work is to use dual notches to estimate the relative permittivity of material under test (MUT). Electromagnetic simulation elucidates the origin of dual mode resonance of all the three resonators. Sensitivity analysis is performed on each sensor by using fifteen MUTs with relative permittivity ranges from 1.006 to 16.5 and constant dimensions 10 mm × 10 mm × 1 mm. To verify the concept, a sensor is fabricated and its response is measured using a vector network analyzer (AV3672). Using curve fitting technique the shift in the resonance frequencies of the fabricated sensor due to interaction with MUT is presented as a function of permittivity. Simulated, measured and formulated results are in good agreement with each other.

Dual broadband absorber based on graphene metamaterial in the terahertz range

In this paper, a dual broadband terahertz (THz) graphene metamaterial absorber (MA) is presented. The absorber consists of patterned graphene, a gating layer, and gold ground separated by a SiO2 substrate. Multiple resonances and broadband absorption are obtained simultaneously with a simple asymmetric electrical split ring structure. The numerical calculations indicate that there are two broad absorption bands (with an absorptance larger than 95%) from 1.4 THz to 1.9 THz and 4.5 THz to 5.1 THz, respectively. Under the regulation of the graphene chemical potential, the absorption strength can be controlled electrically from 95% to 10%. Furthermore, it possesses a good tolerance of polarization and incidence angles. The design scheme provides a new perspective to design dual broadband absorbers and is scalable to develop various graphene absorbers at other frequencies.