C-shaped Resonators Research Articles

Design of tunable terahertz metamaterial for variable optical attenuation and sensing applications

In this work, an actively tunable terahertz metamaterial (TTM) is proposed to realize variable optical attenuation and sensing applications. The unit cell of TTM is composed of H-shaped resonator (HSR) and C-shaped resonator (CSR). The resonant frequency can be tuned from 0.60 THz to 0.82 THz and show an analog electromagnetically induced transparency (EIT) phenomenon. By adjusting the geometry parameters of HSR and CSR, the enhanced quality (Q) factor is obtained from 2 to 14. Moreover, the CSR can be rotated from 0° to 90° to show the potential in the variable optical attenuator (VOA) application. The resonant intensity at 0.60 THz can be gradually decreased and then disappeared eventually when the CSR rotated from 0° to 90° in TE mode. While the resonant intensity at 0.60 THz can be gradually increased and then reach maximum value from 0° to 90° in TM mode. To demonstrate the proposed TTM can be used for the environmental sensing application, the TTM is exposed on the ambient environment with different refractive indexes from 1.0 to 2.2. The maximum sensitivity is 67 GHz. This work offers a novel approach for the THz metamaterial using for the VOA, optical switching, and sensing applications.

MEMS reconfigurable terahertz meta-absorber with polarization-dependent and sensing characteristics

This study presents and demonstrates a design of a micro-electro-mechanical system (MEMS) reconfigurable terahertz (THz) metamaterial absorber (TMA) that shows tunable free spectra range with a maximum value as 166 GHz in transverse magnetic mode and switchable resonance from 1.24 THz to 1.53 THz. The unit structure of TMA consists of two movable rectangular resonators aside and an x-shaped resonator at the center comprising two c-shaped resonators connected upside down. The material configuration of TMA is typical of the metal–insulator-metal sandwich configuration. That is a bottom Au layer preventing the penetration of the THz wave, a dielectric SiO2 layer to accommodate and absorb the incident wave, and a patterned Au structure array to select the resonant frequency. The electromagnetic response of TMA could be flexibly modulated when the structure is reconfigured by changing the gaps between x-shaped resonators and rectangular resonators based on the MEMS platform. Meanwhile, owing to the asymmetry of resonant structure, the electromagnetic response of TMA could be tuned from single-, dual-, triple- quad-resonances by varying the polarization state of incidence. Furthermore, the sensitivity of TMA is explored and demonstrated with remarkable linearity under a variation of the surrounding environment refractive index from 1.0 to 2.0. The feasibility of TMA is validated as the experimental absorption spectra are well meet the simulation results. The above results indicate the proposed TMA possesses great potential applications in multiple resonance switches, polarization switches, and highly efficient environmental sensors.

Frequency-Selective Surface-Based MIMO Antenna Array for 5G Millimeter-Wave Applications.

In this paper, a radiating element consisting of a modified circular patch is proposed for MIMO arrays for 5G millimeter-wave applications. The radiating elements in the proposed 2 × 2 MIMO antenna array are orthogonally configured relative to each other to mitigate mutual coupling that would otherwise degrade the performance of the MIMO system. The MIMO array was fabricated on Rogers RT/Duroid high-frequency substrate with a dielectric constant of 2.2, a thickness of 0.8 mm, and a loss tangent of 0.0009. The individual antenna in the array has a measured impedance bandwidth of 1.6 GHz from 27.25 to 28.85 GHz for S11 ≤ -10 dB, and the MIMO array has a gain of 7.2 dBi at 28 GHz with inter radiator isolation greater than 26 dB. The gain of the MIMO array was increased by introducing frequency-selective surface (FSS) consisting of 7 × 7 array of unit cells comprising rectangular C-shaped resonators, with one embedded inside the other with a central crisscross slotted patch. With the FSS, the gain of the MIMO array increased to 8.6 dBi at 28 GHz. The radiation from the array is directional and perpendicular to the plain of the MIMO array. Owing to the low coupling between the radiating elements in the MIMO array, its Envelope Correlation Coefficient (ECC) is less than 0.002, and its diversity gain (DG) is better than 9.99 dB in the 5G operating band centered at 28 GHz between 26.5 GHz and 29.5 GHz.

Multi Frequency Controllable In-Band Suppressions in a Broad Bandwidth Microstrip Filter Design for 5G Wi-Fi and Satellite Communication Systems Utilizing a Quad-Mode Stub-Loaded Resonator

The key elements used for receiving and processing signals in communication systems are the bandpass filters. Initially, a common operating mechanism was applied for the design of broadband filters, i.e., by cascading low-pass filters or high-pass filters using multiple line resonators with length quarter-half- or full-wavelength with central frequency, but using these approaches, the design topology becomes expensive and complex. The above mechanisms can be possibly overcome using a planar microstrip transmission line structure due to its simple design fabrication procedure and low cost. So, pointing out the above problems in bandpass filters such as low-cost, low insertion loss, and good out-of-band performance, this article presents a broadband filter with multifrequency suppression capability at 4.9 GHz, 8.3 GHz, and 11.5 GHz using a T-shaped shorted stub-loaded resonator with a central square ring coupled to the basic broadband filter. Initially, the C-shaped resonator is utilized for the formation of a stopband at 8.3 GHz for a satellite communication system, and then a shorted square ring resonator is added to the existing C-shaped structure for the realization of two more stopbands at 4.9 GHz and 11.5 GHz for 5G (WLAN 802.11j) communication, respectively. The overall circuit area covered with the proposed filter is 0.52 λg × 0.32 λg (λg is the wavelength of the feed lines at frequency 4.9 GHz). All the loaded stubs are folded in order to save the circuit area, which is an important requirement of next-generation wireless communication systems. The proposed filter has been analyzed using a well-known transmission line theory, even–odd-mode, and simulated with the 3D software HFSS. After the parametric analysis, some attractive features were obtained, i.e., compact structure, simple planar topology, low insertion losses of 0.4 dB over the entire band, good return loss greater than 10 dB, and independently controlled mutli stopbands, which make the proposed design unique and can be used in various wireless communication system applications. Finally, a Rogers RO-4350 substrate is selected for the fabrication of the prototype using an LPKF S63 ProtoLaser machine and then measured using a ZNB20 vector network analyzer for matching the simulated and measured results. After testing the prototype, a good agreement was found between the results.

Sensing Performance Analysis of Spiral Metasurface Utilizing Phase Spectra Measurement Technique

We have demonstrated both numerically and experimentally a 2D plasmonic metamaterial the unit cell of which comprised an Archimedean spiral with a C-shaped resonator. Such metasurface enables the excitation of spoof localized plasmon resonances (LPRs) in the terahertz frequency range, similar in properties to the familiar LPRs in the visible range. We have compared the thin-film sensing potentials of the fundamental and dark resonant modes supported by the metasurface in the range of 0.2–0.5 THz. Both the amplitude and phase transmission spectra have been studied. A sensitivity of 21.1%/RIU (78.7 GHz/RIU) and a figure of merit (FOM) of 14.4 RIU−1 have been achieved. The FOM and Q factor obtained from the phase transmission spectra were shown to be about twice higher than those obtained from the amplitude spectra.

Parameter extraction method for reflection-asymmetric bianisotropic metamaterial slabs using one-port measurements

An effective microwave method is developed for extracting electromagnetic properties of bianisotropic metamaterial (MM) slabs. As a non-iterative method, it utilizes only the one-port (air-backed and metal-backed MM slab) scattering (S-) parameter measurements. This is a useful feature especially for vector network analyzers, such as MS46131A, MS46121B, and R60, performing only one-port S-parameter measurements. Electromagnetic properties of a bianisotropic MM slab formed by C-shaped resonators are measured by an X-band (8.2–12.4 GHz) rectangular waveguide setup. The effect of any gap between the short-circuit termination for metal-backed measurements and the end of the MM slab on extracted electromagnetic properties is also examined for evaluating the possible error for proposed method.

Dual plasmon-Induced transparency in two-dimensional metamaterials with coupled resonators

We herein report a numerical investigation of dual plasmon induced transparency (PIT) effect in two-dimensional plasmonic metamaterials composed of two C-shaped resonators and one U-shaped resonator. The finite difference time domain (FDTD) method is herein introduced to analyze transmission properties and electromagnetic field distributions of certain resonance wavelengths. It was found that, the PIT spectral response was affected by parameters of plasmonic metamaterials, such as the widths of C-shaped resonator and U-shaped resonator, and edge-to-edge distance between the adjacent resonators. The dual PIT effect can be attributed to the near field coupling between the dark and bright modes. The plasmonic nanostructures can provide useful guidelines for dual PIT effect in metamaterials, to support applications in selective storage, bio-chemical nanosensors, plasmonic-based all-optical information processing and optical switches.

Ultra-wideband efficient polarization conversion of terahertz wave in a planar metamaterial

We have investigated numerically and experimentally an ultra-wideband efficient polarization conversion of a linearly polarized terahertz wave in a planar metamaterial (MM) geometry comprising an array of C-shaped resonators. Both the co- and cross-polarization transmission spectra of the proposed MM configuration have been studied for numerous polarizations of incident terahertz starting from 0∘ to 90∘ in steps of 15∘. Our design promises a cross-polarization conversion of 40% from 1.22 to 2.75 THz with a maximum of 45% when the MM design is rotated by 45∘ with respect to the incident light. The change in rotation angle excites an electric dipole in the C resonator, which radiates an electric field in the co-and cross-polarization. The polarization conversion has been explained with induced electric field profiles as well as surface current profiles. It is shown that the change in dimension of the C-shaped resonator causes a shift in the transmission spectrum. Numerically simulated results are in good agreement with the experimental results. Our design eliminates the complexity of the fabrication of multilayer structures and opens avenues to design ultra-wideband and efficient polarization rotation devices in the terahertz frequency regime.

Strategies to Enhance the Data Density in Synchronous Electromagnetic Encoders.

In this paper, we report two different strategies to enhance the data density in electromagnetic encoders with synchronous reading. One approach uses a periodic chain of rectangular metallic patches (clock chain) that determines the encoder velocity, and dictates the instants of time for retrieving the bits of the identification (ID) code. However, contrary to previous electromagnetic encoders, the ID is inferred at both the rising and the falling edges of the clock signal generated by the clock chain. Moreover, the bits of information are not given by the presence or absence of metallic patches at their predefined positions in the so-called ID code chain. With this novel encoding system, a bit state corresponding to a certain instant of time is identical to the previous bit state, unless there is a change in the envelope function of the ID code signal, determined by the additional non-periodic ID code chain. The other encoding strategy utilizes a single chain of C-shaped resonators, and encoding is achieved by considering four different resonator dimensions, corresponding to four states and, hence, to two bits per resonator of the chain. Thus, with these two strategies, the data density is twice the one achievable in previously reported synchronous electromagnetic encoders.

A Band-Switchable and Tunable THz Metamaterial Based on an Etched Vanadium Dioxide Thin Film

A band-switchable and tunable terahertz (THz) metamaterial based on a vanadium dioxide (VO2) thin film was proposed in the THz frequency regime. The VO2 thin film with a high conductivity change rate and smooth phase transition characteristics was deposited. To obtain band switching characteristics and reduce THz wave loss, the VO2 thin film was etched in the form of a line. Two rectangular C-shaped resonators were configured to face each other, with an etched VO2 thin film line in between. When the VO2 thin film was in the insulator phase, the two resonators individually resonated, and when the VO2 thin film was in the metal phase, they were connected and resonated as one, showing band switching characteristics. According to the state of the VO2 thin film, the fabricated metamaterial resonated at 1.29 THz when the two resonators were electrically separated and resonated at 0.65 THz when the two resonators were electrically connected. In the band-switching process, the THz wave transmission characteristics were continuously tunable. The measurement results of the proposed structure clearly showed that the rectangular C-shaped metamaterial based on the etched VO2 thin film is capable of band switching and continuous transmission control. In the near future, band-switchable and tunable THz metamaterials based on etched VO2 thin films can be employed as key devices in THz wave 6G wireless communication technology.

Broadband high-efficiency polarization-encoded meta-holograms based on 3-bit spin-decoupled reflective meta-atoms

Metasurfaces are engineered planar surfaces consisting of arrays of resonators for tailoring the electromagnetic wavefronts in a desirable way. However, the spin-locked issue of the geometric metasurfaces hinders simultaneous manipulation of both spins. In this work, a spin-decoupled information metasurface composed of simple C-shaped resonators is proposed to realize two different information channels under the orthogonal circularly polarized (CP) incidences. Based on the encoded digit '0' or '1', the diffusion scattering or a holographic image could be realized under the CP excitation in a broadband frequency range from 9 to 12 GHz. As an illustrative example, a 3-bit polarization-encoded meta-hologram is designed, fabricated, and characterized. The measured results agree very well with the numerical results, which gives the proposed method great potential in numerous applications such as holographic displays and information processing.

Dual-frequency multiple compact vortex beams generation based on single-layer Bi-spectral metasurface

Vortex beams carrying orbital angular momentum (OAM) have attracted considerable attention owing to the potential to expand channel capacity of microwave and optical communication. However, the OAM generations usually suffer from divergence along propagation. In this work, we proposed a strategy to generate multiple vortex beams with compact energy distributions based on a single-layer reflective metasurface. First, the mechanism is developed for the generation of multiple compact vortex beams. Then, an advanced single-cell bi-spectral meta-atom, which is composed of a double C-shaped slot resonator and a modified double C-shaped resonator, is proposed to actualize independent geometric phase controls at two frequencies. As an illustrative example, a dual-frequency metasurface that can achieve four compact vortex beams (two beams at each frequency) with different OAM modes at 9 and 13 GHz is designed, and each OAM beam features a much more compact energy distribution compared to the conventional OAM beam. The measured results agree very well with the simulated results, which validate the proposed design methodology.

Switchable Resonator Based Reconfigurable Bandpass / Bandstop Microstrip Filter

ABSTRACT This paper presents a reconfigurable loop-based filter performing compound reconfiguration. The designed filter is capable of functioning as a bandstop or bandpass filter by a simple approach of cascading the resonator to the transmission line connecting source and load. Shifting of operating frequencies is characterised by switching between a loop and a C-shaped resonator. To demonstrate the working of the proposed concept, a compound reconfigurable filter operating at 1.5 GHz and 1.6 GHz is developed. The filter is developed on a compact layout of size 0.27 × 0.12 λg 2. The proposed prototype is fabricated and measured for real-time validation.

Compact shorted C‐shaped patch antenna for ultrahigh frequency radio frequency identification tags mounted on a metallic plate

This paper describes an ultrahigh frequency (UHF) tag antenna mounted on a metallic plate for radio frequency identification (RFID). The impedance of the proposed antenna can be tuned using various methods. The compact patch antenna consists of a C-shaped resonator and a ground plane connected to a small shorting wall and is connected to a feeding loop in the middle of the C-shaped resonator. Etching a couple of slits close to the shorting wall and a slot in the center of the C-shaped resonator provided a flexible method for adjustment to match the conjugate impedance with the UCODE 8/8 m chip (13 − j191 Ω at 915 MHz). The optimal design with an overall size of 30 × 30 × 3 mm3 (0.092 λ0 × 0.092 λ0 × 0.0092 λ0) yielded a high power transmission coefficient of 91% and reading range of 6.4 m for the effective isotropic radiated power (EIRP) of 4.0 W when the tag antenna was mounted on a 220 × 220 mm2 metal plate. The proposed antenna was designed at standard frequency bands of the Federal Communications Commission (FCC, 902-928 MHz) for North America and Taiwan. Antenna fabrication and testing were performed, which revealed that the measured data were in good agreement with the simulation results.

Coupling Plasmonic System for Efficient Wavefront Control.

Efficient and flexible manipulation of electromagnetic waves using metasurfaces has attracted continuous attention in recent years. However, previous studies mainly apply sole resonance effect to accomplish the task. Here, we show that introducing a meta-coupling effect would reveal further physical insights in the electromagnetic wave control. To demonstrate this, a reflection-type coupling system composed by two identical linear resonances in a metal-insulator-metal configuration is theoretically proposed using the coupled-mode theory, whose phase diagram can be well controlled upon the coupling changes. Such intriguing optical property is verified by a double C-shaped resonator in the terahertz regime, where the coupling effect can be tuned by changing their either relative distance or rotation. More importantly, the reflection phase shift around the working frequency can be efficiently engineered without having to change the dimensions of the resonators. Two efficient anomalous metasurface deflectors are designed and experimentally characterized, whose maximum measured efficiency is more than 70%. The proposed controlling strategy further enriches the designing freedoms of metasurfaces and may find broad applications in realizing efficient and tunable functional devices.

Design of SWB Antenna with Triple Band Notch Characteristics for Multipurpose Wireless Applications

A compact concentric structured monopole antenna for super wide band (SWB) applications with triple notch band characteristics is designed and experimentally validated. The antenna covers an immense impedance bandwidth (1.6–47.5 GHz) with sharp triple notch bands at 1.8–2.2 GHz, 4–7.2 GHz, and 9.8–10.4 GHz to eliminate interference from co-existing advanced wireless services (AWS), C bands, and X bands, respectively. By loading an E-shaped stub connected at the top of the patch and by etching a split elliptical slot at the lower end of the radiating patch, the band rejection characteristics from 1.8–2.2 GHz for the AWS and 4–7.2 GHz for the C band are achieved, respectively. Further, by making use of a C-shaped resonator near the feed line, band rejection from 9.8–10.4 GHz for the X band is obtained. By varying the parameters of the antenna, the notch bands are controlled independently over a wide range of frequencies. The antenna provides good radiation characteristics, constant group delay response, and better gain over the pass band. The experimental results indicate that the designed antenna offers a remarkable reduction in gain and high variation in group delay over the stop bands. To characterize the wideband property and linear phase response of the designed antenna, its time-domain performance is extensively described and evaluated, which assure pulse transmission with minimum distortion.

Wideband Flexible/Transparent Connected-Ground MIMO Antennas for Sub-6 GHz 5G and WLAN Applications

A flexible transparent wideband four-element MIMO antenna with a connected ground plane is proposed with numerical computation and experimental measurement studies. The optical transparency is obtained using flexible conductive oxide material AgHT-4 and Melinex substrate. The radiating elements are in the form of circular stub-loaded C-shaped resonators, which are positioned in a carefully structured flexible Melinex substrate with an interconnected partial ground plane structured in the form of an L-shaped resonator, attaining an overall antenna size of 0.33λ × 0.48λ at the lowest operating frequency. The proposed antenna spans over a.10 dB impedance bandwidth of 2.21-6 GHz (92.32%) with an isolation level greater than 15dB among all elements. The maximum gain is 0.53dBi with a minimum efficiency of 41%, respectively which is satisfactory considering flexible structure and sheet impedance of 4Ω/sq. MIMO antenna parameters in terms of the envelope correlation coefficient (ECC) and diversity gain (DG) are also extracted where all the values are satisfactory for MIMO applications. The bending analysis of the proposed transparent MIMO antenna along the X and Y axis has revealed good performance in terms of scattering parameters and radiation pattern along with MIMO diversity performance. All of these technical points make the flexible MIMO antenna suitable for smart devices using sub-6 GHz 5G and WLAN band in IoT applications where visual clutter and co-site location issues need to be mitigated with the integration ease of conformal placement on the curved component/device surfaces.

A Metasurface-Based Single-Layered Compact AMC-Backed Dual-Band Antenna for Off-Body IoT Devices

In this article, a compact printed monopole dual-band antenna using artificial magnetic conductor (AMC)-plane with improved gain and broader bandwidth, applicable for off-body internet of things (IoT) devices is presented. The monopole antenna consists of two C-shaped resonators connected through a U-shaped monopole, parasitic elements, discrete ground circular rings and a co-planar waveguide (CPW) feedline. Each artificial magnetic conductor (AMC) unit cell consists of a slotted circular and a square stubs, designed with two zero-crossing phases for improving the radiation characteristics and to achieve the high gain. The overall size of the proposed AMC-backed antenna is 44.4 mm × 44.4 mm × 1.6 mm with electrical dimensions of 0.75λg×0.75λg×0.027λg. This AMC-backed antenna featured measured bandwidths of 9.6% and 12.4% with improved measured gain values of 4.88 dB and 4.73 dB at 2.45 GHz and 5.8 GHz, respectively. The specific absorption rate (SAR) values are analysed and found to be 1.58 W/kg at 2.45 GHz and 0.9 W/kg at 5.8 GHz. Therefore, the proposed AMC-backed antenna is useful for off-body IoT devices operating at 2.45 and 5.8 GHz industrial, scientific, and medical (ISM) band applications.

Fano resonances in the corrugated disk resonator and their applications

We have experimentally excited terahertz multipolar Fano resonances in two asymmetrical metal particles: a defective corrugated metallic disk(CMD) structure and a hybrid structure consisted of a C-shaped resonator and a CMD. Furthermore, the Fano resonance modes can also be excited by the interaction between plasmonic waveguide and CMD. Our findings have shed light into the terahertz multipolar Fano resonances in asymmetrical CMD and opened the way to the design of terahertz plasmonic devices.

Low-profile folded dipole UHF RFID tag antenna with outer strip lines for metal mounting application

A metal mountable UHF RFID tag antenna with a low-profile folded dipole structure is proposed. It is fabricated on a single layer of polytetrafluoroethylene (PTFE) dielectric laminate. It is composed of two symmetrical C-shape resonators integrated with the outer strip lines. The IC chip's terminals are connected directly to the center of the C-shaped resonators. The outer strip lines are integrated with the C-shaped resonators, which function to lower the reflection coefficient so as to match the IC chip impedance. In particular, the outer strip lines increase the inductive reactance of the antenna impedance in order to realize IC chip impedance. The design has a simple structure, due to the fact that it lacks any metallic vias. Moreover, the gain of the tag antenna can be enhanced when positioned on a large metallic sheet. Installed on a $40$ $\times$ $40$ $cm^{2}$ metallic plate, the total gain of the proposed tag antenna is $3$ dB, with a maximum reading range of $5.77$ m, while it is $3.92$ m in free space. The simulation results were in excellent agreement with the fabricated design results.